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A68k - a freely distributable assembler for the Amiga
by Charlie Gibbs
with special thanks to
Brian R. Anderson and Jeff Lydiatt
(Version 2.6 - November 2, 1989)
Note: This program is Freely Distributable, as opposed to Public
Domain. Permission is given to freely distribute this program provided no
fee is charged, and this documentation file is included with the program.
This assembler is based on Brian R. Anderson's 68000 cross-
assembler published in Dr. Dobb's Journal, April through June 1986.
I have converted it to produce AmigaDOS-format object modules, and
have made many enhancements, such as macros and INCLUDE files.
My first step was to convert the original Modula-2 code into C.
I did this for two reasons. First, I had access to a C compiler, but
not a Modula-2 compiler. Second, I like C better anyway.
The executable code generator code (GetObjectCode and MergeModes)
is essentially the same as in the original article, aside from its
translation into C. I have almost completely rewritten the remainder
of the code, however, in order to remove restrictions, add enhancements,
and adapt it to the AmigaDOS environment. Since the only reference book
available to me was the AmigaDOS Developer's Manual (Bantam, February
1986), the assembler and the remainder of this document work in terms
of that book.
RESTRICTIONS
Let's get these out of the way first:
o The verification file (-v) option is not supported. Diagnostic
messages always appear on the console. They also appear in the
listing file, however (see extensions below). You can produce
an error file by redirecting console output to a file - the
line number counter and final summary are displayed on stderr
so you can still see what's happening.
o The file names in the INCLUDE directory list (-i) must be
separated by commas. The list may not be enclosed in quotes.
o Labels assigned by EQUR and REG directives are case-sensitive.
o Strange things will happen if your source code (including
INCLUDE files and macro expansions) exceeds 32,766 lines.
Tough darts. Break up your source file. Can you actually
read that monster? :-)
o The following directives are not supported, and will be flagged
as invalid op-codes:
OFFSET
NOPAGE
LLEN
PLEN
NOOBJ
FAIL
FORMAT
NOFORMAT
MASK2
I feel that NOPAGE, LLEN, and PLEN should not be defined within
a source module. It doesn't make sense to me to have to change
your program just because you want to print your listings on
different paper. The command-line switch "-p" (see below) can
be used as a replacement for PLEN; setting it to a high value
(like 32767) is a good substitute for NOPAGE. The effect of
LLEN can be obtained by running the listing file through an
appropriate filter.
EXTENSIONS
Now for the good stuff:
o Labels can be any length that will fit onto one source line
(currently 127 bytes maximum). Since labels are stored on the
heap, the number of labels that can be processed is limited
only by available memory.
o Since section data and user macro definitions are stored in
the symbol table (see above), they too are limited only by
available memory. (Actually, there is a hard-coded limit of
32,767 sections, but I doubt anyone will run into that one.)
o The only values a label cannot take are the register names -
A68k can distinguish between the same name used as a label,
instruction name or directive, macro name, or section name.
o Section and user macro names appear in the symbol table dump,
and will also be cross-referenced. Their names can be the same
as any label (see above); they will be listed separately.
o INCLUDEs and macro calls can be nested indefinitely, limited
only by available memory. The message "Secondary heap
overflow - assembly terminated" will be displayed if memory
is exhausted. You can increase the size of this heap using
the -w switch (see below). Recursive macros are supported;
recursive INCLUDEs will, of course, result in a loop that
will be broken only when the heap overflows.
o The EVEN directive forces alignment on a word (2-byte)
boundary. It does the same thing as CNOP 0,2.
(This one is left over from the original code.)
o Backward references to labels within the current CODE section
will be converted to PC-relative addressing with displacement
if this mode is legal for the instruction. This feature is
disabled by the -n switch.
o If a MOVEM instruction only specifies one register, it is
converted to the corresponding MOVE instruction. Instructions
such as MOVEM D0-D0,label will not be converted, however.
This feature is disabled by the -n switch.
o ADD, SUB, and MOVE instructions will be converted to ADDQ,
SUBQ, and MOVEQ respectively if possible. Instructions coded
explicitly (e.g. ADDA or ADDI) will not be converted. This
feature is disabled by the -n switch.
o ADD, CMP, SUB, and MOVE to an address register are converted to
ADDA, CMPA, SUBA, and MOVEA respectively, unless (for ADD, SUB,
or MOVE) they have already been converted to quick form.
o ADD, AND, CMP, EOR, OR, and SUB of an immediate value are
converted to ADDI, ANDI, CMPI, EORI, ORI, and SUBI respectively
(unless the address register or quick conversion above has
already been done).
o If both operands of a CMP instruction are postincrement mode,
the instruction is converted to CMPM.
o Operands of the form 0(An) will be treated as (An) except for
the MOVEP instruction, which always requires a displacement.
This feature is disabled by the -n switch.
o The SECTION directive allows a third parameter. This can be
specified as either CHIP or FAST (upper or lower case). If
this parameter is present, the hunk will be written with the
MEMF_CHIP or MEMF_FAST bit set. This allows you to produce
"pre-ATOMized" object modules.
o The synonyms DATA and BSS are accepted for SECTION directives
starting data or BSS hunks. The CHIP and FAST options (see
above) can also be used, e.g. BSS name,CHIP.
o The following synonyms have been implemented for compatibility
with the Aztec assembler:
CSEG is treated the same as CODE or SECTION name,CODE
DSEG is treated the same as DATA or SECTION name,DATA
PUBLIC is treated as either XDEF or XREF, depending on
whether or not the symbol in question has been
defined in the current source module.
A single PUBLIC directive can name a mixture
internally- and externally-defined symbols.
o The ability to produce Motorola S-records is retained from the
original code. The -s switch causes the assembler to produce
S-format instead of AmigaDOS format. Relocatable code cannot
be produced in this format.
o Error messages consist of three parts.
The position of the offending line is given as a line
number within the current module. If the line is within a
macro expansion or INCLUDE file, the position of the macro
call or INCLUDE statement in the outer module is given as
well. This process is repeated until the outermost source
module is reached.
Next, the offending source line itself is listed.
Finally, the errors for that line are displayed. A flag
(^) is placed under the column where the error was detected.
o Named local labels are supported. These work the same as the
local labels supported by the Metacomco assembler (nnn$) but
are formed in the same manner as normal labels, except that
they must be preceded by a backslash (\).
o The following synonyms have been implemented for compatibility
with the Assempro assembler:
ENDIF is treated the same as ENDC
= is treated the same as EQU
| is treated the same as ! (logical OR)
o Quotation marks (") can be used as string delimiters
as well as apostrophes ('). Any given string must begin
and end with the same delimiter. This allows such statements
as the following:
MOVEQ "'",D0
DC.B "This is Charlie's assembler."
Note that you can still define an apostrophe within a string
delimited by apostrophes if you double it, e.g.
MOVEQ '''',D0
DC.B 'This is Charlie''s assembler.'
o If any errors are found in the assembly, the object code file
will be scratched, unless you specified the -k (keep) flag
on the command line.
o The symbols .A68K, .a68k, .a68K, and .A68k are automatically
defined as SET symbols having absolute values of 1.
This enables a source program to determine whether it is
being assembled by this assembler, and is effectively
insensitive as to whether or not it is checked in upper case.
o A zeroth positional macro parameter (\0) is supported. It
is replaced by the length of the macro call (B, W, or L,
defaulting to W). For instance, given the macro:
moov MACRO
move.\0 \1,\2
ENDM
the macro call
moov.l d0,d1
would be expanded as
move.l d0,d1
o If an INCLUDE file doesn't generate any code and no listing
file is required (including suppression of the listing using
NOLIST), it won't be read again in pass 2. The statement
numbers will be bumped to keep in proper alignment. This
can really speed up assemblies that INCLUDE lots of EQUates.
o The ORG directive is supported. It works like RORG, except
that it takes the actual address to be jumped to, rather
than an offset from the start of the current section.
The given address must be in the current section.
As far as A68k is concerned, the only real difference
between ORG and RORG is that the ORG value must be
relocatable, while the RORG value must be absolute.
o Branch (Bcc, including BRA and BSR) instructions will be
converted to short form if possible. Shortening a branch
may bring other branches within range of shortening - this
can set up a ripple effect, and A68k may not catch all
branches that could theoretically be optimized. Any branches
which A68k misses (there shouldn't be too many under normal
circumstances) can be displayed by specifying the -f switch
(see below). Branch optimization is disabled by the -n switch.
THE SMALL CODE / SMALL DATA MODEL
Version 2.4 implements a rudimentary small code/data model.
It consists of converting any data reference to one of the following
three addressing modes:
address register indirect with displacement using a
specified address register, defaulting to A4
(for references to the DATA or BSS section)
program counter indirect with displacement
(for references to the CODE section)
absolute word
(for absolute and 16-bit relocatable values)
These conversions do not take place unless a NEAR directive is
encountered. The NEAR directive can take one operand, which
must be either an address register or a symbol which has been
equated (using EQUR) to an address register. Register A7 (SP)
may not be used. If no register is given, A4 is assumed.
Conversion is done for all operands until a FAR directive
is encountered. NEAR and FAR directives can occur any number
of times, enabling conversion to be turned on and off at will.
Backward references which cannot be converted (e.g. external
labels declared as XREF) will remain as absolute long addressing.
All forward references are assumed to be convertible, since during
pass 1 A68k has no way of telling whether conversion is possible.
If conversion turns out to be impossible, invalid object code will
be generated - an error message ("Invalid forward reference") will
indicate when this occurs.
Although the small code/data model can greatly reduce the
size of assembled programs, several restrictions apply:
o Small code and small data models are active simultaneously.
You can't have one without the other, since during pass 1
A68k doesn't know whether forward references are to CODE
or to DATA/BSS.
o Programs can consist of a maximum of two sections,
one CODE, the other DATA or BSS. If you try to define
a third section, the message "Too many SECTIONs" will
be displayed. The NEAR directive is active only within
the CODE section.
o While the NEAR directive is active, external labels (XREF)
must be declared before they are used, CODE section references
must be with 32K of the current position (i.e. expressible as
PC-relative), and DATA/BSS section references must be in the
first 64K of the DATA/BSS section (i.e. expressible as
address register indirect with displacement). Any instructions
which do not satisfy these requirements cannot be detected in
pass 1, so A68k has no choice but to display an error message
in pass 2 ("Invalid forward reference") which in this case
indicates that invalid code has been generated. To properly
assemble such instructions, you can temporarily disable
conversion with a FAR directive, then resume afterwards
with another NEAR directive.
o Conversion cannot be done for references between modules.
All external references must be left as absolute long.
o A68k assumes that the base register (normally A4) points to
the start of the DATA/BSS section plus 32768 bytes. The
register must be preloaded with this value before executing
any code converted by the NEAR directive. One way to do this
is to code the instruction that loads the register prior to
the NEAR directive. Another way is to use a MOVE.L with
immediate mode, which is never converted. Here are examples
of the two methods:
LEA data+32768,a4 NEAR
NEAR ;defaults to A4 MOVE.L #data+32768,a4
<remainder of code> <remainder of code>
BSS BSS
data: data:
<data areas> <data areas>
END END
I'll be the first to admit that this is a very crude and ugly
implementation. I hope to improve it in future versions.
FILES
A68k uses the following files:
o The source code file - this file contains the program to be
assembled. This file is an ASCII text file whose last line
must be an END statement.
o The object code file - this file is created by A68k, replacing
any previous version which may exist. If any errors are
encountered during the assembly, this file will be scratched,
unless the -k (keep) switch is specified (see below).
Although this file is normally written in AmigaDOS format,
the -s switch (see below) will cause it to be written in
Motorola S-record format instead.
o The listing file - this file is optionally created by A68k
and contains a listing complete with page headings (including
form feeds), generated object code, and error messages if any.
It is suitable for feeding to a printer.
o An equate file - this file is optionally created by A68k
and consists of a leading comment line followed by EQU
statements, one for each symbol encountered by A68k whose
value is absolute. This file is only created if the -e
command-line switch is specified (see below).
o A header file - if requested, this file is read by A68k
immediately prior to the source code file. It treated
exactly as if it were requested by an INCLUDE statement
at the front of the source file, but is selected only if
the -h command-line switch is specified (see below).
o Include files are selected by INCLUDE directives within the
source file, and are inserted in the source code in place
of the INCLUDE directive itself. A68k first searches the
current directory for INCLUDE files; the -i command-line
switch (see below) specifies additional directories which
can be searched.
FILE NAMES
The names of the above files can be explicitly specified.
However, A68k will generate default file names in the following cases:
o If the -o switch is omitted, a default name will be assigned
to the object code file.
o If the -e switch is specified with no file name, a default
name will be assigned to the equate file.
o If the -l or -x switch is specified with no file name, a
default name will be assigned to the listing file.
A default name is generated by deriving a stem name from the source
code file name, and appending .o for an object code file name (.s
if the -s switch is specified to produce Motorola S-records), .equ
for an equate file name, or .lst for a listing file name. The stem
name consists of all characters of the source file name up to the
last period (or the entire source file name if it contains no period).
Here are some examples:
Default names
--------------------------------------------
Source file Object file Equate file Listing file
----------- ----------- ----------- ------------
myprog.asm myprog.o myprog.equ myprog.lst
myprog myprog.o myprog.equ myprog.lst
new.prog.asm new.prog.o new.prog.equ new.prog.lst
HOW TO USE A68k
The command-line syntax to run the assembler is as follows:
a68k <source file name>
[<object file name>]
[<listing file name>]
[-d[[!]<prefix>]]
[-e[<equate file name>]]
[-f]
[-h<header file name>]
[-i<INCLUDE directory list>]
[-k]
[-l[<listing file name>]]
[-n]
[-o<object file name>]
[-p<page depth>]
[-q[<quiet interval>]]
[-s]
[-t]
[-w[<hash table size>][,<secondary heap size>]]
[-x[<listing file name>]]
[-y]
[-z[<debug start line>][,<debug end line>]]
These options can be given in any order. Any parameter which is not
a switch (denoted by a leading hyphen) is assumed to be a file name;
up to three file names (assumed to be source, object, and listing file
names respectively) can be given. A source file name is always required.
If a switch is being given a value, that value must immediately follow
the switch letter with no intervening spaces. For instance, to specify
a page depth of 40 lines, the specification "-p40" should be used;
"-p 40" will be rejected.
Switches perform the following actions:
-d causes symbol table entries (hunk_symbol) to be written
to the object module for the use of symbolic debuggers.
If the switch is followed by a string of characters, only
those symbols beginning with that prefix string will be
written. This can be used to suppress internal symbols
generated by compilers. If the first character is an
exclamation mark (!), only symbols which do NOT begin
with the following characters are written out.
Here are some examples:
-d writes all symbols
-dabc writes only symbols beginning with "abc"
-d!x writes symbols which do not begin with "x"
-e causes an equate file (see above) to be produced. A file
name can be specified; otherwise a default name will be used.
-f causes any branches (Bcc, BRA, BSR) that could be converted
to short form to be flagged. A68k will convert as many
branches as possible to short form (unless the -n switch is
is specified), but certain combinations of instructions may
set up a ripple effect where shortening one branch brings
another one into range. This switch will cause A68k to
flag any branches that it may have missed; during pass 2
it is possible to tell this, although during pass 1 it might
not be. If the -n switch (see below) is specified along
with this switch (suppressing all optimization), no branches
will be shortened, but all branches which could be shortened
will be flagged.
-h causes a header file to be read prior to the source code file.
A file name must be given. The action is the same as if the
first statement of the source file were an INCLUDE statement
naming the header file. To find the header file, the same
directories will be searched as for INCLUDE files (see the
-i switch below).
-i specifies directories to be searched for INCLUDE files in
addition to the current directory. Several names, separated
by commas, may be specified. No embedded blanks are allowed.
For example, the specification
-imylib,df1:another.lib
will cause INCLUDE files to be searched for first in the
current directory, then in "mylib", then in "df1:another.lib".
-k causes the object file to be kept even if any errors were
found. Otherwise, it will be scratched if any errors occur.
-l causes a listing file to be produced. If you want the listing
file to include a symbol table dump and cross-reference, use
the -x switch instead (see below).
-n causes all object code optimization (see above) to be disabled.
-o allows the default name for the object code file (see above)
to be overridden.
-p causes the page depth to be set to the specified value.
This takes the place of the PLEN directive in the Metacomco
assembler. Page depth defaults to 60 lines (-p60).
-q changes the interval at which A68k displays the line number
it has reached in its progress through the assembly. The
default is to display every 10 lines (-q10). Specifying
larger values reduces console I/O, making assemblies run
slightly faster.
If you specify a negative number (e.g. -q-10), line numbers
will be displayed at an interval equal to the absolute value
of the specified number, but will be given as positions
within the current module (source, macro, or INCLUDE) rather
than as a total statement count - the module name will also
be displayed.
A special case is the value zero (-q0 or just -q) - this
will cause all console output, except for error messages,
to be suppressed.
-s causes the object file to be written in Motorola S-record
format, rather than AmigaDOS format. The default name for
an S-record file ends with ".s" rather than ".o"; this can
still be overridden with the -o switch, though.
-t allows tabs in the source file to be passed through to the
listing file, rather than being expanded. In addition, tabs
will be generated in the listing file to skip from the object
code to the source statement, etc. This can greatly reduce
the size of the listing file, as well as making it quicker to
produce. Do not use this option if you will be displaying or
listing the list file on a device which does not assume a tab
stop at every 8th position.
-w specifies the sizes of fixed memory areas that A68k allocates
for its own use. You should normally never have to specify
this switch, but it may be useful for tuning.
The first parameter gives the number of entries that the hash
table (used for searching the symbol table) will contain.
The default value of 2047 should be enough for all but the
very largest programs. The assembly will not fail if this
value is too small, but may slow down if too many long hash
chains must be searched. The hashing statistics displayed by
the -y switch (see below) can be used to tune this parameter.
I've heard that you should really specify a prime number for
this parameter, but I haven't gone into hashing theory enough
to know whether it's actually necessary.
The second parameter of the -w switch specifies the size (in
bytes) of the secondary heap, which is used to store nested
macro and INCLUDE file information (see below). It defaults
to 1024, which should be enough unless you use very deeply
nested macros and/or INCLUDE files with long path names.
You can specify either or both parameters. For example:
-w4093 secondary heap size remains at 1024 bytes
-w,2000 hash table size remains at 2047 entries
-w4093,2000 increases the size of both areas
If you're really tight for memory, and are assembling small
modules, you can use this switch to shrink these areas below
their default sizes. At the end of an assembly, a message
will be displayed giving the sizes actually used, in the form
of the -w command you would have to enter to allocate that much
space. This is primarily useful to see how much secondary
heap space was used.
NOTE: All other memory used by A68k (e.g. the actual symbol
table) is allocated as required (currently in 8K chunks).
-x works the same as -l (see above), except that a symbol table
dump, including cross-reference information, will be added
to the end of the listing file.
-y causes hashing statistics to be displayed at the end of the
assembly. First the number of symbols in the table is given,
followed by a summary of hash chains by length. Chains with
length zero denote unused hash table entries. Ideally (i.e.
if there were no collisions) there should be as many chains
with length 1 as there are symbols, and there should be no
chains of length 2 or greater. I added this option to help
me tune my hashing algorithm, but you can also use it to see
whether you should allocate a larger hash table (using the
first parameter of the -w switch, see above).
-z was provided to help debug A68k itself. It causes A68k to
list a range of source lines, complete with line number and
current location counter value, during both passes. Lines
are listed immediately after they have been read from the
source file, before any processing occurs.
Here are some examples of the -z switch:
-z lists all source lines
-z100,200 lists lines 100 through 200
-z100 lists all lines starting at 100
-z,100 lists the first 100 lines
TECHNICAL INFORMATION
The actual symbol table entries (pointed to by the hash table,
colliding entries are linked together) are stored in 8K chunks which
are allocated as required. The first entry of each chunk is reserved
as a link to the next chunk (or NULL in the last chunk) - this makes
it easy to find all the chunks to free them when we're finished. All
symbol table entries are stored in pass 1. During pass 2, cross-
reference table entries are built in the same group of chunks,
immediately following the last symbol table entry. Additional chunks
will continue to be linked in if necessary.
Symbol names and macro text are stored in another series of linked
chunks. These chunks consist of a link pointer followed by strings
(terminated by nulls) laid end to end. Symbols are independent entries,
linked from the corresponding symbol table entry. Macros are stored as
consecutive strings, one per line - the end of the macro is indicated by
an ENDM statement. If a macro spans two chunks, the last line in the
original chunk is followed by a newline character to indicate that the
macro is continued in the next chunk.
Relocation information is built during pass 2 in yet another
series of linked chunks. If more than one chunk is needed to hold one
section's relocation information, all additional chunks are released
at the end of the section.
The secondary heap is built from both ends, and it grows and
shrinks according to how many macros and INCLUDE files are currently
open. At all times there will be at least one entry on the heap, for
the original source code file. The expression parser also uses the
secondary heap to store its working stacks - this space is freed as
soon as an expression has been evaluated.
The bottom of the heap holds the names of the source code file
and any macro or INCLUDE files that are currently open. The full path
is given. A null string is stored for user macros. Macro arguments
are stored by additional strings, one for each argument in the macro
call line. All strings are stored in minimum space, similar to the
labels and user macro text on the primary heap. File names are
pointed to by the fixed table entries (see below) - macro arguments
are accessed by stepping past the macro name to the desired argument,
unless NARG would be exceeded.
The fixed portion of the heap is built down from the top. Each
entry occupies 16 bytes. Enough information is stored to return to
the proper position in the outer file once the current macro or
INCLUDE file has been completely processed.
The diagram below illustrates the layout of the secondary heap.
Heap2 + maxheap2 -----------> ___________________________
| |
| Input file table |
struct InFCtl *InF ---------> |___________________________|
| |
| Parser operator stack |
struct OpStack *Ops --------> |___________________________|
| |
| (unused space) |
struct TermStack *Term -----> |___________________________|
| |
| Parser term stack |
char *NextFNS --------------> |___________________________|
| |
| Input file name stack |
char *Heap2 ----------------> |___________________________|
The "high-water mark" for NextFNS is stored in char *High2,
and the "low-water mark" (to stretch a metaphor) for InF is stored
in struct InFCtl *LowInF. These figures are used only to determine
the maximum heap usage.
AND FINALLY...
Please send me any bug reports, flames, etc. I can be reached
on Mind Link (604/533-2312), at any meeting of the Commodore
Computer Club / Panorama (PAcific NORthwest AMiga Association),
or via Jeff Lydiatt or Larry Phillips. I don't have the time
or money to live on Compuserve or BIX, but my Usenet address is
Charlie_Gibbs@mindlink.UUCP (...uunet!van-bc!rsoft!mindlink!a218).
Charlie Gibbs
2121 Rindall Avenue
Port Coquitlam, B.C.
Canada
V3C 1T9
