nasmdoc.src

nasm早期的源代码,比较简单是学习汇编和编译原理的好例子

generate completely different opcodes, despite having
identical-looking syntaxes.

NASM avoids this undesirable situation by having a much simpler
syntax for memory references. The rule is simply that any access to
the \e{contents} of a memory location requires square brackets
around the address, and any access to the \e{address} of a variable
doesn't. So an instruction of the form \c{mov ax,foo} will
\e{always} refer to a compile-time constant, whether it's an \c{EQU}
or the address of a variable; and to access the \e{contents} of the
variable \c{bar}, you must code \c{mov ax,[bar]}.

This also means that NASM has no need for MASM's \i\c{OFFSET}
keyword, since the MASM code \c{mov ax,offset bar} means exactly the
same thing as NASM's \c{mov ax,bar}. If you're trying to get
large amounts of MASM code to assemble sensibly under NASM, you
can always code \c{%idefine offset} to make the preprocessor treat
the \c{OFFSET} keyword as a no-op.

This issue is even more confusing in \i\c{a86}, where declaring a
label with a trailing colon defines it to be a `label' as opposed to
a `variable' and causes \c{a86} to adopt NASM-style semantics; so in
\c{a86}, \c{mov ax,var} has different behaviour depending on whether
\c{var} was declared as \c{var: dw 0} (a label) or \c{var dw 0} (a
word-size variable). NASM is very simple by comparison:
\e{everything} is a label.

NASM, in the interests of simplicity, also does not support the
\i{hybrid syntaxes} supported by MASM and its clones, such as
\c{mov ax,table[bx]}, where a memory reference is denoted by one
portion outside square brackets and another portion inside. The
correct syntax for the above is \c{mov ax,[table+bx]}. Likewise,
\c{mov ax,es:[di]} is wrong and \c{mov ax,[es:di]} is right.


\S{qstypes} NASM Doesn't Store \i{Variable Types}

NASM, by design, chooses not to remember the types of variables you
declare. Whereas MASM will remember, on seeing \c{var dw 0}, that
you declared \c{var} as a word-size variable, and will then be able
to fill in the \i{ambiguity} in the size of the instruction \c{mov
var,2}, NASM will deliberately remember nothing about the symbol
\c{var} except where it begins, and so you must explicitly code
\c{mov word [var],2}.

For this reason, NASM doesn't support the \c{LODS}, \c{MOVS},
\c{STOS}, \c{SCAS}, \c{CMPS}, \c{INS}, or \c{OUTS} instructions,
but only supports the forms such as \c{LODSB}, \c{MOVSW}, and
\c{SCASD}, which explicitly specify the size of the components of
the strings being manipulated.


\S{qsassume} NASM Doesn't \i\c{ASSUME}

As part of NASM's drive for simplicity, it also does not support the
\c{ASSUME} directive. NASM will not keep track of what values you
choose to put in your segment registers, and will never
\e{automatically} generate a \i{segment override} prefix.


\S{qsmodel} NASM Doesn't Support \i{Memory Models}

NASM also does not have any directives to support different 16-bit
memory models. The programmer has to keep track of which functions
are supposed to be called with a \i{far call} and which with a
\i{near call}, and is responsible for putting the correct form of
\c{RET} instruction (\c{RETN} or \c{RETF}; NASM accepts \c{RET}
itself as an alternate form for \c{RETN}); in addition, the
programmer is responsible for coding CALL FAR instructions where
necessary when calling \e{external} functions, and must also keep
track of which external variable definitions are far and which are
near.


\S{qsfpu} \i{Floating-Point} Differences

NASM uses different names to refer to floating-point registers from
MASM: where MASM would call them \c{ST(0)}, \c{ST(1)} and so on, and
\i\c{a86} would call them simply \c{0}, \c{1} and so on, NASM
chooses to call them \c{st0}, \c{st1} etc.

As of version 0.96, NASM now treats the instructions with
\i{`nowait'} forms in the same way as MASM-compatible assemblers.
The idiosyncratic treatment employed by 0.95 and earlier was based
on a misunderstanding by the authors.


\S{qsother} Other Differences

For historical reasons, NASM uses the keyword \i\c{TWORD} where MASM
and compatible assemblers use \i\c{TBYTE}.

NASM does not declare \i{uninitialized storage} in the same way as
MASM: where a MASM programmer might use \c{stack db 64 dup (?)},
NASM requires \c{stack resb 64}, intended to be read as `reserve 64
bytes'. For a limited amount of compatibility, since NASM treats
\c{?} as a valid character in symbol names, you can code \c{? equ 0}
and then writing \c{dw ?} will at least do something vaguely useful.
\I\c{RESB}\i\c{DUP} is still not a supported syntax, however.

In addition to all of this, macros and directives work completely
differently to MASM. See \k{preproc} and \k{directive} for further
details.


\C{lang} The NASM Language

\H{syntax} Layout of a NASM Source Line

Like most assemblers, each NASM source line contains (unless it
is a macro, a preprocessor directive or an assembler directive: see
\k{preproc} and \k{directive}) some combination of the four fields

\c label:    instruction operands        ; comment

As usual, most of these fields are optional; the presence or absence
of any combination of a label, an instruction and a comment is allowed.
Of course, the operand field is either required or forbidden by the
presence and nature of the instruction field.

NASM uses backslash (\\) as the line continuation character; if a line
ends with backslash, the next line is considered to be a part of the
backslash-ended line.

NASM places no restrictions on white space within a line: labels may
have white space before them, or instructions may have no space
before them, or anything. The \i{colon} after a label is also
optional. (Note that this means that if you intend to code \c{lodsb}
alone on a line, and type \c{lodab} by accident, then that's still a
valid source line which does nothing but define a label. Running
NASM with the command-line option
\I{orphan-labels}\c{-w+orphan-labels} will cause it to warn you if
you define a label alone on a line without a \i{trailing colon}.)

\i{Valid characters} in labels are letters, numbers, \c{_}, \c{$},
\c{#}, \c{@}, \c{~}, \c{.}, and \c{?}. The only characters which may
be used as the \e{first} character of an identifier are letters,
\c{.} (with special meaning: see \k{locallab}), \c{_} and \c{?}.
An identifier may also be prefixed with a \I{$, prefix}\c{$} to
indicate that it is intended to be read as an identifier and not a
reserved word; thus, if some other module you are linking with
defines a symbol called \c{eax}, you can refer to \c{$eax} in NASM
code to distinguish the symbol from the register. Maximum length of 
an identifier is 4095 characters.

The instruction field may contain any machine instruction: Pentium
and P6 instructions, FPU instructions, MMX instructions and even
undocumented instructions are all supported. The instruction may be
prefixed by \c{LOCK}, \c{REP}, \c{REPE}/\c{REPZ} or
\c{REPNE}/\c{REPNZ}, in the usual way. Explicit \I{address-size
prefixes}address-size and \i{operand-size prefixes} \c{A16},
\c{A32}, \c{O16} and \c{O32} are provided - one example of their use
is given in \k{mixsize}. You can also use the name of a \I{segment
override}segment register as an instruction prefix: coding
\c{es mov [bx],ax} is equivalent to coding \c{mov [es:bx],ax}. We
recommend the latter syntax, since it is consistent with other
syntactic features of the language, but for instructions such as
\c{LODSB}, which has no operands and yet can require a segment
override, there is no clean syntactic way to proceed apart from
\c{es lodsb}.

An instruction is not required to use a prefix: prefixes such as
\c{CS}, \c{A32}, \c{LOCK} or \c{REPE} can appear on a line by
themselves, and NASM will just generate the prefix bytes.

In addition to actual machine instructions, NASM also supports a
number of pseudo-instructions, described in \k{pseudop}.

Instruction \i{operands} may take a number of forms: they can be
registers, described simply by the register name (e.g. \c{ax},
\c{bp}, \c{ebx}, \c{cr0}: NASM does not use the \c{gas}-style
syntax in which register names must be prefixed by a \c{%} sign), or
they can be \i{effective addresses} (see \k{effaddr}), constants
(\k{const}) or expressions (\k{expr}).

For x87 \i{floating-point} instructions, NASM accepts a wide range of
syntaxes: you can use two-operand forms like MASM supports, or you
can use NASM's native single-operand forms in most cases.
\# Details of
\# all forms of each supported instruction are given in
\# \k{iref}.
For example, you can code:

\c         fadd    st1             ; this sets st0 := st0 + st1
\c         fadd    st0,st1         ; so does this
\c
\c         fadd    st1,st0         ; this sets st1 := st1 + st0
\c         fadd    to st1          ; so does this

Almost any x87 floating-point instruction that references memory must
use one of the prefixes \i\c{DWORD}, \i\c{QWORD} or \i\c{TWORD} to
indicate what size of \i{memory operand} it refers to.


\H{pseudop} \i{Pseudo-Instructions}

Pseudo-instructions are things which, though not real x86 machine
instructions, are used in the instruction field anyway because that's
the most convenient place to put them. The current pseudo-instructions
are \i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT} and \i\c{DO};
their \i{uninitialized} counterparts \i\c{RESB}, \i\c{RESW},
\i\c{RESD}, \i\c{RESQ}, \i\c{REST} and \i\c{RESO}; the \i\c{INCBIN}
command, the \i\c{EQU} command, and the \i\c{TIMES} prefix.


\S{db} \c{DB} and friends: Declaring initialized Data

\i\c{DB}, \i\c{DW}, \i\c{DD}, \i\c{DQ}, \i\c{DT} and \i\c{DO} are
used, much as in MASM, to declare initialized data in the output
file. They can be invoked in a wide range of ways:
\I{floating-point}\I{character constant}\I{string constant}

\c       db    0x55                ; just the byte 0x55
\c       db    0x55,0x56,0x57      ; three bytes in succession
\c       db    'a',0x55            ; character constants are OK
\c       db    'hello',13,10,'$'   ; so are string constants
\c       dw    0x1234              ; 0x34 0x12
\c       dw    'a'                 ; 0x61 0x00 (it's just a number)
\c       dw    'ab'                ; 0x61 0x62 (character constant)
\c       dw    'abc'               ; 0x61 0x62 0x63 0x00 (string)
\c       dd    0x12345678          ; 0x78 0x56 0x34 0x12
\c       dd    1.234567e20         ; floating-point constant
\c       dq    0x123456789abcdef0  ; eight byte constant
\c       dq    1.234567e20         ; double-precision float
\c       dt    1.234567e20         ; extended-precision float

\c{DT} and \c{DO} do not accept \i{numeric constants} as operands.
\c{DB} does not accept \i{floating-point} numbers as operands.


\S{resb} \c{RESB} and friends: Declaring \i{Uninitialized} Data

\i\c{RESB}, \i\c{RESW}, \i\c{RESD}, \i\c{RESQ}, \i\c{REST} and
\i\c{RESO} are designed to be used in the BSS section of a module:
they declare \e{uninitialized} storage space. Each takes a single
operand, which is the number of bytes, words, doublewords or whatever
to reserve.  As stated in \k{qsother}, NASM does not support the
MASM/TASM syntax of reserving uninitialized space by writing
\I\c{?}\c{DW ?} or similar things: this is what it does instead. The
operand to a \c{RESB}-type pseudo-instruction is a \i\e{critical
expression}: see \k{crit}.

For example:

\c buffer:         resb    64              ; reserve 64 bytes
\c wordvar:        resw    1               ; reserve a word
\c realarray       resq    10              ; array of ten reals


\S{incbin} \i\c{INCBIN}: Including External \i{Binary Files}

\c{INCBIN} is borrowed from the old Amiga assembler \i{DevPac}: it
includes a binary file verbatim into the output file. This can be
handy for (for example) including \i{graphics} and \i{sound} data
directly into a game executable file. It can be called in one of
these three ways:

\c     incbin  "file.dat"             ; include the whole file
\c     incbin  "file.dat",1024        ; skip the first 1024 bytes
\c     incbin  "file.dat",1024,512    ; skip the first 1024, and
\c                                    ; actually include at most 512


\S{equ} \i\c{EQU}: Defining Constants

\c{EQU} defines a symbol to a given constant value: when \c{EQU} is
used, the source line must contain a label. The action of \c{EQU} is
to define the given label name to the value of its (only) operand.
This definition is absolute, and cannot change later. So, for
example,

\c message         db      'hello, world'
\c msglen          equ     $-message

defines \c{msglen} to be the constant 12. \c{msglen} may not then be
redefined later. This is not a \i{preprocessor} definition either:
the value of \c{msglen} is evaluated \e{once}, using the value of
\c{$} (see \k{expr} for an explanation of \c{$}) at the point of
definition, rather than being evaluated wherever it is referenced
and using the value of \c{$} at the point of reference. Note that
the operand to an \c{EQU} is also a \i{critical expression}
(\k{crit}).


\S{times} \i\c{TIMES}: \i{Repeating} Instructions or Data

The \c{TIMES} prefix causes the instruction to be assembled multiple
times. This is partly present as NASM's equivalent of the \i\c{DUP}
syntax supported by \i{MASM}-compatible assemblers, in that you can
code

\c zerobuf:        times 64 db 0

or similar things; but \c{TIMES} is more versatile than that. The
argument to \c{TIMES} is not just a numeric constant, but a numeric
\e{expression}, so you can do things like

\c buffer: db      'hello, world'
\c         times 64-$+buffer db ' '

which will store exactly enough spaces to make the total length of
\c{buffer} up to 64. Finally, \c{TIMES} can be applied to ordinary
instructions, so you can code trivial \i{unrolled loops} in it:

\c         times 100 movsb

Note that there is no effective difference between \c{times 100 resb
1} and \c{resb 100}, except that the latter will be assembled about
100 times faster due to the internal structure of the assembler.

The operand to \c{TIMES}, like that of \c{EQU} and those of \c{RESB}
and friends, is a critical expression (\k{crit}).