ASM
This assembler, written in BCPL, runs on the Alto and produces BCPL-
compatible relocatable binary output files, suitable for input to BLDR,
the BCPL loader. The Alto Hardware manual describes the source
language and the virtual machine.
1. Symbols
Symbols may be up to 130 characters in length, and every character of a
symbol must be used to identify it. By default upper- and lower-case
characters are different, and two character strings represent the same
symbol only if the same letters and cases are used in both. However,
the /U switch causes all lower-case letters in symbols to be changed to
upper case (even in external symbols). Thus if you want an assembly-
language program to link to symbols containing lower-case letters, you
must either default lower-case conversion in ASM or map all symbols to
upper case in BLDR using its /U switch.
2. Strings
Strings follow BCPL conventions. They may not extend from one line to
the next.
3. Assembly Regions
This assembler can assemble into three regions: two static regions (one
in page 0) and one code region. The directives .NREL, .SREL, and .ZREL
cause the assembler to begin placing code in the code region, the non-
page-0 static region, and the page 0 static region, respectively. The
BCPL loader causes the restrictions that the code area may not contain
pointers into the code area, that the first word of the code area may
not point to a static area, and that no static area may contain
pointers to a static area. The only external symbols are statics.
Arithmetic is not allowed on symbols denoting statics, and the symbol
"." is undefined in .SREL and .ZREL. Any absolute or code- relative
expression (including such goodies as JMP@ 62) may be placed in .SREL
or .ZREL. Any absolute expression, static reference, or instruction
reference to .ZREL may appear in .NREL.
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Copyright Xerox Corporation 1979
ASM February 10, 1979 2
4. Text
There are two text modes, .TXTM B and .TXTM L. Mode B causes the
generation of standard BCPL strings. Mode L causes the generation of
long strings, a full word length followed by the string characters, two
per word.
5. .GET
The directive .GET "FOO" causes the file FOO to be inserted into the
source text at that point. .GET can be used up to two levels deep.
Its primary utility is likely to be for lists of externals and for
canned entry and exit sequences.
6. .GETNOLIST
Works exactly like .GET, except that the "gotten" file is not included
in the listing, nor are any files which it .GET's.
7. .BEXT
In addition to .EXTN and .EXTD and .ENT, I have added two directives
.BEXT and .BEXTZ which work exactly as BCPL's External works for non-
page-0 and page 0 statics, respectively. This should increase the
utility of the .GET feature above.
8. Expressions
Parentheses (but not precedence) are supported. Constructs like "K and
$*N and 5 and 17. and 3B10 are all primaries. Most BCPL and customary
assembler operators are allowed. The construct 1B10 means 40(octal),
unlike BCPL's convention. I am willing to be convinced on this point.
9. Predefined Symbols
All predefined symbols and directives and opcodes are defined both in
all upper-case and all lower-case letters. For example, both LDA and
lda are predefined, but Lda is not. The following Alto-specific
opcodes are preloaded in the symbol table:
JSRII JSRIS CYCLE CONVERT DIR EIR BRI
RCLK SIO BLT BLKS SIT RDRM WTRM
JMPRM MUL DIV
ASM February 10, 1979 3
In addition, the following pile of skips which test various conditions
has been added, courtesy of Dan Ingalls. Only the names have been
changed to confuse the innocent:
Two operands:
SZE SZ SNZ SP SGZ SN SEQ
SE SNE SLT SLE SGT SGE SGTU
SLEU SGEU SLTU SODD SKEVEN SNIL SNNIL
MKZERO MKONE MKNIL MKMINUSONE
No Operands:
NOP SKIP
It should be explained that U stands for unsigned, and that Dan thinks
of NIL as -1.
10. Operation
If the source file is called FOO.ASM, type
ASM FOO.ASM
If you just type ASM FOO it will first try to use FOO and, failing in
that, try FOO.ASM. The assembler will usually want to construct
several files, which it will do by substituting various extensions on
FOO unless you specify otherwise. There are a lot of switches which
apply to ASM:
/L Construct a listing file
/S Include the symbols defined by the user, for what they're worth
/A Include all symbols, even the predefined ones
/R Include a printout of the .BR file
/N Don't make a .BR file
/E Make an .ER file which is a copy of the error messages
sent to the terminal
/D Print debugging messages (as errors, in fact)
/P Pause after printing each error message (continue with CR)
/U Map all lower-case letters in symbols to upper-case
There are also a lot of switches which apply to file names, and which
tell the assembler to use this name instead of the one it was about to
invent:
/L Names the listing file
/E Names the error file
/S Names the source file (also no switches)
/T Names the temporary file
/B Names the relocatable binary file