Copyright (c) 1982 The Regents of the University of California.
All rights reserved.

%sccs.include.redist.roff%

@(#)asdocs4.me 5.1 (Berkeley) 04/17/91

delim $$ .EN .pp The syntax of machine instruction statements accepted by .i as is generally similar to the syntax of \*(DM. There are differences, however. .pp .i As uses the character .q \*(DL instead of .q # for immediate constants, and the character .q * instead of .q @ for indirection. Opcodes and register names are spelled with lower-case rather than upper-case letters. .pp Under certain circumstances, the following constructs are (optionally) recognized by .i as to indicate the number of bytes to allocate for the displacement used when constructing displacement and displacement deferred addressing modes: .(b

primary alternate length

B\` B^ byte (1 byte)

W\` W^ word (2 bytes)

L\` L^ long word (4 bytes)

.)b .pp One can also use lower case .b b , .b w or .b l instead of the upper case letters. There must be no space between the size specifier letter and the .q "^" or .q "\`" . The constructs .b "S^" and .b "G^" are not recognized by .i as , as they are by the \*(DM assembler. It is preferred to use the .q "\`" displacement specifier, so that the .q "^" is not misinterpreted as the .b xor operator. .pp Literal values (including floating-point literals used where the hardware expects a floating-point operand) are assembled as short literals if possible, hence not needing the .b "S^" \*(DM directive. .pp If the displacement length modifier is present, then the displacement is .b always assembled with that displacement, even if it will fit into a smaller field, or if significance is lost. If the length modifier is not present, and if the value of the displacement is known exactly in .i as 's first pass, then .i as determines the length automatically, assembling it in the shortest possible way, Otherwise, .i as will use the value specified by the .b -d argument, which defaults to 4 bytes. .pp .i As considers the instructions .b caseb , .b casel , .b casew to have three operands. The displacements must be explicitly computed by .i as , using one or more .b .word statements. .pp These opcodes (formed in general by substituting a .q j for the initial .q b of the standard opcodes) take as branch destinations the name of a label in the current subsegment. It is an error if the destination is known to be in a different subsegment, and it is a warning if the destination is not defined within the object module being assembled. .pp If the branch destination is close enough, then the corresponding short branch .q b instruction is assembled. Otherwise the assembler choses a sequence of one or more instructions which together have the same effect as if the .q b instruction had a larger span. In general, .i as chooses the inverse branch followed by a .b brw , but a .b brw is sometimes pooled among several .q j instructions with the same destination. .pp .i As is unable to perform the same long/short branch generation for other instructions with a fixed byte displacement, such as the .b sob , .b aob families, or for the .b acbx family of instructions which has a fixed word displacement. This would be desirable, but is prohibitive because of the complexity of these instructions. .pp If the .b -J assembler option is given, a .b jmp instruction is used instead of a .b brw instruction for .b ALL .q j instructions with distant destinations. This makes assembly of large (>32K bytes) programs (inefficiently) possible. .i As does not try to use clever combinations of .b brb , .b brw and .b jmp instructions. The .b jmp instructions use PC relative addressing, with the length of the offset given by the .b -d assembler option. .pp These are the extended branch instructions .i as recognizes: .(b

jeql jeqlu jneq jnequ

jgeq jgequ jgtr jgtru

jleq jlequ jlss jlssu

jbcc jbsc jbcs jbss

jlbc jlbs

jcc jcs

jvc jvs

jbc jbs

jbr

.)b .pp Note that .b jbr turns into .b brb if its target is close enough; otherwise a .b brw is used. .pp Diagnostics are intended to be self explanatory and appear on the standard output. Diagnostics either report an .i error or a .i warning. Error diagnostics complain about lexical, syntactic and some semantic errors, and abort the assembly. .pp The majority of the warnings complain about the use of \*(VX features not supported by all implementations of the architecture. .i As will warn if new opcodes are used, if .q G or .q H floating point numbers are used and will complain about mixed floating conversions. .(b

limit what

Arbitrary\** Files to assemble

BUFSIZ Significant characters per name

Arbitrary Characters per input line

Arbitrary Characters per string

Arbitrary Symbols

4 Text segments

4 Data segments

.)b .(f \**Although the number of characters available to the argv line is restricted by \*(UX to 10240. .)f .pp Most of the annoyances deal with restrictions on the extended branch instructions. .pp .i As only uses a two level algorithm for resolving extended branch instructions into short or long displacements. What is really needed is a general mechanism to turn a short conditional jump into a reverse conditional jump over one of .b two possible unconditional branches, either a .b brw or a .b jmp instruction. Currently, the .b -J forces the .b jmp instruction to .i always be used, instead of the shorter .b brw instruction when needed. .pp The assembler should also recognize extended branch instructions for .b sob , .b aob , and .b acbx instructions. .b Sob instructions will be easy, .b aob will be harder because the synthesized instruction uses the index operand twice, so one must be careful of side effects, and the .b acbx family will be much harder (in the general case) because the comparison depends on the sign of the addend operand, and two operands are used more than once. Augmenting .i as with these extended loop instructions will allow the peephole optimizer to produce much better loop optimizations, since it currently assumes the worst case about the size of the loop body. .pp The string temporary file is not put in memory when the -V flag is set. The string table in the generated a.out contains some strings and names that are never referenced from the symbol table; the loader removes these unreferenced strings, however.