This document describes the language accepted by the 80386 assem-
bler that is part of the Amsterdam Compiler Kit. Note that only
the syntax is described, only a few 386 instructions are shown as
examples. The syntax of numbers is the same as in C. The con-
stants 32, 040, and 0x20 all represent the same number, but are
written in decimal, octal, and hex, respectively. The rules for
character constants and strings are also the same as in C. For
example, 'a' is a character constant. A typical string is
"string". Expressions may be formed with C operators, but must
use [ and ] for parentheses. (Normal parentheses are claimed by
the operand syntax.) Symbols contain letters and digits, as well
as three special characters: dot, tilde, and underscore. The
first character may not be a digit or tilde. The names of the
80386 registers are reserved. These are: ~~~al, bl, cl, dl
~~~ah, bh, ch, dh
~~~ax, bx, cx, dx, eax, ebx, ecx, edx
~~~si, di, bp, sp, esi, edi, ebp, esp
~~~cs, ds, ss, es, fs, gs The xx and exx variants of the eight
general registers are treated as synonyms by the assembler. Nor-
mally ";ax" is the 16-bit low half of the 32-bit "eax" register.
The assembler determines if a 16 or 32 bit operation is meant
solely by looking at the instruction or the instruction prefixes.
It is however best to use the proper registers when writing as-
sembly to not confuse those who read the code. The last group of
6 segment registers are used for selector + offset mode address-
ing, in which the effective address is at a given offset in one
of the 6 segments. Names of instructions and pseudo-ops are not
reserved. Alphabetic characters in opcodes and pseudo-ops must
be in lower case. Commas, blanks, and tabs are separators and
can be interspersed freely between tokens, but not within tokens.
Commas are only legal between operands. The comment character is
!. The rest of the line is ignored. The opcodes are listed be-
low. Notes: (1) Different names for the same instruction are
separated by /. (2) Square brackets ([]) indicate that 0 or 1 of
the enclosed characters can be included. (3) Curly brackets ({})
work similarly, except that one of the enclosed characters must
be included. Thus square brackets indicate an option, whereas
curly brackets indicate that a choice must be made.
mov[b] dest, source ! Move word/byte from source to dest
pop dest ! Pop stack
push source ! Push stack
xchg[b] op1, op2 ! Exchange word/byte
xlat ! Translate
o16 ! Operate on a 16 bit object instead of 32 bit
in[b] source ! Input from source I/O port
in[b] ! Input from DX I/O port
out[b] dest ! Output to dest I/O port
out[b] ! Output to DX I/O port
lds reg,source ! Load reg and DS from source
les reg,source ! Load reg and ES from source
lea reg,source ! Load effect address of source to reg and DS
{cdsefg}seg ! Specify seg register for next instruction
a16 ! Use 16 bit addressing mode instead of 32 bit
lahf ! Load AH from flag register
popf ! Pop flags
pushf ! Push flags
sahf ! Store AH in flag register
aaa ! Adjust result of BCD addition
add[b] dest,source ! Add
adc[b] dest,source ! Add with carry
daa ! Decimal Adjust after addition
inc[b] dest ! Increment by 1
aas ! Adjust result of BCD subtraction
sub[b] dest,source ! Subtract
sbb[b] dest,source ! Subtract with borrow from dest
das ! Decimal adjust after subtraction
dec[b] dest ! Decrement by one
neg[b] dest ! Negate
cmp[b] dest,source ! Compare
aam ! Adjust result of BCD multiply
imul[b] source ! Signed multiply
mul[b] source ! Unsigned multiply
aad ! Adjust AX for BCD division
o16 cbw ! Sign extend AL into AH
o16 cwd ! Sign extend AX into DX
cwde ! Sign extend AX into EAX
cdq ! Sign extend EAX into EDX
idiv[b] source ! Signed divide
div[b] source ! Unsigned divide
and[b] dest,source ! Logical and
not[b] dest ! Logical not
or[b] dest,source ! Logical inclusive or
test[b] dest,source ! Logical test
xor[b] dest,source ! Logical exclusive or
sal[b]/shl[b] dest,CL! Shift logical left
sar[b] dest,CL ! Shift arithmetic right
shr[b] dest,CL ! Shift logical right
rcl[b] dest,CL ! Rotate left, with carry
rcr[b] dest,CL ! Rotate right, with carry
rol[b] dest,CL ! Rotate left
ror[b] dest,CL ! Rotate right
cmps[b] ! Compare string element ds:esi with es:edi
lods[b] ! Load from ds:esi into AL, AX, or EAX
movs[b] ! Move from ds:esi to es:edi
rep ! Repeat next instruction until ECX=0
repe/repz ! Repeat next instruction until ECX=0 and ZF=1
repne/repnz ! Repeat next instruction until ECX!=0 and ZF=0
scas[b] ! Compare ds:esi with AL/AX/EAX
stos[b] ! Store AL/AX/EAX in es:edi
As accepts a number of special jump opcodes that can assemble to
instructions with either a byte displacement, which can only
reach to targets within -126 to +129 bytes of the branch, or an
instruction with a 32-bit displacement. The assembler automati-
cally chooses a byte or word displacement instruction. The Eng-
lish translation of the opcodes should be obvious, with l(ess)
and g(reater) for signed comparisions, and b(elow) and
a(bove)*(CQ for unsigned comparisions. There are lots of syn-
onyms to allow you to write ";jump if not that" instead of "jump
if this";. The call, jmp, and ret instructions can be either in-
trasegment or intersegment. The intersegment versions are indi-
cated with the suffix f.
jmp[f] dest ! jump to dest (8 or 32-bit displacement)
call[f] dest ! call procedure
ret[f] ! return from procedure
ja/jnbe ! if above/not below or equal (unsigned)
jae/jnb/jnc ! if above or equal/not below/not carry (uns.)
jb/jnae/jc ! if not above nor equal/below/carry (unsigned)
jbe/jna ! if below or equal/not above (unsigned)
jg/jnle ! if greater/not less nor equal (signed)
jge/jnl ! if greater or equal/not less (signed)
jl/jnqe ! if less/not greater nor equal (signed)
jle/jgl ! if less or equal/not greater (signed)
je/jz ! if equal/zero
jne/jnz ! if not equal/not zero
jno ! if overflow not set
jo ! if overflow set
jnp/jpo ! if parity not set/parity odd
jp/jpe ! if parity set/parity even
jns ! if sign not set
js ! if sign set
jcxz dest ! jump if ECX = 0
loop dest ! Decrement ECX and jump if CX != 0
loope/loopz dest! Decrement ECX and jump if ECX = 0 and ZF = 1
loopne/loopnz dest! Decrement ECX and jump if ECX != 0 and ZF = 0
int n ! Software interrupt n
into ! Interrupt if overflow set
iretd ! Return from interrupt
clc ! Clear carry flag
cld ! Clear direction flag
cli ! Clear interrupt enable flag
cmc ! Complement carry flag
stc ! Set carry flag
std ! Set direction flag
sti ! Set interrupt enable flag
The special symbol . is the location counter and its value is the
address of the first byte of the instruction in which the symbol
appears and can be used in expressions. There are four different
assembly segments: text, rom, data and bss. Segments are de-
clared and selected by the .sect pseudo-op. It is customary to
declare all segments at the top of an assembly file like this:
~~~.sect .text; .sect .rom; .sect .data; .sect .bss The assembler
accepts up to 16 different segments, but expects only four to be
used. Anything can in principle be assembled into any segment,
but the bss segment may only contain uninitialized data. Note
that the . symbol refers to the location in the current segment.
There are two types: name and numeric. Name labels consist of a
name followed by a colon (:). The numeric labels are single dig-
its. The nearest 0: label may be referenced as 0f in the forward
direction, or 0b backwards. Each line consists of a single
statement. Blank or comment lines are allowed. The most general
form of an instruction is ~~~label: opcode operand1, operand2
! comment The following operators can be used: + - * / & | ^ ~ <<
(shift left) >> (shift right) - (unary minus). 32-bit integer
arithmetic is used. Division produces a truncated quotient. Be-
low is a list of the addressing modes supported. Each one is
followed by an example.
constant mov eax, 123456
direct access mov eax, (counter)
register mov eax, esi
indirect mov eax, (esi)
base + disp. mov eax, 6(ebp)
scaled index mov eax, (4*esi)
base + index mov eax, (ebp)(2*esi)
base + index + disp. mov eax, 10(edi)(1*esi)
Any of the constants or symbols may be replacement by expres-
sions. Direct access, constants and displacements may be any
type of expression. A scaled index with scale 1 may be written
without the 1*. The call and jmp instructions can be interpreted
as a load into the instruction pointer.
call _routine ! Direct, intrasegment
call (subloc) ! Indirect, intrasegment
call 6(ebp) ! Indirect, intrasegment
call ebx ! Direct, intrasegment
call (ebx) ! Indirect, intrasegment
callf (subloc) ! Indirect, intersegment
callf seg:offs ! Direct, intersegment
Symbols can acquire values in one of two ways. Using a symbol as
a label sets it to . for the current segment with type relocat-
able. Alternative, a symbol may be given a name via an assign-
ment of the form symbol = expression in which the symbol is
assigned the value and type of its arguments. Space can be re-
served for bytes, words, and longs using pseudo-ops. They take
one or more operands, and for each generate a value whose size is
a byte, word (2 bytes) or long (4 bytes). For example: .data1
2, 6 ! allocate 2 bytes initialized to 2 and 6
.data2 3, 0x10 ! allocate 2 words initialized to 3 and
16
.data4 010 ! allocate a longword initialized to 8
.space 40 ! allocates 40 bytes of zeros allocates
50 (decimal) bytes of storage, initializing the first two bytes
to 2 and 6, the next two words to 3 and 16, then one longword
with value 8 (010 octal), last 40 bytes of zeros. The pseudo-ops
.ascii and .asciz take one string argument and generate the ASCII
character codes for the letters in the string. The latter auto-
matically terminates the string with a null (0) byte. For exam-
ple, .ascii "hello"
.asciz "world
" Sometimes it is necessary to force the next
item to begin at a word, longword or even a 16 byte address
boundary. The .align pseudo-op zero or more null byte if the
current location is a multiple of the argument of .align. Every
item assembled goes in one of the four segments: text, rom, data,
or bss. By using the .sect pseudo-op with argument .text, .rom,
.data or .bss, the programmer can force the next items to go in a
particular segment. A symbol can be given global scope by in-
cluding it in a .define pseudo-op. Multiple names may be listed,
separate by commas. It must be used to export symbols defined in
the current program. Names not defined in the current program
are treated as ";undefined external" automatically, although it is
customary to make this explicit with the .extern pseudo-op. The
.comm pseudo-op declares storage that can be common to more than
one module. There are two arguments: a name and an absolute ex-
pression giving the size in bytes of the area named by the sym-
bol. The type of the symbol becomes external. The statement can
appear in any segment. If you think this has something to do
with FORTRAN, you are right. In the kernel directory, there are
several assembly code files that are worth inspecting as exam-
ples. However, note that these files, are designed to first be
run through the C preprocessor. (The very first character is a #
to signal this.) Thus they contain numerous constructs that are
not pure assembler. For true assembler examples, compile any C
program provided with using the -S flag. This will result in an
assembly language file with a suffix with the same name as the C
source file, but ending with the .s suffix.