Documentation and tutorials.
It is an assembly engine designed as a successor of the one used in flat assembler 1, one of the recognized assemblers for x86 processors. This is a bare engine that by itself has no ability to recognize and encode instructions of any processor, however it has an ability to become an assembler for any CPU architecture. It has a macroinstruction language that is substantially improved compared to the one provided by flat assembler 1 and it allows to easily implement instruction encoders in form of customizable macroinstructions. This approach has a great flexibility at the cost of performance.
If it happened that very fast assembly comparable to flat assembler 1 was required and the performance drawback of macroinstructions was not acceptable, it would be possible to address the issue by building a custom assembler based on this engine, and the complete source code is available to anyone who would like to try it. But the focus of this package is on the use of flat assembler g in its pure form.
The source code of this tool can be compiled with flat assembler 1, but it is also possible to use flat assembler g itself to compile it. The source contains clauses that include different header files depending on the assembler used. When flat assembler g compiles itself, it uses the macroinstructions that come with the supplied example programs, since they implement x86 instructions and formats with a syntax compatible with flat assembler 1.
The macroinstructions that process the syntax of x86 instructions are complex and take a long time to assemble, but incidentally the time it takes for flat assembler g to compile itself on an ordinary modern machine is comparable to the time that an early version of flat assembler 1 needed to assemble itself a decade and a half earlier on a computer that was then similarly mediocre. This can be seen as an interesting demonstration of how the software may be getting slower at the same rate as the hardware becomes faster.
The example programs for x86 architecture that come in this package are the selected samples that originally came with flat assembler 1, with an addition of the sets of the macroinstructions that implement instruction encoders and output formatters required to assemble them just like the original flat assembler did. While they are not complete, they are intended to encourage the creation of further sets of macroinstructions that would provide more instructions and output formats.
To demonstrate how the instruction sets of different architectures may be implemented, there are some example programs for the microcontrollers, 8051 and AVR. They have been kept simple and therefore they do not provide a complete framework for programming such CPUs, though they may provide a solid base for the creation of such environments.
There is also an example of assembling the JVM bytecode, which is a conversion of the sample originally created for flat assembler 1. For this reason it is somewhat crude and does not fully utilize the capabilities offered by the new engine. However it is good at visualising the structure of a class file.
The essential function of flat assembler g is to generate output defined by the instructions in the source code. Given the one line of text as shown below, the assembler would generate a single byte with the stated value:
The macroinstructions can be defined to generate some specific sequences of data depending on the provided parameters. They may correspond to the instructions of chosen machine language, as in the following example, but they could as well be defined to generate other kinds of data, for various purposes.
macro int number if number = 3 db 0CCh else db 0CDh, number end if end macro int 20h ; generates two bytes
The assembly as seen this way may be considered a kind of interpreted language, and the assembler certainly has many characteristics of the interpreter. However it also shares certain aspects with a compiler. It is possible for an instruction to use the value which is defined later in the source and may depend on the instructions that come before that definition, as demonstrated by the following sample.
macro jmpi target if target-($+2) < 80h & target-($+2) >= -80h db 0EBh db target-($+1) else db 0E9h dw target-($+2) end if end macro jmpi start db 'some data' start:
The "jmpi" defined above produces the code of jump instruction as in 8086 architecture. Such code contains the relative offset of the target of a jump, stored in either single byte or 16-bit word. The relative offset is computed as a difference between the address of the target and the address of the next instruction. The special symbol "$" provides the address of current instruction and it is used to calculate the relative offset and determine whether it may fit in a single byte.
Therefore the code generated by "jmpi start" in the above sample depends on the value of an address labeled as "start", and this in turn depends on the length of the output of all the instructions that precede it, including the said jump. This creates a loop of dependencies and the assembler needs to find a solution that fulfills all the constraints created by the source text. This would not be possible if assembler was just an imperative interpreter. Its language is thus in some aspects declarative.
Finding a solution for such circular dependencies may resemble solving an equation, and it is even possible to construct an example where flat assembler g is indeed capable of solving one:
x = (x-1)*(x+2)/2-2*(x+1) db x
The circular reference has been reduced here to a single definition that references itself to construct the value. The flat assembler g is able to find a solution in this case, though in many others it may fail. The method used by this assembler is to perform multiple passes over the source text and then try to predict all the values with the knowledge gathered this way. This approach is in most cases good enough for the assembly of machine codes, but rarely suffices to solve the complex equations and the above sample is one of the exceptions.
Not all instructions have a simple syntax like then ones in the previous examples. To aid in the processing of arguments that may contain special constructions, flat assembler g provides a few capable tools, demonstrated below on the examples that implement selected few instructions of the Z80 processor. The rules governing the use of presented features are found in the manual.
When an instruction has a very small set of allowed arguments, each one of them can be treated separately with the "match" construction:
macro EX? first,second match (=SP?), first match =HL?, second db 0E3h else match =IX?, second db 0DDh,0E3h else match =IY?, second db 0FDh,0E3h else err "incorrect second argument" end match else match =AF?, first match =AF'?, second db 08h else err "incorrect second argument" end match else match =DE?, first match =HL?, second db 0EBh else err "incorrect second argument" end match else err "incorrect first argument" end match end macro EX (SP),HL EX (SP),IX EX AF,AF' EX DE,HL
The "?" character appears in many places to mark the names as case-insensitive and all these occurrences could be removed to further simplify the example.
When the set of possible values of an argument is larger but has some regularities, the textual substitutions can be defined to replace some of the symbols with carefully chosen constructions that can then be recognized and parsed:
A? equ [:111b:] B? equ [:000b:] C? equ [:001b:] D? equ [:010b:] E? equ [:011b:] H? equ [:100b:] L? equ [:101b:] macro INC? argument match [:r:], argument db 100b + r shl 3 else match (=HL?), argument db 34h else match (=IX?+d), argument db 0DDh,34h,d else match (=IY?+d), argument db 0FDh,34h,d else err "incorrect argument" end match end macro INC A INC B INC (HL) INC (IX+2)
This approach has a trait that may not always be desirable: it allows to use an expression like "[:0:]" directly in an argument. But it is possible to prevent exploiting the syntax in such way by using a prefix in the "match" construction:
REG.A? equ [:111b:] REG.B? equ [:000b:] REG.C? equ [:001b:] REG.D? equ [:010b:] REG.E? equ [:011b:] REG.H? equ [:100b:] REG.L? equ [:101b:] macro INC? argument match [:r:], REG.argument db 100b + r shl 3 else match (=HL?), argument db 34h else match (=IX?+d), argument db 0DDh,34h,d else match (=IY?+d), argument db 0FDh,34h,d else err "incorrect argument" end match end macro
In case of an argument structured like "(IX+d)" it could sometimes be desired to allow other algebraically equivalent forms of the expression, like "(d+IX)" or "(c+IX+d)". Instead of parsing every possible variant individually, it is possible to let the assembler evaluate the expression while treating the selected symbol in a distinct way. When a symbol is declared as an "element", it has no value and when it is used in an expression, it is treated algebraically like a variable term in a polynomial.
element HL? element IX? element IY? macro INC? argument match [:r:], argument db 100b + r shl 3 else match (a), argument if a eq HL db 34h else if a relativeto IX db 0DDh,34h,a-IX else if a relativeto IY db 0FDh,34h,a-IY else err "incorrect argument" end if else err "incorrect argument" end match end macro INC (3*8+IX+1) virtual at IX x db ? y db ? end virtual INC (y)
There is a small problem with the above macroinstruction. A parameter may contain any text and when such value is placed into an expression, it may induce erratic behavior. For example if "INC (1|0)" was processed, it would turn the "a eq HL" expression into "1|0 eq HL" and this logical expression is correct and true even though the argument was malformed. To prevent this from happening, a local variable may be used as a proxy holding the value of an argument:
macro INC? argument match [:r:], argument db 100b + r shl 3 else match (a), argument local value value = a if value eq HL db 34h else if value relativeto IX db 0DDh,34h,a-IX else if value relativeto IY db 0FDh,34h,a-IY else err "incorrect argument" end if else err "incorrect argument" end match end macro
There is an additional advantage of such proxy variable, thanks to the fact that its value is computed before the macroinstruction begins to generate any output. When an expression contains a symbol like "$", it may give different values depending where it is calculated and the use of proxy variable ensures that the value taken is the one obtained by evaluating the argument before generating the code of an instruction.
When the set of symbols allowed in expressions is larger, it is better to have a single construction to process an entire family of them. An "element" declaration may associate an additional value with a symbol and this information can then be retrieved with the "metadata" operator applied to a linear polynomial that contains given symbol as a variable. The following example is another variant of the previous macroinstruction that demonstrates the use of this feature:
element register element A? : register + 111b element B? : register + 000b element C? : register + 001b element D? : register + 010b element E? : register + 011b element H? : register + 100b element L? : register + 101b element HL? element IX? element IY? macro INC? argument local value match (a), argument value = a if value eq HL db 34h else if value relativeto IX db 0DDh,34h,a-IX else if value relativeto IY db 0FDh,34h,a-IY else err "incorrect argument" end if else match any more, argument err "incorrect argument" else value = argument if value eq value element 1 & value metadata 1 relativeto register db 100b + (value metadata 1 - register) shl 3 else err "incorrect argument" end if end match end macro
The "any more" pattern is there to catch any argument that contains a complex expressions consisting of more than one token. This prevents the use of syntax like "INC A+0" or "INC A+B-A". But in case of some of the instructions sets, the inclusion of such constraint may depend on a personal preference.
The "value eq value element 1" condition ensures that the value does not contain any terms other than the name of a register. Even when an argument is forced to contain no more than a single token, it is still possible that is has a complex value, for instance if there were definitions like "X = A + B" or "Y = 2 * A". Both "INC X" and "INC Y" would then cause the operator "element 1" to return the value "A", which differs from the value checked in either case.
If an instruction takes a variable number of arguments, a simple way to recognize its various forms is to declare an argument with "&" modifier to pass the complete contents of the arguments to "match":
element CC NZ? := CC + 000b Z? := CC + 001b NC? := CC + 010b C? := CC + 011b PO := CC + 100b PE := CC + 101b P := CC + 110b M := CC + 111b macro CALL? arguments& local cc,nn match condition =, target, arguments cc = condition - CC nn = target db 0C4h + cc shl 3 else nn = arguments db 0CDh end match dw nn end macro CALL 0 CALL NC,2135h
This approach also allows to handle other, more difficult cases, like when the arguments may contain commas or are delimited in different ways.
A standard way of defining a label is by following its name with ":" (this also acts like a line break and any other command, including another label, may follow in the same line). Such label simply defines a symbol with the value equal to the current address, which initially is zero and increases when any bytes are added into the output.
In some variants of assembly language it may be desirable to allow label to precede an instruction without an additional ":" inbetween. It is then necessary to create a labeled macroinstruction that after defining a label passes processing to the original macroinstruction with the same name:
struc INC? argument .: INC argument end struc start INC A INC B
This has to be done for every instruction that needs to allow this kind of syntax. A simple loop like the following one would suffice:
iterate instruction, EX,INC,CALL struc instruction? argument .: instruction argument end struc end iterate
Every built-in instruction that defines data already has the labeled variant.
By defining a labeled instruction that has "?" in place of name it is possible to intercept every line that starts with an identifier that is not a known instruction and is therefore assumed to be a label. The following one would allow a label without ":" to begin any line in the source text (it also handles the special cases so that labels followed with ":" or with "=" and a value would still work):
struc ? tail& match :, tail .: else match : instruction, tail .: instruction else match == value, tail . = value else .: tail end match end struc
Obviously, it is no longer needed to define any specific labeled macrointructions when a global effect of this kind is applied. A variant should be chosen depending on the type of syntax that needs to be allowed.
Intercepting even the labels defined with ":" may become useful when the value of current address requires some additional processing before being assigned to a label - for example when a processor uses addresses with a unit larger than a byte. The intercepting macroinstruction might then look like this:
struc ? tail& match :, tail label . at $ shr 1 else match : instruction, tail label . at $ shr 1 instruction else . tail end match end struc
The value of current address that is used to define labels may be altered with "org". If the labels need to be differentiated from absolute values, a symbol defined with "element" may be used to form an address:
element CODEBASE org CODEBASE + 0 macro CALL? argument local value value = argument if value relativeto CODEBASE db 0CDh dw value - CODEBASE else err "incorrect argument" end if end macro
To define labels in an address space that is not going to be reflected in the output, a "virtual" block should be declared. The following sample prepares macroinstructions "DATA" and "CODE" to switch between generating program instructions and data labels. Only the instruction codes would go to the output:
element DATA DATA_OFFSET = 2000h element CODE CODE_OFFSET = 1000h macro DATA? _END virtual at DATA + DATA_OFFSET end macro macro CODE? _END org CODE + CODE_OFFSET end macro macro _END? if $ relativeto DATA DATA_OFFSET = $ - DATA end virtual else if $ relativeto CODE CODE_OFFSET = $ - CODE end if end macro postpone _END end postpone CODE
The "postpone" block is used here to ensure that the "virtual" block always gets closed correctly, even if source text ends with data definitions.
Within the environment prepared by the above sample any instruction would be able to distinguish data labels from the ones defined within program. For example a branching instruction could be made to accept an argument being either a label within a program or an absolute value, but to disallow any label of data:
macro CALL? argument local value value = argument if value relativeto CODE db 0CDh dw value - CODE else if value relativeto 0 db 0CDh dw value else err "incorrect argument" end if end macro DATA variable db ? CODE routine:
In this context either "CALL routine" or "CALL 1000h" would be allowed, while "CALL variable" would not be.
When the labels have values that are not absolute numbers, it is possible to generate relocations for instructions that use them. A special "virtual" block may be used to store the offsets of values inside the program that need to be relocated when its base changes:
virtual at 0 Relocations:: rw RELOCATION_COUNT end virtual RELOCATION_INDEX = 0 postpone RELOCATION_COUNT := RELOCATION_INDEX end postpone macro WORD? value if value relativeto CODE store $ - CODE : 2 at Relocations : RELOCATION_INDEX shl 1 RELOCATION_INDEX = RELOCATION_INDEX + 1 dw value - CODE else dw value end if end macro macro CALL? argument local value value = argument if value relativeto CODE | value relativeto 0 db 0CDh word value else err "incorrect argument" end if end macro
The table of relocations that is created this way can then be accessed with "load". The following two lines could be used to put the table in its entirety somewhere in the output:
load RELOCATIONS : RELOCATION_COUNT shl 1 from Relocations : 0 dw RELOCATIONS
The "load" reads the whole table into a single string, then "dw" writes it into output (padded to multiple of a word, but in this case the string never requires such padding).
For more complex types of relocations additional modifier may need to be employed. For example, if upper and lower portions of an address needed to be stored in separate places (for example across two instructions) and relocated separately, necessary modifiers could be implemented as follows:
element MOD.HIGH element MOD.LOW HIGH? equ MOD.HIGH + LOW? equ MOD.LOW + macro BYTE? value if value relativeto MOD.HIGH + CODE ; register HIGH relocation db (value - MOD.HIGH - CODE) shr 8 else if value relativeto MOD.LOW + CODE ; register LOW relocation db (value - MOD.LOW - CODE) and 0FFh else if value relativeto MOD.HIGH db (value - MOD.HIGH) shr 8 else if value relativeto MOD.LOW db (value - MOD.LOW) and 0FFh else db value end if end macro
The commands that would register relocation have been omitted for clarity, in this case not only offset within code but some additional information would need to registered in appropriate structures. With such preparation, relocatable units in code might be generated like:
BYTE HIGH address BYTE LOW address
Such approach allows to easily enable syntax with modifiers in any instruction that internally uses "byte" macroinstruction when generating code.
In some cases a command that assembler needs to parse may begin with something different than a name of instruction or a label. It may be that a name is preceded by a special character, like "." or "!", or that it is an entirely different kind of construction. It is then necessary to use "macro ?" to intercept whole lines of source text and process any special syntax of such kind.
For example, if it was required to allow a command written as ".CODE", it would not be possible to implement it directly as a macroinstruction, because initial dot causes the symbol to be interpreted as a local one and globally defined instruction could never be executed this way. The intercepting macroinstruction provides a solution:
macro ? line& match .=CODE?, line CODE else match .=DATA?, line DATA else line end match end macro
The lines that contain either ".CODE" or ".DATA" text are processed here in such a way, that they invoke the global macroinstruction with corresponding name, while all other intercepted lines are executed without changes. This method allows to filter out any special syntax and let the assembler process the regular instructions as usual.
Sometimes unconventional syntax is expected only in a specific area of source text, like inside a block with defined boundaries. The parsing macroinstruction should then be applied only in this place, and removed with "purge" when the block ends:
macro concise macro ? line& match =end =concise, line purge ? else match dest+==src, line ADD dest,src else match dest-==src, line SUB dest,src else match dest==src, line LD dest,src else match dest++, line INC dest else match dest--, line DEC dest else match any, line err "syntax error" end match end macro end macro concise C=0 B++ A+=2 end concise
A macroinstruction defined this way does not intercept lines that contain directives controlling the flow of the assembly, like "if" or "repeat", and they can still be used freely inside such a block. This would change if the declaration was in the form "macro ?! line&". Such a variant would intercept every line with no exception.
This assembly engine has a single main output that has to be generated sequentially. This may seem problematic when the file needs to contains distinct sections for code and data, but the contents of these sections is to be collected from interleaved pieces that may be spread across multiple source files. However, there are some relatively simple methods that allow to do it, all based in one way or the other on forward-referencing capabilities of the assembler.
A natural approach is to define contents of auxiliary section in "virtual" block and copy it to appropriate position in the output with a single operation. When a "virtual" block is labeled, it can be re-opened multiple times to append more data to it.
include '8086.inc' org 100h jmp CodeSection DataSection: virtual Data:: end virtual postpone virtual Data load Data.OctetString : $ - $$ from $$ end virtual end postpone db Data.OctetString CodeSection: virtual Data Hello db "Hello!",24h end virtual mov ah,9 mov dx,Hello int 21h virtual Data ExitCode db 37h end virtual mov ah,4Ch mov al,[ExitCode] int 21h
This leads to a relatively simple syntax even without help of any additional macros.
Another method could be to put the pieces of the section into macros and execute them all at the required position in source. A disadvantage of such approach is that tracing errors in definitions might become a bit cumbersome.
The techniques that allow to easily append to a section generated in parallel can also be very useful to generate data structures like relocation tables. Instead of "store" commands used earlier when demonstrating the concept, regular data directives could be used inside a re-opened "virtual" block to create relocation records.
Copyright © 1999-2018, Tomasz Grysztar.
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