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This document describes the syntax of flat assembler g language, with basic examples. It was written with an assumption that it would be read sequentially and at any moment it uses only the concepts and constructions that have been introduced earlier. However it should be possible to jump right to the section that interests the reader, and then go back to earlier parts only when it is needed in order to better understand the later ones.
To start assembly from the command line it is necessary to provide two parameters: the first one should be name of a source file, the second one - name of the destination file. If the assembly is successful, the generated output is written into the destination and a short summary is displayed, otherwise an information about errors is shown. The maximum number of presented errors can be controlled with an additional "-e" switch (by default no more than one error is presented). The "-p" switch controls the maximum number of passes the assembler is going to attempt. This limit is by default set to 100. The "-r" switch allows to set up the limit of the recursion stack, that is the maximum allowed depth of entering macroinstructions and including additional source files. The "-v" switch can enable showing all the lines from this stack when reporting an error (by default the assembler tries to select only the lines that are likely the most informative, but this simple heuristic may not always be correct).
Every command in the assembly language occupies a single line of text. If a line contains the semicolon character, everything from that character up to the end of the line is treated as a comment and ignored by the assembler. The main part of a line (i.e. excluding the comment) may end with the backslash character and in such case the next line from the source text is going to be appended to this one. This allows to split any command across multiple lines, when needed. From now on we will refer to a source line as an entity obtained by stripping comments and joining the lines of text connected with backslash characters.
The text of source line is divided into syntactical units called tokens. There is a number of special characters that become separate tokens all by themselves. Any of the characters listed below is such a syntactical unit:
Any contiguous (i.e. not broken by whitespace) sequence of characters other than the above ones becomes a single token, which can be a name or a number. The exception to this rule is when a sequence starts with the single or the double quote character. This defines a quoted string and it may contain any of the special characters, whitespace and even semicolons, as it ends only when the same character that was used to start it is encountered. The quotes that are used to enclose the string do not become a part of the string themselves. If it is needed to define a string containing the same character that is used to enclose it, the character needs to be doubled inside the string - only one copy of the character will become a part of the string, and the sequence will continue.
Numbers are distinguished from names by the fact that they either begin with a decimal digit, or with the "$" character followed by any hexadecimal digit. This means that a token can be considered numeric even when it is not a valid number. To be a correct one it must be one of the following: a decimal number (optionally with the letter "d" attached at the end), a binary number followed by the letter "b", an octal number followed by the letter "o" or "q", or a hexadecimal number either prepended with "$" or "0x", or followed by the character "h". Because the first digit of a hexadecimal number can be a letter, it may be needed to prepend it with the digit zero in order to make it recognizable as a number. For example, "0Ah" is a valid number, while "Ah" is a name.
Any name can become a defined symbol by having some meaning (a value) assigned to it. One of the simplest methods of creating a symbol with a given value is to use the "=" command:
a = 1
The ":" command defines a label, that is a symbol with a value equal to the current address in the generated output. At the beginning of the source text this address is always zero, so when the following two commands are the first ones in the source file, they define symbols that have identical values:
first: second = 0
Labels defined with ":" command are special constructs in assembly language, since they allow any other command (including another label definition) to follow in the same line. This is the only kind of command that allows this.
What comes before the ":" or "=" character in such definition is a symbol identifier. It can be a simple name, like in the above samples, but it may also contain some additional modifiers, described below.
When a name in a symbol definition has the "?" character appended to it (with no whitespace between them), the symbol is case-insensitive (otherwise it would be defined as case-sensitive). This means that the value of such symbol may be referred to (as in an expression to the right of the "=" character) by the name being any variant of the original name that differs only in the case of letters. Only the cases of the 26 letters of the English alphabet are allowed to differ, though.
It is possible to define a case-sensitive symbol that clashes with a case-insensitive one. Then the case-sensitive symbol takes precedence and the more general one is used only when corresponding case-sensitive symbol is not defined. This can be remedied by using the "?" modifier, since it always means that the name followed by it refers to the case-insensitive symbol.
tester? = 0 tester = 1 TESTER = 2 x = tester ; x = 1 y = Tester ; y = 0 z = TESTER ; z = 2 t = tester? ; t = 0
Every symbol has its own namespace of descendants, called child namespace. When two names are connected by a dot with no whitespace in between, such identifier refers to the symbol with the second name in a namespace of descendants to the symbol specified by the first name. This operation can be repeated many times within a single identifier, allowing to refer to descendants of descendants in a chain of any length.
space: space.x = 1 space.y = 2 space.color: space.color.r = 0 space.color.g = 0 space.color.b = 0
Any of the names in such chain may optionally be followed by the "?" character to mark that it refers to a case-insensitive symbol. If "?" is inserted in the middle of the name (effectively splitting it into separate tokens) such identifier is considered a syntactical error.
When an identifier starts with a dot (in other words: when the name of the parent symbol is empty), it refers to the symbol in the namespace of the latest label that was defined in the current base namespace. This allows to rewrite the above sample like this:
space: .x = 1 .y = 2 .color: .color.r = 0 .color.g = 0 .color.b = 0
After the "space" label is defined, it becomes the latest label defined within the base namespace, so the following ".x" refers to the "space.x" symbol and then the ".color" refers to the "space.color". This symbol becomes the latest label defined within the child namespace of "space", but it does not affect the current namespace, where "space" remains the newest label.
The "namespace" command followed by a symbol identifier changes the base namespace for a section of source text. It must be paired with the "end namespace" command later in the source to mark the end of such section. This can be used to again rewrite the above sample in a different way:
space: namespace space x = 1 y = 2 color: .r = 0 .g = 0 .b = 0 end namespace
When a name is not preceded by a dot, and as such it does not have explicitly specified in what namespace the symbol resides, the assembler looks for defined symbol in the current namespace, and if none is found, in the consecutive namespaces of parent symbols, starting from the namespace containing the parent symbol of current namespace. If no defined symbol with such name is found, it is assumed that the name refers to the symbol in the current namespace (and unless there is "?" character after such name, it is assumed that the symbol is case-sensitive). A definition that does not specify the namespace where the new symbol should be created, always makes the new symbol in the current base namespace.
global = 0 regional = 1 namespace regional regional = 2 ; regional.regional = 2 x = global ; regional.x = 0 regional.x = regional ; regional.regional.x = 2 global.x = global ; global.x = 0 end namespace
The comments in the above sample show the equivalent definitions with respect to the original base namespace. Note that when a name is used to specify the namespace, the assembler looks for a defined symbol with such name to lookup in its namespace, but when it is a name of a symbol to be defined, it is always created within the current base namespace.
When the final dot of an identifier is not followed by any name, it refers to the parent symbol of the namespace that would be searched for a symbol if there was a name after this dot. Adding such dot at the end of an identifier may appear redundant, but it can be used to alter the way the definition of a symbol works, because it forces the assembler to look for an already existing symbol that it can alter instead of squarely creating a new one in the current namespace. For instance, if in the fourth line of the previous example "regional." was put in place of "regional", it would define a new value for the original "regional" symbol instead of making a new symbol in the child namespace. Similarly, a definition formed this way may assign a new value to a symbol regardless of whether it was previously defined as case-insensitive or not.
If an identifier is just a single dot, by the above rules it refers to the latest label defined in the current base namespace. This can be applied to rewrite the earlier example in yet another way:
space: namespace . x = 1 y = 2 color: namespace . r = 0 g = 0 b = 0 end namespace end namespace
It also demonstrates how namespace sections can be nested one within another.
The "#" may be inserted anywhere inside an identifier without changing its meaning. When "#" is the only character separating two name tokens, it causes them to be interpreted as a single name formed by concatenating the tokens.
variable = 1 varia#ble = var#iable + 2 ; variable = 3
This can also be applied to numbers.
There is a couple of additional rules concerning the use of dots in identifiers. When an identifier starts with a dot, but there has been no label defined in the current base namespace, the identifier refers to the descendant of a special symbol that resides in the current namespace but has no name. If an identifier starts with a sequence of two or more dots, the identifier refers to the descedant of a similar unnamed symbol, but it is a distinct one for any given number of dots. While the namespace accessed with a single starting dot changes every time a new label is defined within the current namespace, the special namespace accessed with two or more dots in the beginning of an identifier remains the same everywhere within the current base namespace:
first: .child = 1 ..other = 0 second: .child = 2 ..another = ..other
In this example the meaning of the ".child" identifier changes from place to place, but the "..other" identifier means the same everywhere.
When two names inside an identifier are connected with a sequence of two or more dots, the identifier refers to the descendant of such special unnamed symbol in the namespace specified by the identifier before that sequence of dots. The unnamed child namespace is chosen depending on a number of dots and in this case the number of required dots is increased by one. The following example demonstrates the two methods of identifying such symbol:
namespace base ..other = 1 end namespace result = base.#..other
The "#" character has been inserted into the last identifier for a better readability, but the plain sequence of three dots would do the same.
The unnamed symbol that hosts a special namespace can itself be accessed when an identifier ends with a sequence of two or more dots - thanks to the rule that an identifier which ends in a dot refers to the parent symbol of the namespace that would be accessed if there was a name after this dot. So in the context of the previous example the "base..." (or "base.#..") would refer to the unnamed parent of the namespace where the "other" symbol resides, and it would be the same symbol as identified by simple ".." inside the namespace of the "base" symbol.
When a symbol is defined as a label, it must be the only definition of this symbol in the entire source. A value that is in this way assigned to the symbol can be accesed from every place in the source, even before the label is actually defined. When a symbol is used before it is defined (this is often called forward-referencing) the assembler tries to correctly predict the value of the symbol by doing multiple passes over the source text. Only when all predictions prove to be correct, the assembler generates the final output.
This kind of symbols, which can only be defined once and thus have a universal value that can always be forward-referenced, is called a constant symbol. All labels are constants.
When a symbol is defined with a "=" command, it may have multiple definitions of this kind. Such symbol is called variable and when it is used, the value from its latest definition is accessed. A symbol defined with such command can also be forward-referenced, but only when it is defined exactly once in the entire source and as such has a single unambiguous value.
a = 1 ; a = 1 a = a + 1 ; a = 2 a = b + 1 ; a = 3 b = 2
A special case of forward-referencing is self-referencing, when the value of a symbol is used in its own definition. The assembly of such construct is successful only when the assembler is able to find a value that is stable under such evaluation, effectively solving an equation. But due to the simplicity of the resolving algorithm based on predictions a solution may not be found even when it exists.
x = (x-1)*(x+2)/2-2*(x+1) ; x = 6 or x = -1
The ":=" defines a constant value. It may be used instead of "=" to ensure that the given symbol is defined exactly once and that it can be forward-referenced.
The "=:" defines a variable symbol like "=", but it differs in how it treats the previous value (when such exists). While "=" discards the previous value, "=:" preserves it so it can later be brought back with the "restore" command:
a = 1 a =: 2 ; preserves a = 1 a = 3 ; discards a = 2 and replaces it with a = 3 restore a ; brings back a = 1
The "restore" may be followed by multiple symbol identifiers separated with commas, and it discards the latest definition of every one of them. It is not considered an error to use "restore" with a symbol that has no active definition (either because it was never defined or because all of its definitions were already discarded earlier). If a symbol is treated with the "restore" command, it can never be forward-referenced - for this reason "restore" cannot be applied to constants.
The "label" keyword followed by a symbol identifier is an alternative way of defining a label. In this basic form it is equivalent to a definition made with ":", but it occupies an entire line. However with this command it is possible to provide more settings for the defined label. The identifier may be optionally followed by the ":" token and then an additional value to be associated with this label (usually denoting the size of the labeled entity). The assembler has a number of built-in constants defining various sizes for this purpose, but this value can also be provided as a plain number.
label character:byte label char:1
The ":" character may be omitted in favor of a plain whitespace, but it is recommended for clarity. After an identifier and an optional size, the "at" keyword may follow and then a value that should be assigned to the label instead of the current address.
label wchar:word at char
The built-in size constants are equivalent to the following set of definitions:
byte? = 1 ; 8 bits word? = 2 ; 16 bits dword? = 4 ; 32 bits fword? = 6 ; 48 bits pword? = 6 ; 48 bits qword? = 8 ; 64 bits tbyte? = 10 ; 80 bits tword? = 10 ; 80 bits dqword? = 16 ; 128 bits xword? = 16 ; 128 bits qqword? = 32 ; 256 bits yword? = 32 ; 256 bits dqqword? = 64 ; 512 bits zword? = 64 ; 512 bits
The "element" keyword followed by a symbol identifier defines a special constant that has no fixed value and can be used as a variable in the linear polynomials. The identifier may be optionally followed by the ":" token and then a value to be associated with this symbol, called metadata of the element.
element A element B:1
The metadata assigned to a symbol can be extracted with a special operator, defined in the next section.
In every construction described so far where a value of some kind was provided, like after the "=" command or after the "at" keyword, it could be a literal value (a number or a quoted string) or a symbol identifier. A value can also be specified through an expression containing built-in operators.
The "+", "-" and "*" perform standard arithmetic operations on integers ("+" and "-" can also be used in a unary form - with only one argument). "/" and "mod" perform division with remainder, giving a quotient or a remainder respectively. Of these arithmetic operators "mod" has the highest precedence (it is calculated first), "*" and "/" come next, while "+" and "-" are evaluated last (even in their unary variants). Operators with the same precedence are evaluated from left to right. Parentheses can be used to enclose sub-expressions when a different order of operations is required.
The "xor", "and" and "or" perform bitwise operations on numbers. "xor" is addition of bits (exclusive or), "and" is multiplication of bits, and "or" is inclusive or (logical disjunction). These operators have higher precedence than any arithmetic operators.
The "shl" and "shr" perform bit-shifting of the first argument by the amount of bits specified by the second one. "shl" shifts bits left (towards the higher powers of two), while "shr" shifts bits right (towards zero), dropping bits that fall into the fractional range. These operators have higher precedence than other binary bitwise operations.
The "not", "bsf" and "bsr" are unary operators with even higher precedence. "not" inverts all the bits of a number, while "bsf" and "bsr" search for the lowest or highest set bit respectively, and give the index of that bit as a result.
All the operations on numbers are performed as if they were done on the infinite 2-adic representations of those numbers. For example the "bsr" with a negative number as an argument gives no valid result, since such number has an infinite chain of set bits extending towards infinity and as such contains no highest set bit (this is signaled as an error).
The "bswap" operator allows to create a string of bytes containing the representation of a number in a reverse byte order (big endian). The second argument to this operator should be the length in bytes of the required string. This operator has the same precedence as the "shl" and "shr" operators.
When a string value is used as an argument to any of the operations on numbers, it is treated as a sequence of bits and automatically converted into a positive number (extended with zero bits towards the infinity). The consecutive characters of a string correspond to the higher and higher bits of a number.
To convert a number back to a string, the "string" unary operator may be used. This operator has the lowest possible precedence, so when it precedes an expression, all of it is evaluated prior to the conversion. When conversion in the opposite direction is needed, simple unary "+" is enough to make a string become a number.
The length of a string may be obtained with the "lengthof" unary operator. This operator can only be applied to a string and it is one of the operators with the highest precedence.
When a symbol defined with the "element" command is used in an expression the result may be a linear polynomial in a variable represented by the symbol. Only simple arithmetic operations are allowed on the terms of a polynomial, and it must stay linear - so, for example, it is only allowed to multiply a polynomial by a number, but not by another polynomial.
There are a few operators that have the highest possible precedence and allow to extract the information about the terms of linear polynomial. The polynomial should come as the first argument, and the index of the term as the second one. The "element" operator extracts the variable of a polynomial term (with the coefficient of one), the "scale" operator extracts the coefficient (a number by which the variable is multiplied) and "metadata" operator gives back the metadata associated with the variable.
When the second argument is an index higher than the index of the last term of the polynomial, all three operators return zero. When the second argument is zero, "element" and "scale" give information about the constant term - "element" returns numeric 1 and "scale" returns the value of the constant term.
element A linpoly = A + A + 3 vterm = linpoly scale 1 * linpoly element 1 ; vterm = 2 * A cterm = linpoly scale 0 * linpoly element 0 ; cterm = 3 * 1
The "metadata" operator with an index of zero returns the size that is associated with the first argument. This value is definite only when the first argument is a symbol that has a size associated with it (or an arithmetic expression that contains such symbol), otherwise it is zero. There exists an additional unary operator "sizeof", which gives the same value as "metadata 0".
label table : 256 length = sizeof table ; length = 256
The order of the terms of the linear polynomial depends on the way in which the value was constructed. Every arithmetic operation preserves the order of the terms in the first argument, and the terms that were not present in the first argument are attached at the end in the same order in which they occurred in the second argument. This order only matters when extracting terms with "element", "scale" or "metadata" operator.
The "elementsof" is another unary operator of the highest precedence, it counts the number of variable terms of a linear polynomial.
An expression may also contain a literal value that defines a floating-point number. Such number must be in decimal notation, it may contain "." character as a decimal mark and may be followed by the "e" character and then a decimal value of the exponent (optionally preceded by "+" or "-" to mark the sign of exponent). When "." or "e" is present, it must be followed by at least one digit. The "f" character can be appended at the end of such literal value. If a number contains neither "." nor "e", the final "f" is the only way to ensure that it is treated as floating-point and not as a simple decimal integer.
The floating-point numbers are handled by the assembler in the binary form. Their range and precision are at least as high as they are in the longest floating-point format that the assembler is able to produce in the output.
Basic arithmetic operations are allowed to have a floating-point number as any of the arguments, but none of the arguments may contain a non-scalar (linear polynomial) terms then. The result of such operation is always a floating-point number.
The unary "float" operator may be used to convert an integer value to floating-point. This operator has the highest precedence.
The "trunc" is another unary operator with the highest precedence and it can be applied to floating-point numbers. It extracts the integer part of a number (it is a truncation toward zero) and the result is always a plain integer, not a floating-point number. If the argument was already a plain integer, this operation leaves it unchanged.
The "bsr" operator can be applied to floating-point numbers and it returns the exponent of such number, which is the exponent of the largest power of two that is not larger than the given number. The sign of the floating-point value does not affect the result of this operation.
It is also allowed to use a floating-point number as the first argument to the "shl" and "shr" operators. The number is then multiplied or divided by the power of two specified by the second argument.
There are three distinct classes of symbols, determining the position in source line at which the symbol may be recognized. A symbol belonging to the instruction class is recognized only when it is the first identifier of the command, while a symbol from the expression class is recognized only when used to provide a value of arguments to some command.
All the types of definitions that were described in the earlier sections create the expression class symbols. The "label" and "restore" are the examples of built-in symbols belonging to the instruction class.
In any namespace it is allowed for symbols of different classes to share the same name, for example it is possible to define the instruction named "shl", while there is also an operator with the same name - but an operator belongs to the expression class. It is however recommended to avoid name clashes of this kind. An existing instruction may actually prevent definition of a symbol with the same name using a command like "=", because such definition requires the name to be the first identifier in command, and then it would be automatically interpreted as an instruction.
The third class of symbols are the labeled instructions. A symbol belonging to this class may be recognized only when the first identifier of the command is not an instruction - in such case the first identifier becomes a label to the instruction defined by the second one. If we treat "=" as a special kind of identifer, it may serve as an example of labeled instruction.
The rules concerning namespace apply equally to the symbols of all classes, for example symbol of instruction class belonging to the child namespace of latest label can be executed by preceding its name with dot. It should be noted, however, that when a namespace is specified through its parent symbol, it is always a symbol belonging to the expression class. It is not possible to refer to a child namespace of an instruction, only to the namespace belonging to the expression class symbol with the same name.
The assembler contains built-in symbols of all classes. Their names are always case-insensitive and they may be redefined, but it is not possible to remove them. When all the values of such symbol are removed with a command like "restore", the built-in value persists.
The "db" instruction allows to generate bytes of data and put them into the output. It should be followed by one or more values, separated with commas. When the value is numeric, it defines a single byte. When the value is a string, it puts the string of bytes into output.
db 'Hello',13,10 ; generate 7 bytes
The "dup" keyword may be used to generate the same value multiple times. The "dup" should be preceded by numeric expression defining the number of repetitions, and the value to be repeated should follow. A sequence of values may also be duplicated this way, in such case "dup" should be followed by the entire sequence enclosed in parentheses (with values separated with commas).
db 4 dup 90h ; generate 4 bytes db 2 dup ('abc',10) ; generate 8 bytes
When a special identifier consisting of a lone "?" character is used as a value in the arguments to "db", it reserves a single byte. This advances the address in the output where the next data are going to be put, but the reserved bytes are not generated themselves unless they are followed by some other data. Therefore if the bytes are reserved at the end of output, they do not increase the size of generated file. This kind of data is called uninitialized, while all the regular data are said to be initialized.
The "rb" instruction reserves a number of bytes specified by its argument.
db ? ; reserve 1 byte rb 7 ; reserve 7 bytes
Every built-in instruction that generates data (traditionally called a data directive) is paired with a labeled instruction of the same name. Such command in addition to generating data defines a label at address of generated data, with associated size equal to the size of data unit used by this instruction. In case of "db" and "rb" this size is 1.
some db sizeof some ; generate a byte with value 1
The "dw", "dd", "dp", "dq", "dt", "ddq", "dqq" and "ddqq" are instructions analogous to "db" with a different sizes of data unit. The order of bytes within a single generated unit is always little-endian. When a string of bytes is provided as the value to any of these instructions, the generated data is extended with zero bytes to the length which is the multiple of data unit. The "rw", "rd", "rp", "rq", "rt", "rdq", "rqq" and "rdqq" are the instructions that reserve a specified number of data units. The unit sizes associated with all these instructions are listed in table 1.
The "dw", "dd", "dq", "dt" and "ddq" instructions allow floating-point numbers as data units. Any such number is then converted into floating-point format appropriate for a given size.
The "emit" (with a synonym "dbx") is a data directive that uses the size of unit specified by its first argument to generate data defined by the remaining ones. The size may be separated from the next argument with a colon instead of a comma, for better readability. When the unit size is such that it has a dedicated data directive, the definition made with "emit" has the same effect as if these values were passed to the instruction tailored for this size.
emit 2: 0,1000,2000 ; generate three 16-bit values
The "file" instruction reads the data from an external file and writes it into output. The argument must be a string containing the path to the file, it may optionally be followed by ":" and the numeric value specifying an offset within the file, next it may be followed by comma and the numeric value specifying how many bytes to copy.
file 'data.bin' ; insert entire file excerpt file 'data.bin':10h,4 ; insert selected four bytes
|Size (bytes)||Generate data||Reserve data|
The "if" instruction causes a block of source text to be assembled only under certain condition, specified by a logical expression that is an argument to this instruction. The "else if" command in the following lines ends the previous conditionally assembled block and opens a new one, assembled only when the previous conditions were not met and the new condition (an argument to "else if") is true. The "else" command ends the previous conditionally assembled block and begins a block that is assembled only when none of the previous conditions was true. The "end if" command should be used to end the entire construction. There may be many or none "else if" commands inside and no more than one "else".
A logical expression is a distinct syntactical entity from the basic expressions that were described earlier. A logical expression consists of logical values connected with logical operators. The logical operators are: unary "~" for negation, "&" for conjunction and "|" for alternative. The negation is evaluated first, while "&" and "|" are simply evaluated from left to right, with no precedence over each other.
A logical value in its simplest form may be a basic expression, it then corresponds to true condition if and only if its value is not constant zero. Another way to create a logical value is to compare the values of two basic expressions with one of the following operators: "=" (equal), "<" (less than), ">" (greater than), "<=" (less or equal), ">=" (greater or equal), "<>" (not equal).
count = 2 if count > 1 db '0' db count-1 dup ',0' else if count = 1 db '0' end if
When linear polynomials are compared this way, the logical value is valid only when they are comparable, which is whey they differ in constant term only. Otherwise the condition like equality is neither universally true nor universally false, since it depends on the values substituted for variables, and assembler signals this as an error.
The "relativeto" operator creates a logical value that is true only when the difference of compared values does not contain any variable terms. Therefore it can be used to check whether two linear polynomials are comparable - the "relativeto" condition is true only when both compared polynomials have the same variable terms.
Because logical expressions are lazily evaluated, it is possible to create a single condition that will not cause an error when the polynomials are not comparable, but will compare them if they are:
if a relativeto b & a > b db a - b end if
The "eqtype" operator can also be used to compare two basic expressions, it makes a logical value which is true when the values of the expressions are of the same type - either both are algebraic, both are strings or both are floating-point numbers. An algebraic type covers the linear polynomials and it includes the integer values.
The "eq" operator compares two basic expressions and creates a logical value which is true only when their values are of the same type and equal. This operator can be used to check whether a value is a certain string, a certain floating-point number or a certain linear polynomial. It can compare values that are not comparable with "=" operator.
The "defined" operator creates a logical value combined with a basic expression that follows it. This condition is true when the expression does not contain symbols that have no accessible definition. The expression is only tested for the availability of its components, it does not need to have a computable value. This can be used to check whether a symbol of expression class has been defined, but since the symbol can be accessible through forward-referencing, this condition may be true even when the symbol is defined later in source. The basic expression that follows "defined" is also allowed to be empty and the condition is then trivially satisfied.
The "used" operator forms a logical value if it is followed by a single identifier. This condition is true when the value of specified symbol has been used anywhere in the source.
The "assert" is an instruction that signalizes an error when a condition specified by its argument is not met.
The "macro" command allows to define a new instruction, in form of a macroinstruction. The block of source text between the "macro" and "end macro" command becomes the text of macroinstruction and this sequence of lines is assembled in place of the original command that starts with identifier of instruction defined this way.
macro null db 0 end macro null ; "db 0" is assembled here
The macroinstruction is allowed to have arguments only when the definition contains them. After the "macro" and the identifier of defined symbol optionally may come a list of simple names separated with commas, these names define the parameters of macroinstruction. When this instruction is then used, it may be followed by at most the same number of arguments separated with commas, and their values are assigned to the consecutive parameters. Before any line of text inside the macroinstruction is interpreted, the name tokens that correspond to any the parameters are replaced with their assigned values.
macro lower name,value name = value and 0FFh end macro lower a,123h ; a = 23h
The value of a parameter can be any text, not necessarily a correct expression. If a line calling the macroinstruction contains fewer arguments than the number of defined parameters, the excess parameters receive the empty values.
When the name of parameter is defined, it may be followed by "?" character to denote that it is case-insensitive, analogously to a name in a symbol identifier. There must be no whitespace between the name and "?". A definition of a parameter may also be followed by "*" to denote that it requires a value that is not empty, or alternatively by ":" character followed by a default value, which is assigned to the parameter instead of an empty one when no other value is provided.
macro prepare name*,value:0 name = value end macro prepare x ; x = 0 prepare y,1 ; y = 1
If an argument to macroinstruction needs to contain a comma character, the entire argument must be enclosed between the "<" and ">" characters (they do not become a part of the value). If another "<" character is encountered inside such value, it must be balanced with corresponding ">" character inside the same value.
macro data name,value name: .data db value .end: end macro data example, <'abc',10>
The last defined parameter may be followed by "&" character to denote that this parameter should be assigned a value containing the entire remaining part of line, even if it normally would define multiple arguments. Therefore when macroinstruction has just one parameter followed by "&", the value of this parameter is the entire text of arguments following the instruction.
macro id first,rest& dw first db rest end macro id 2, 7,1,8
When the name of a parameter is to be replaced with its value and it is preceded by "`" character (without any whitespace inbetween), the text of the value is embedded into a quoted string and this string replaces both the "`" character and the name of parameter.
macro text line& db `line end macro text x+1 ; db 'x+1'
The "local" is a command that may only be used inside the macroinstruction. It should be followed by one or more names separated with commas, and it declares that the names from this list should in context of current macroinstruction be interpreted as belonging to the special namespace associated with this macroinstruction instead of current base namespace. This allows to create some unique symbols every time the macroinstruction is called. Such declaration defines an additional parameter with the specified name and it only affects the uses of that name that follow within the same macroinstruction. Declaring the same name as local multiple times within the same macroinstruction gives no additional effect.
macro measured name,string local top name db string top: name.length = top - name end macro measured hello, 'Hello!' ; hello.length = 6
Just like an expression symbol may be redefined and refer to its previous value in the definition of the new one, the macroinstructions can also be redefined, and use the previous value of this instruction symbol in its text:
macro zero db 0 end macro macro zero name label name:byte zero end macro zero x
And just like other symbols, a macroinstruction may be forward-referenced when it is defined exactly once in the entire source. Such macroinstruction may then use its own value in a recursive way, analogously to self-referential definition of an expression symbol.
macro factorial n if n factorial n-1 result = result * (n) else result = 1 end if end macro
A caution should be taken when redefining the built-in instructions, since they are always case-insensitive and if a macroinstruction is defined with the same name, but case-sensitive, it will create a symbol that may be forward-referenced, while overriding one of the case variants of assembly command. Trying to call the original instruction inside such macroinstruction may lead to infinite recursion. Therefore when redefining a case-insensitive instruction, it is important to not forget the "?" character after the name of macroinstruction.
The "purge" command discards the definition of a symbol just like "restore", but it does so for the symbol of instruction class. It behaves in the same way as "restore" in all the other aspects. A macroinstruction can remove its own definition with "purge".
A macroinstruction may in turn define another macroinstruction or a number of them. The blocks designated by "macro" and "end macro" must be properly nested one within the other for such definition to be accepted by the assembler.
macro enum enclosing counter = 0 macro item name name := counter counter = counter + 1 end macro macro enclosing purge item,enclosing end macro end macro enum x item a item b item c x
When it is required that macroinstruction generates unpaired "macro" or "end macro" command, it can be done with special "esc" instruction. Its argument becomes a part of macroinstruction, but is not being taken into account when counting the nested "macro" and "end macro" pairs.
macro xmacro name esc macro name x& end macro xmacro text db `x end macro
When an identifer of macroinstruction in its definition is followed by "!" character, it defines the unconditional macroinstruction. This is a special kind of instruction class symbol, which is evaluated even in places where the assembly is suspended - like inside the conditional block whose condition is false, or inside a definition of another macroinstruction. This allows to define instructions that can be used where otherwise a directly stated "end if" or "end macro" would be required, as in the following example:
macro proc name name: if used name end macro macro endp! end if .end: end macro proc tester db ? endp
If the macroinstruction "proc" in the above sample was not defined as an unconditional one and the block started with "if" was being skipped, the macroinstruction would not get evaluated, and this would lead to an error because "end if" would be missing.
It should be noted that "end" command executes an instruction identified by its argument in context of the child namespace of case-insensitive "end" symbol. Therefore command like "end if" could be alternatively invoked with an "end.if" identifier, and it is possible to override any such instruction by redefining a symbol in the "end?" namespace. Moreover, any instruction defined within the "end?" namespace can then be called with the "end" command. This slighly modified variant of the above sample puts these facts to use:
macro proc name name: if used name end macro macro end?.proc! end if .end: end macro proc tester db ? end proc
A similar rule applies to the "else" command and the instructions in the "else?" namespace.
When an instruction is unconditional, it is not allowed to override it with a regular instruction - all new definitions of such instruction must be unconditional, too. Some of the built-in instructions are unconditional, like "end", and redefining them is only possible through an unconditional macroinstruction.
When an identifier consisting of a lone "?" character is used as an instruction symbol in the definition of macroinstruction, it defines a special instruction that is then called every time a line to be assembled does not begin with an unconditional instruction, and the complete text of line becomes the arguments to this macroinstruction. This special symbol can also be defined as an unconditional instruction, and then it is called for every following line with no exception. This allows to completely override the assembly process on portions of the text. The following sample defines a macroinstruction which allows to define a block of comments by skiping all the lines of text until it encounters a line with content equal to the argument given to "comment".
macro comment?! ender macro ?! line& if `line = `ender purge ? end if end macro end macro comment ~ Any text may follow here. ~
The "struc" command allows to define a labeled instruction, in form of a macroinstruction. Except for the fact that such definition must be closed with "end struc" instead of "end macro", these macroinstructions are defined in the same way as with "macro" command. A labeled instruction is evaluated when the first identifier of a command is not an instruction and the second identifier is of the labeled instruction class:
struc some db 1 end struc get some ; "db 1" is assembled here
Inside the labeled macroinstruction identifiers starting with dot no longer refer to the namespace of latest label in the current base namespace. Instead they refer to the namespace of label with which the instruction was labeled.
struc POINT label . : qword .x dd ? .y dd ? end struc my POINT ; defines my.x and my.y
Note that the parent symbol, which can be refered by "." identifier, is not defined unless an appropriate definition is generated by the macroinstruction. In the above example it is done with the "label" instruction. For an easier use of this feature, other syntaxes may be defined with macroinstructions, like in this sample:
macro struct? definition& struc definition label . : .%top - . namespace . end macro macro ends?! %top: end namespace end struc end macro struct POINT vx:?,vy:? x dd vx y dd vy ends my POINT 10,20
Note that it is not necessary to use "esc" instruction to generate unpaired "struc" or "end struc" through a macroinstruction defined with "macro". In the same manner unpaired "macro" or "end macro" can occur freely in a definition made with "struc".
The "restruc" command is analogous to "purge", but it operates on symbols from the class of labeled instructions.
As with "macro", it is possible to use an identifier consisting of a lone "?" character with "struc". It defines a special labeled macroinstruction that is called every time the first symbol of a line is not recognized as an instruction. Everything that follows that first identifier becomes the arguments to labeled macroinstruction. The following sample uses this feature to catch any orphaned labels (the ones that are not followed by any character) and treat them as regular ones instead of causing an error. It achieves it by making ":" the default value for "def" parameter:
struc ? def::& . def end struc orphan regular: assert orphan = regular
This special variant does not override unconditional labeled instructions unless it is unconditional itself.
While "." provides an efficient method of accessing the label symbol, sometimes it may be needed to process the actual text of the label. A special parameter can be defined for this purpose and its name should be inserted enclosed in parentheses before the name of labeled macroinstruction:
struc (name) SYMBOL . db `name,0 end struc test SYMBOL
The "equ" is a built-in labeled instruction that defines symbol of expression class with a symbolic value. Such value can contain any text (even an empty one) and when it uses in expression, it is equivalent to inserting the text of its value in place of its identifier, with an effect similar to evaluation of a parameter of macroinstruction.
This can lead to a different results than when a standard variable defined with "=" is used, as the following example demonstrates:
numeric = 2 + 2 symbolic equ 2 + 2 x = numeric*3 ; x = 4*3 y = symbolic*3 ; y = 2 + 2*3
While "x" is assigned the value of 12, the value of "y" is 8. This shows that the use of such symbols can lead to unintended interactions and therefore definitions of this type should be avoided unless really necessary.
The "equ" allows redefinitions, and it preserves the previous value of symbol analogously to the "=:" command, so the earlier value can be brought back with "restore" instruction. To replace the symbolic value (analogously to how "=" overwrites the regular value) the "reequ" command should be used instead of "equ".
The symbolic value, in addition to retaining the exact text it was defined with, preserves the context in which the symbols contained in this text are to be interpreted. Therefore it can effectively become a reliable link to value of some other symbol, lasting even when it is used in a different context (this includes change of the base namespace or a symbol referred by a starting dot):
first: .x = 1 link equ .x .x = 2 second: .x = 3 db link ; db 2
It should be noted that the same applies to the parameters of any macroinstruction. If during the execution of macroinstruction the context changes, the identifiers within the text of parameter still refer to the same symbols as in the line that called this instruction:
x = 1 namespace x x = 2 end namespace macro prodx value namespace x db value*x end namespace end macro prodx x ; db 1*2
Furthermore, the parameters defined with "local" command use the same mechanism to alter the context in which given name is interpreted, without altering the text of the name. However, such modified context is not relevant if the value of parameter is inserted in a middle or at the end of a complex identifier, because it is the structure of an identifier that dictates how its later parts are interpreted and only the context for an initial part matters.
If the text following "equ" viewed as a sequence of symbols contains any identifiers of known symbolic values, each of them is replaced with the text of its value before creating a new one that is defined. The corresponding sections of text still preserve the original context, though.
The "define" is a regular instruction that also creates a symbolic value, but as opposed to "equ" it does not evaluate symbolic variables in the assigned text. It should be followed by an identifier of symbol to be defined and then by the text of the value.
The difference between "equ" and "define" is often not noticeable, because when used in final expression the symbolic variables are nestedly evaluated until only the usable constituents of expressions are left. A possible use of "define" is to create a link to another symbolic variable, like the following example demonstrates:
a equ 0* x equ -a define y -a a equ 1* db x 2 ; db -0*2 db y 2 ; db -1*2
The other uses of "define" will arise in the later sections, with the introduction of other instructions that operate on symbolic values.
The "define", like "equ", preserves the previous value of symbol. The "redefine" is a variant of this instruction that discards the earlier value, analogously to "reequ".
Note that while symbolic variables belong to the expression class of symbols, their state cannot be determined with operators like "defined" or "used", because an logical expression containing such operators is evaluated as if every symbolic variable was replaced with the text of corresponding value. Therefore "defined" followed by an identifer of symbolic variable is going to be applied to the content of this variable, whatever it is. For example if you create a symbolic variable which is simply a link to a regular symbol, the "defined" or "used" followed by the identifier of this symbolic variable is going to determine the status of linked symbol.
The "repeat" instruction allows to assemble a block of instructions multiple times, with the number of repetitions specified by the value of its argument. The block of instructions should be ended with "end repeat" command. A synonym "rept" can be used instead of "repeat".
a = 2 repeat a + 3 a = a + 1 end repeat assert a = 7
The "while" instruction causes the block of instructions to be assembled repeatedly as long as the condition specified by its argument is true. Its argument should be a logical expression, like an argument for "if" or "assert". The block should be closed with "end while" command.
a = 7 while a > 4 a = a - 2 end while assert a = 3
The "%" is a special parameter which is preprocessed inside the repeated block of instructions and is replaced with a decimal number being the number of current repetition (starting with 1). It works in a similar way to a parameter of macroinstruction, so it is replaced with its value before the actual command is processed and so it can be used to create symbol identifiers containing the number as a part of name:
repeat 16 f#% = 1 shl % end repeat
The above example defines symbols "f1" to "f16" with values being the consecutive powers of two.
The "repeat" instruction can have additional arguments, separated with commas, each containing a name of supplementary parameters specific to this block. Each of the names can be followed by ":" character and the expression specifying the base value from which the parameter is going to start counting the repetitions. This allows to easily change the previous sample to define the range of symbols from "f0" to "f15":
repeat 16, i:0 f#i = 1 shl i end repeat
The "%%" is another special parameter that has a value equal to the total number of repetitions planned. This parameter is undefined inside the "while" block. The following example uses it to create the sequence of bytes with values descending from 255 to 0:
repeat 256 db %%-% end repeat
The "break" instruction allows to stop the repeating prematurely. When it is encountered, it causes the rest of repeated block to be skipped and no further repetitions to be executed. It can be used to stop the repeating if a certain condition is met:
s = x/2 repeat 100 if x/s = s break end if s = (s+x/s)/2 end repeat
The above sample tries to find the square root of the value of symbol "x", which is assumed defined elsewhere. It can easily be rewritten to perform the same task with "while" instead of "repeat":
s = x/2 while x/s <> s s = (s+x/s)/2 if % = 100 break end if end while
The "iterate" instruction (with a synonym "irp") repeats the block of instructions while iterating through the list of values separated with commas. The first argument to "iterate" should be a name of parameter, folowed by the comma and then a list of values. During each iteration the parameter receives one of the values from the list.
iterate value, 1,2,3 db value end iterate
Like it is in the case of an argument to macroinstruction, the value of parameter that contains commas needs to be enclosed with "<" and ">" characters. It is also possible to enclose the first argument to "iterate" with "<" and ">", in order to define multiple parameters. The list of values is then divided into section containing as many values as there are parameters, and each iteration operates on one such section, assigning to each parameter a corresponding value:
iterate <name,value>, a,1, b,2, c,3 name = value end iterate
The name of a parameter can also, like in the case of macroinstructions, be followed by "*" to require that the parameter has a value that is not empty, or ":" and a default value.
The "break" instruction plus both the "%" and "%%" parameters can be used inside the "iterate" block with the same effects as in case of "repeat".
The "indx" is an instruction that can be only be used inside the iterated block and it changes the values of all the iterated parameters to the ones corresponding to iteration with number specified by the argument to "indx" (but when the next iteration is started, the values of parameters are again assigned in the normal way). This allows to process the iterated values in a different order. In the following example the values are processed from the last to the first:
iterate value, 1,2,3 indx 1+%%-% db value end iterate
With "indx" it is even possible to move the view of iterated values many times during the single repetition. In the following example the entire processing is done during the first repetition of iterated block and then the "break" instruction is used to prevent further iterations:
iterate str, 'alpha','beta','gamma' repeat %% dw offset#% end repeat repeat %% indx % offset#% db str end repeat break end iterate
The parameters defined by "iterate" do not attach the context to iterated values, but neither do they remove the original context if such is already attached to the text of arguments. So if the values given to "iterate" were themselves created from another parameter that preserved the original context for the symbol identifiers (like the parameter of macroinstruction), then this context is preserved, but otherwise "iterate" defines just a plain text substitution.
The parameters defined by instructions like "iterate" or "repeat" are processed everywhere in the text of associated block, but with some limitations if the block is defined partly by the text of macroinstruction and partly in other places. In that case the parameters are only accessible in the parts of the block that are defined in the same place as the initial command.
Every time a parameter is defined, its name can have the "?" character attached to it to indicate that this parameter is case-insensitive. However when parameters are recognized inside the preprocessed line, it does not matter whether they are followed by "?" there. The only modifier that is recognized by preprocessor when it replaces the parameter with its value is the "`" character.
The repeating instructions together with "if" belong to the group called control directives. They are the instructions that control the flow of assembly. Each of them defines its own block of subordinate instructions, closed with corresponding "end" command, and if these blocks are nested within each other, it always must be a proper nesting - the inner block must always be closed before the outer one. All control directives are therefore the unconditional instructions - they are recognized even when they are inside an otherwise skipped block.
The "postpone" is another control directive, which causes the block of instructions to be assembled later, when all of the source text has already been processed.
dw final_count postpone final_count = counter end postpone counter = 0
The above sample postpones the definition of "final_count" symbol until the entire source has been processed, so that it can access the final value of "counter" variable.
The "irpv" is another repeating instruction and an iterator. It has just two arguments, first being the name of parameter and second the identifier of symbolic variable. It iterates through all the stacked values of symbolic variable, starting from the oldest one.
var equ 1 var equ 2 var equ 3 var reequ 4 irpv param, var db param end irpv
In the above example there are three iterations, with values 1, 2, and 4.
"irpv" can effectively convert a value of symbolic variable into a parameter, and this can be useful all by itself, because the symbolic variable is only evaluated in the expressions inside the arguments of instructions (labeled or not), while the parameters are preprocessed in the entire line before any processing of command is started. This allows, for example, to redefine a regular value that is linked by symbolic variable:
x = 1 var equ x irpv symbol, var indx %% symbol = 2 break end irpv assert x = 2
The combination of "indx" and "break" was added to the above sample to limit the iteration to the latest value of symbolic variable. In the next section a better solution to the same problem will be presented.
The "match" is a control directive which causes its block of instructions to be assembled only when the text specified by its second argument matches the pattern given by the first one. A text is separated from a pattern with a comma character, and it includes everything that follows this separator up to the end of line.
Every special character (except for the "," and "=", which have a specific meaning in the pattern) is matched literally - it must be paired with the identical token in the text. In the following example the content of the first block is assembled, while the content of the second one is not.
match +,+ assert 1 ; positive match end match match +,- assert 0 ; negative match end match
The quoted strings are also matched literally, but name tokens in the pattern are treated differently. Every name acts as a wildcard and can match any sequence of tokens which is not empty. If the match is successful, the parameters with such names are created, and each is assigned the value equal to the text this wildcard was matched with.
match a[b], 100h dw a+b ; dw 100h+3 end match
A parameter name in pattern can have an extra "?" character attached to it to indicate that it is a case-insensitive name.
The "=" character causes the token that follows it to be matched literally. It allows to perform the matching of name tokens, and also of the special characters that would otherwise have a different meaning, like "," or "=", or "?" following the name.
match =a==a, a=8 db a ; db 8 end match
If the "=" is followed by name token with "?" character attached to it, this element is matched literally but in a case-insensitive way:
match =a?==a, A=8 db a ; db 8 end match
When there are many wildcards in the pattern, each consecutive one is matched with as few tokens as possible and the last one takes what is left. If the wildcards follow each other without any literally matched elements between them, the first one is matched with just a single token, and the second one with the remaining text:
match car cdr, 1+2+3 db car ; db 1 db cdr ; db +2+3 end match
In the above sample the matched text must contain at least two tokens, because each wildcard needs at least one token to be not empty. In the next example there are additional constraints, but the same general rules applies and the first wildcard consumes as little as possible:
match first:rest, 1+2:3+4:5+6 db `first ; db '1+2' db 13,10 db `rest ; db '3+4:5+6' end match
While any whitespace next to the wildcards is ignored, the presence or absence of whitespace between the literally matched elements is meaningful. If such elements have no whitespace between them, their counterparts must contain no whitespace between them either. But if there is a whitespace between the elements in pattern, it places no constraints on the use of whitespace in the corresponding text - it can be present of not.
match ++,++ assert 1 ; positive match end match match ++,+ + assert 0 ; negative match end match match + +,++ assert 1 ; positive match end match match + +,+ + assert 1 ; positive match end match
The presence of whitespace in the text becomes required when the pattern contains the "=" character followed by a whitespace:
match += +, ++ assert 0 ; negative match end match match += +, + + assert 1 ; positive match end match
The "match" command is analogous to "if" in that it allows to use the "else" or "else match" to create a selection of blocks from which only one is executed:
macro let param match dest+==src, param dest = dest + src else match dest-==src, param dest = dest + src else match dest++, param dest = dest + 1 else match dest--, param dest = dest + 1 else match dest==src, param dest = src else assert 0 end match end macro let x=3 ; x = 3 let x+=7 ; x = x + 7 let x++ ; x = x + 1
It is even possible to mix the "if" and "match" conditions in a sequence of "else" blocks. The entire construction must be closed with "end" command corresponding to whichever of the two was used last:
macro record text match any, text recorded equ `text else if RECORD_EMPTY recorded equ '' end if end macro
The "match" is able to recognize symbolic variables and before the matching is started, their identifiers in the text of the second argument are replaced with corresponding values (just like they are replaced in the text that follows the "equ" command):
var equ 2+3 match a+b, var db a xor b end match
This means that the "match" can be used instead of "irpv" to convert the latest value of a symbolic variable to parameter. The sample from the previous section, where "irpv" was used with "break" to perform just one iteration on the last value, can be rewritten to use "match" instead:
x = 1 var equ x match symbol, var symbol = 2 end match assert x = 2
The difference between them is that "irpv" would execute its block even for an empty value, while in the case of "match" the "else" block would have to be added to handle the empty text.
When the evaluation of symbolic variables in the matched text is undesirable, the symbol created with "define" can be used as a proxy to preserve the text, because the replacement is not recursive:
macro drop value local temporary define temporary value match =A, temporary db A restore A else db value end match end macro A equ 1 A equ 2 drop A drop A
A concern could arise that "define" may modify the meaning of text by equipping it with a local context. But when the value for "define" comes from the parameter of macroinstruction (as in the above sample), it already carries its original context and "define" does not alter it.
The "rmatch" directive is very similar to "match", but it operates on the raw text of the second argument. Not only it does not evaluate the symbolic variables, but it also strips the text of any additional context it could have carried.
struc has instruction rmatch raw, instruction namespace . raw end namespace end rmatch end struc define x x has a = 3 assert x.a = 3
In the above sample the identifier of "a" would be interpreted in the context effective for the line calling the "has" macroinstruction if it was not converted back into the raw text by "rmatch".
The "org" instruction starts a new area of output. The content of such area is written into the destination file next to the previous data, but the addresses in the new area are based on the value specified in the argument to "org". The area is closed automatically when the next one is started or when the source ends.
org 100h start: ; start = 100h
The "$" is a built-in symbol of expression class which is always equal to the value of current address. Therefore definition of a constant with the value specified by "$" symbol is equivalent to defining a label at the same point:
org 100h start = $ ; start = 100h
The "$$" symbol is always equal to the base of current addressing space, so in the area started with "org" it has the same value as the base address from the argument of "org". The difference between "$" and "$$" is thus the current position relative to the start of the area:
org 2000h db 'Hello!' size = $ - $$ ; size = 6
The "$@" symbol evaluates to the base address of current block of uninitialized data. When there was no such data defined just before the current position, this value is equal to "$", otherwise it is equal to "$" minus the length of said data inside the current addressing space. Note that reserved data no longer counts as such when it is followed by an initialized one.
The "section" instruction is similar to "org", but it additionally trims all the reserved data that precedes it analogously to how the uninitialized data is not written into output when it is at the end of file. The "section" can therefore be followed by initialized data definitions without causing the previously reserved data to be initialized with zeros and written into output. In this sample only the first of the three reserved buffers is actually converted into zeroed data and written into output, because it is followed by some initialized data. The second one is trimmed because of the "section", and the third one is cut off since it lies at the end of file:
data1 dw 1 buffer1 rb 10h ; zeroed and present in the output org 400h data dw 2 buffer2 rb 20h ; not in the output section 1000h data3 dw 3 buffer3 rb 30h ; not in the output
The "$%" is a built-in symbol equal to the offset within the output file at which the initialized data would be generated if it was defined at this point. The "$%%" symbol is the current offset within the output file. These two values differ only when they are used after some data has been reserved - the "$%" is then larger than "$%%" by the length of unitialized data which would be generated into output if it was to be followed by some initialized one.
db 'Hello!' rb 4 position = $%% ; position = 6 next = $% ; next = 10
The values in the comments of the above sample assume that the source contains no other instructions generating output.
The "virtual" creates a special output area which is not written into output file. This kind of area must reside between the "virtual" and "end virtual" commands, and after it is closed, the output generator comes back to the area it was previously operating on, with position and address the same as there were just before opening the "virtual" block. This allows also to nest the "virtual" blocks within each other.
When "virtual" has no argument, the base address of this area is the same as current address in the outer area. An argument to "virtual" can have a form of "at" keyword followed by an expression defining the base address for the enclosed area:
int dw 1234h virtual at int low db ? high db ? end virtual
The "load" instruction defines the value of a variable by loading the string of bytes from the data generated in an output area. It should be followed by an identifier of symbol to define, then optionally the ":" character and a number of bytes to load, then the "from" keyword and an address of the data to load. This address can be specified in two modes. If it is simply a numeric expression, it is an address within the current area. In that case the loaded bytes must have already been generated, so it is only possible to load from the space between "$$" and "$" addresses.
virtual at 100h db 'abc' load b:byte from 101h ; b = 'b' end virtual
When the number of bytes is not specified, the length of loaded string is determined by the size associated with address.
The other variant of "load" needs a special kind of label, which is created with "::" instead of ":". Such label has a value that cannot be used directly, but it can be used with "load" instruction to access the data of the area in which this label has been defined. The address for "load" has then to be specified as the area label followed by ":" and then the address within that area:
virtual at 0 hex_digits:: db '0123456789ABCDEF' end virtual load a:byte from hex_digits:10 ; a = 'A'
This variant of "load" can access the data which is generated later, even within the current area:
area:: db 'abc' load sub:3 from area:$-2 ; sub = 'bcd' db 'def'
The "store" instruction can modify already generated data in the output area. It should be followed by a value (automatically converted to string of bytes), then optionally the ":" character followed by a number of bytes to write (when this setting is not present, the length of string is determined by the size associated with address), then the "at" keyword and the address of data to replace, in one of the same two modes as allowed by "load". However the "store" is not allowed to modify the data that has not been generated yet, and any area that has been touched by "store" becomes a variable area, forbidding also the "load" to read a data from such area in advance.
The following example uses the combination of "load" and "store" to encrypt the entire contents of the current area with a simple "xor" operation:
db "Text" key = 7Bh repeat $-$$ load a : byte from $$+%-1 store a xor key : byte at $$+%-1 end repeat
If the final data of an area that has been modified by "store" needs to be read earlier in the source, it can be achieved by copying this data into a different area that would not be constrained in such way. This is analogous to defining a constant with a final value of some variable:
load char from const:0 virtual var:: db 'abc' .length = $ end virtual store 'A' at var:0 virtual const:: repeat var.length load a : byte from var:%-1 db a end repeat end virtual
The area label can be forward-referenced by "load", but it can never be forward-referenced by "store", even if it refers to the current output area.
The "include" instruction reads the source text from another file and processes it before proceeding further in the current source. Its argument should be a string defining the path to a file (the format of the path may depend on the operating system). If after the instruction and before the argument the "!" character is added, the other file is read and processed unconditionally, even when it is inside a skipped block (the unconditional instructions from the other file may then get recognized).
The "eval" instruction takes a sequence of bytes defined by its arguments, treats it as a source text and assembles it. The arguments are either strings or the numeric values of single bytes, separated with commas. In the next example "eval" is used to generate definitions of symbols named as a consecutive letters of the alphabet:
repeat 26 eval 'A'+%-1,'=',`% end repeat assert B = 2
The "display" instruction causes a sequence of bytes to be written into standard output, next to the messages generated by the assembler. It should be followed by strings or numeric values of single bytes, separated with commas. The following example uses "repeat 1" to define a parameter with a decimal representation of computed number, and then displays it as a string:
macro show description,value repeat 1, d:value display description,`d,13,10 end repeat end macro show '2^64=',1 shl 64
The "err" instruction signalizes an error in the assembly process, with a custom message specified by its argument. It allows the same kind of arguments as the "display" directive.
if $>10000h err 'segment too large' end if
The built-in symbol "%t" has the constant value of the timestamp marking the point in time when the assembly was started.
The "__file__" is a built-in symbol whose value is a string containing the name of currently processed source file. The accompanying "__line__" symbol provides the number of currently processed line in that file.
Copyright © 2004-2016, Tomasz Grysztar.