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CHAPTER 12 COMPATIBILITY WITH OTHER ASSEMBLERS
I gave heavy priority to compatibility when I designed A86; a
priority just a shade behind the higher priorities of
reliability, speed, convenience, and power. For those of you who
feel that "close, but incompatible" is like saying "a little bit
pregnant", I'm sorry to report that A86 will not assemble all
Intel/IBM/MASM programs, unmodified. But I do think that a vast
majority of programs can, with a little massaging, be made to
assemble under A86. Furthermore, the massaging can be done in
such a way as to make the programs still acceptable to that old,
behemoth assembler.
Version 3.00 of A86 has many compatibility features not present
in earlier versions. Among the features added since A86 was
first released are: more general forward references, double
quotes for strings, "=" as a synonym for EQU, the RADIX
directive, and the COMMENT directive. If you tried feeding an
old source file to a previous A86 and were dismayed by the number
of error messages you got, try again: things might be more
manageable now.
Conversion of MASM programs to A86
Following is a list of the things you should watch out for when
converting from MASM to A86:
1. You need to determine whether the program was coded as a COM
program or as an EXE program. All COM programs coded for MASM
will contain an ORG 100H directive somewhere before the start
of the code. EXE programs will contain no such ORG, and will
often contain statements that load named segments into
registers. If the program was coded as EXE, you must either
assemble it (using the +O option) to an OBJ file to be fed to
LINK, or you must eliminate the instructions that load segment
registers-- in a COM program they often aren't necessary
anyway, since COM programs are started with all segment
registers already pointing to the same value.
A good general rule is: when it doubt, try assembling to an
OBJ file.
2. You need to determine whether the program is executing with
all segment registers pointing to the same value. Simple COM
programs that fit into 64K will typically fall into this
category. Most EXE programs, programs that use huge amounts
of memory, and programs (such as memory-resident programs)
that take over interrupts typically have different values in
segment registers.
12-2
If there are different values in the segment registers, then
there may be instructions in the program for which the old
assembler generates segment override prefixes "behind your
back". You will need to find such references, and generate
explicit overrides for them. If there are data tables within
the program itself, a CS-override is needed. If there are
data structures in the stack segment not accessed via a
BP-index, an SS-override is needed. If ES points to its own
segment, then an ES-override is needed for accesses (other
than STOS and MOVS destinations) to that segment. In the
interrupt handlers to memory-resident programs, the "normal"
handler is often invoked via an indirect CALL or JMP
instruction that fetches the doubleword address of the normal
handler from memory, where it was stored by the initialization
code. That CALL or JMP often requires a CS-override-- watch
out!
If you want to remain compatible with the old assembler, then
code the overrides by placing the segment register name, with
a colon, before the memory-access operand in the instruction.
If you do not need further compatibility, you can place the
segment register name before the instruction mnemonic. For
example:
MOV AL,CS:TABLE[SI] ; if you want compatibility do this
CS MOV AL,TABLE[SI] ; if not you can do it this way
3. You should use a couple of A86's switches to maximize
compatibility with MASM. I've already mentioned the +O switch
to produce .OBJ files. You should also assemble with the +D
switch, which disables A86's unique parsing of constants with
leading zeroes as hexidecimal. The RADIX command in your
program will also do this. And you should use the +L15 switch,
that disables a few other A86 features that might have reduced
compatibility. See Chapter 3 for a detailed explanation of
these switches.
4. A86 is a bit more restrictive with respect to forward
references than MASM, but not as much as it used to be. You'll
probably need to resolve just a few ambiguous references by
appending " B" or " W" to the forward reference name. One
common reference that needs a bit more recoding is the
difference of two forward references, often used to refer to
the size of a block of allocated memory. You handle this by
defining a new symbol representing the size, using an EQU
right after the block is declared, and then replacing the
forward-reference difference with the size symbol.
5. A86's macro definition and conditional assembly language is
different than MASM's. Most macros can be translated by
replacing the named parameters of the old macros with the
dedicated names #n of the A86 macro language; and by replacing
ENDM with #EM. Other constructs have straightforward
translations, as illustrated by the following examples. Note
that examples involving macro parameters have double pound
signs, since the condition will be tested when the macro is
expanded, not when it is defined.
12-3
MASM construct Equivalent A86 construct
IFE expr #IF ! expr
IFB <PARM3> ##IF !#S3
IFNB <PARM4> ##IF #S4
IFIDN <PARM1>,<CX> ##IF "#1" EQ "CX"
IFDIF <PARM2>,<SI> ##IF "#2" NE "SI"
.ERR (any undefined symbol)
.ERRcond TRUE EQU 0FFFF
TRUE EQU cond
EXITM #EX
IRP ... ENDM #RX1L ... #ER
REPT 100 ...ENDM #RX1(100) ... #ER
IRPC ... ENDM #CX ... #EC
The last three constructs, IRP, REPT, and IRPC, usually occur
within macros; but in MASM they don't have to. The A86
equivalents are valid only within macros-- if they occur in
the MASM program outside of a macro, you duplicate them by
defining an enclosing macro on the spot, and calling that
macro once, right after it is defined.
To retain compatibility, you isolate the old macro definitions
in an INCLUDE file (A86 will ignore the INCLUDE directive),
and isolate the A86 macro definitions in a separate file, not
used in an MASM assembly of the program.
6. A86 supports the STRUC directive, with named structure
elements, just like MASM, with one exception: A86 does not
save initial values declared in the STRUC definition, and A86
does not allow assembly of instances of structure elements.
For example, the MASM construct
PAYREC STRUC
PNAME DB 'no name given'
PKEY DW ?
ENDS
PAYREC 3 DUP (?)
PAYREC <'Eric',1811>
causes A86 to accept the STRUC definition, and define the
structure elements PNAME and PKEY correctly; but the PAYREC
initializations need to be recoded. If it isn't vital to
initialize the memory with the specific definition values, you
could recode the first PAYREC as:
DB ((TYPE PAYREC) * 3) DUP ?
If you must initialize values, you do so line by line:
DB 'Eric '
DW ?
If there are many such initializations, you could define a
macro INIT_PAYREC containing the DB and DW lines.
12-4
7. A86 does not support a couple of the more exotic features of
MASM assembly language: the RECORD directive and its
associated operators WIDTH and MASK; and the usage of
angle-brackets to initialize structure records. These
features would have added much complication to the internal
structure of symbol tables in A86; degrading the speed and the
reliability of the assembler. I felt that their use was
sufficiently rare that it was not worth including them for
compatibility.
If your old program does use these features, you will have to
re-work the areas that use them. Macros can be used to
duplicate the record and structure initializations. Explicit
symbol declarations can replace the usage of the WIDTH and
MASK operators.
Compatibility symbols recognized by A86
A86 has been programmed to ignore a variety of lines that have
meaning to Intel/IBM/MASM assemblers; but which do nothing for
A86. These include lines beginning with a period (except .RADIX,
which is acted upon), percent sign, or dollar sign; and lines
beginning with ASSUME, INCLUDE, PAGE, SUBTTL, and TITLE. If you
are porting your program to A86, and you wish to retain the
option of returning to the other assembler, you may leave those
lines in your program. If you decide to stay with A86, you can
remove those lines at your leisure.
In addition, there is a class of symbols now recognized by A86 in
its .OBJ mode, but still ignored in .COM mode. This includes
NAME, END, and PUBLIC.
Named SEGMENT and ENDS directives written for other assemblers
are, of course, recognized by A86's .OBJ mode. In non-OBJ mode,
A86 treats these as CODE SEGMENT directives. A special exception
to this is the directive
segname SEGMENT AT atvalue
which is treated by A86 as if it were the following sequence:
segname EQU atvalue
STRUC
This will accomplish what is usually intended when SEGMENT AT is
used in a program intended to be a COM file.
12-5
Conversion of A86 Programs to Intel/IBM/MASM
I consider this section a bit of a blasphemy, since it's a little
silly to port programs from a superior assembler, to run on an
inferior one. However, I myself have been motivated to do so
upon occasion, when programming for a client not familiar with
A86; or whose computer doesn't run A86; who therefore wants the
final version to assemble on Intel's assembler. Since my
assembler/debugger environment is so vastly superior to any other
environment, I develop the program using my assembler, and port
it to the client's environment at the end.
The main key to success in following the above scenarios is to
exercise supreme will power, and not use any of the wonderful
language features that exist on A86, but not on MASM. This is
often not easy; and I have devised some methods for porting my
features to other assemblers:
1. I hate giving long sequences of PUSHes and POPs on separate
lines. If the program is to be ported to a lesser assembler,
then I put the following lines into a file that only A86 will
see:
PUSH2 EQU PUSH
PUSH3 EQU PUSH
POP2 EQU POP
POP3 EQU POP
I define macros PUSH2, PUSH3, POP2, POP3 for the lesser
assembler, that PUSH or POP the appropriate number of
operands. Then, everywhere in the program where I would
ordinarily use A86's multiple PUSH/POP feature, I use one or
more of the PUSHn/POPn mnemonics instead.
2. I refrain from using the feature of A86 whereby constants with
a leading zero are default-hexadecimal. All my hex constants
end with H.
3. I will usually go ahead and use my local labels L0 through L9;
then at the last minute convert them to a long set of labels
in sequence: Z100, Z101, Z102, etc. I take care to remove all
the ">" forward reference specifiers when I make the
conversion. The "Z" is used to isolate the local labels at
the end of the lesser assembler's symbol table listing. This
improves the quality of the final program so much that it is
worth the extra effort needed to convert L0--L9's to Z100--
Zxxx's.
4. I will place declarations B EQU DS:BYTE PTR 0 and W EQU
DS:WORD PTR 0 at the top of the program. Recall that A86 has
a "duplicate definition" feature whereby you can EQU an
already-existing symbol, as long as it is equated to the value
it already has. This feature extends to the built in symbols
B and W, so A86 will look at those equates and essentially
ignore them. On the old assembler, the effect of the
declarations is to add A86's notation to the old language.
Example:
12-6
B EQU DS:BYTE PTR 0
W EQU DS:WORD PTR 0
MOV AX,W[0100] ; replaces MOV AX, DS:WORD PTR 0100
MOV AL,B[BX] ; replaces MOV AL, DS:BYTE PTR [BX]