as.9

操作系统设计与实现源码

.\" unchecked (kjb)
.CD "as \(en assembler"
.SE "AS\(emASSEMBLER [IBM]"
.SP 1
.PP
This document describes the language accepted by the 80386 assembler
that is part of the Amsterdam Compiler Kit.  Note that only the syntax is
described, only a few 386 instructions are shown as examples.
.SS "Tokens, Numbers, Character Constants, and Strings"
.PP
The syntax of numbers is the same as in C.
The constants 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, \(fma\(fm 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.)
.SS "Symbols"
.PP
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.
.PP
The names of the 80386 registers are reserved.  These are:
.HS
~~~al, bl, cl, dl
.br
~~~ah, bh, ch, dh
.br
~~~ax, bx, cx, dx, eax, ebx, ecx, edx
.br
~~~si, di, bp, sp, esi, edi, ebp, esp
.br
~~~cs, ds, ss, es, fs, gs
.HS
The xx and exx variants of the eight general registers are treated as
synonyms by the assembler.  Normally "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 assembly to not confuse those who read the code.
.HS
The last group of 6 segment registers are used for selector + offset mode
addressing, in which the effective address is at a given offset in one of
the 6 segments.
.PP
Names of instructions and pseudo-ops are not reserved.  
Alphabetic characters in opcodes and pseudo-ops must be in lower case.
.SS "Separators"
.PP
Commas, blanks, and tabs are separators and can be interspersed freely 
between tokens, but not within tokens.
Commas are only legal between operands.
.SS "Comments"
.PP
The comment character is \*(OQ!\*(CQ.  
The rest of the line is ignored.
.SS "Opcodes"
.PP
The opcodes are listed below.
Notes: (1) Different names for the same instruction are separated by \*(OQ/\*(CQ.
(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 \fImust\fR be included.
Thus square brackets indicate an option, whereas curly brackets indicate
that a choice must be made.
.sp
.if t .ta 0.25i 1.2i 3i
.if n .ta 2 10 24
.nf
.B "Data Transfer"
.HS
	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

.B "Input/Output"
.HS
	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

.B "Address Object"
.HS
	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

.B "Flag Transfer"
.HS
	lahf		! Load AH from flag register
	popf		! Pop flags 
	pushf		! Push flags 
	sahf		! Store AH in flag register

.B "Addition"
.HS
	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

.B "Subtraction"
.HS
	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

.B "Multiplication"
.HS
	aam		! Adjust result of BCD multiply
	imul[b]	source	! Signed multiply
	mul[b]	source	! Unsigned multiply

.B "Division"
.HS
	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

.B "Logical"
.HS
	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

.B "Shift"
.HS
	sal[b]/shl[b]	dest,CL	! Shift logical left
	sar[b]	dest,CL	! Shift arithmetic right
	shr[b]	dest,CL	! Shift logical right

.B "Rotate"
.HS
	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

.B "String Manipulation"
.HS
	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

.fi
.B "Control Transfer"
.PP
\fIAs\fR accepts a number of special jump opcodes that can assemble to
instructions with either a byte displacement, which can only reach to targets
within \(mi126 to +129 bytes of the branch, or an instruction with a 32-bit
displacement.  The assembler automatically chooses a byte or word displacement
instruction.
.PP
The English translation of the opcodes should be obvious, with
\*(OQl(ess)\*(CQ and \*(OQg(reater)\*(CQ for signed comparisions, and
\*(OQb(elow)\*(CQ and \*(OQa(bove)*(CQ for unsigned comparisions.  There are
lots of synonyms to allow you to write "jump if not that" instead of "jump
if this".
.PP
The \*(OQcall\*(CQ, \*(OQjmp\*(CQ, and \*(OQret\*(CQ instructions can be 
either intrasegment or
intersegment.  The intersegment versions are indicated with 
the suffix \*(OQf\*(CQ.

.if t .ta 0.25i 1.2i 3i
.if n .ta 2 10 24
.nf
.B Unconditional
.HS
	jmp[f]	dest	! jump to dest (8 or 32-bit displacement)
	call[f]	dest	! call procedure
	ret[f]		! return from procedure

.B "Conditional"
.HS
	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

.B "Iteration Control"
.HS
	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

.B "Interrupt"
.HS
	int	n	! Software interrupt n
	into		! Interrupt if overflow set
	iretd		! Return from interrupt

.B "Flag Operations"
.HS
	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

.fi
.SS "Location Counter"
.PP
The special symbol \*(OQ.\*(CQ 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.
.SS "Segments"
.PP
There are four different assembly segments: text, rom, data and bss.
Segments are declared and selected by the \fI.sect\fR pseudo-op.  It is
customary to declare all segments at the top of an assembly file like
this:
.HS
~~~.sect .text; .sect .rom; .sect .data; .sect .bss
.HS
The assembler accepts up to 16 different segments, but
.MX
expects only four to be used.  Anything can in principle be assembled
into any segment, but the
.MX
bss segment may only contain uninitialized data.
Note that the \*(OQ.\*(CQ symbol refers to the location in the current
segment.
.SS "Labels"
.PP
There are two types: name and numeric.  Name labels consist of a name
followed by a colon (:).
.PP
The numeric labels are single digits.  The nearest 0: label may be
referenced as 0f in the forward direction, or 0b backwards.
.SS "Statement Syntax"
.PP
Each line consists of a single statement.
Blank or comment lines are allowed.
.SS "Instruction Statements"
.PP
The most general form of an instruction is
.HS
~~~label: opcode operand1, operand2    ! comment
.HS
.SS "Expression Semantics"
.PP
.tr ~~
The following operators can be used:
+ \(mi * / & | ^ ~ << (shift left) >> (shift right) \(mi (unary minus).
.tr ~
32-bit integer arithmetic is used.  
Division produces a truncated quotient.
.SS "Addressing Modes"
.PP
Below is a list of the addressing modes supported.
Each one is followed by an example.
.HS
.ta 0.25i 3i
.nf
	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)
.HS
.fi
Any of the constants or symbols may be replacement by expressions.  Direct
access, constants and displacements may be any type of expression.  A scaled
index with scale 1 may be written without the \*(OQ1*\*(CQ.
.SS "Call and Jmp"
.PP
The \*(OQcall\*(CQ and \*(OQjmp\*(CQ instructions can be interpreted
as a load into the instruction pointer.
.HS
.ta 0.25i 3i
.nf
	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
.HS
.fi
.SP 1
.SS "Symbol Assigment"
.SP 1
.PP
Symbols can acquire values in one of two ways.
Using a symbol as a label sets it to \*(OQ.\*(CQ for the current
segment with type relocatable.  
Alternative, a symbol may be given a name via an assignment of the form
.HS
~~~symbol = expression 
.HS
in which the symbol is assigned the value and type of its arguments.
.SP 1
.SS "Storage Allocation"
.SP 1
.PP
Space can be reserved 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:
.HS
.if t .ta 0.25i 3i
.if n .ta 2 24
	.data1 2, 6	! allocate 2 bytes initialized to 2 and 6
.br
	.data2 3, 0x10	! allocate 2 words initialized to 3 and 16
.br
	.data4 010	! allocate a longword initialized to 8
.br
	.space 40	! allocates 40 bytes of zeros
.HS
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.
.SS "String Allocation"
.PP
The pseudo-ops \fI.ascii\fR and \fI.asciz\fR
take one string argument and generate the ASCII character
codes for the letters in the string. 
The latter automatically terminates the string with a null (0) byte.
For example,
.HS
~~~.ascii "hello"
.br
~~~.asciz "world\en"
.HS
.SS "Alignment"
.PP
Sometimes it is necessary to force the next item to begin at a word, longword
or even a 16 byte address boundary.
The \fI.align\fR pseudo-op zero or more null byte if the current location
is a multiple of the argument of .align.
.SS "Segment Control"
.PP
Every item assembled goes in one of the four segments: text, rom, data,
or bss.  By using the \fI.sect\fR pseudo-op with argument
\fI.text, .rom, .data\fR or \fI.bss\fR, the programmer can force the
next items to go in a particular segment.
.SS "External Names"
.PP
A symbol can be given global scope by including it in a \fI.define\fR 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 \fI.extern\fR pseudo-op.
.SS "Common"
.PP
The \fI.comm\fR pseudo-op declares storage that can be common to more than 
one module.  There are two arguments: a name and an absolute expression giving
the size in bytes of the area named by the symbol.  
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.
.SS "Examples"
.PP
In the kernel directory, there are several assembly code files that are
worth inspecting as examples.
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 
.MX
using the \fB\(enS\fR 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.