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Like most assemblers, each NASM source line contains (unless it is a macro, a preprocessor directive or an assembler directive: see chapter 4 and chapter 5) some combination of the four fields
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 colon after a label is also optional. (Note that
this means that if you intend to code
on a line, and type
by accident, then
that's still a valid source line which does nothing but define a label.
Running NASM with the command-line option
will cause it to warn you if you
define a label alone on a line without a trailing colon.)
Valid characters in labels are letters, numbers,
. The only characters which may be used as the
first character of an identifier are letters,
(with special meaning: see
. An identifier may also be prefixed with a
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
, you can
in NASM code to distinguish the
symbol from the register.
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
, in the
usual way. Explicit address-size and operand-size prefixes
provided - one example of their use is given in
chapter 9. You can also use the name of a
segment register as an instruction prefix: coding
is equivalent to coding
. We recommend the latter syntax,
since it is consistent with other syntactic features of the language, but
for instructions such as
, which has no
operands and yet can require a segment override, there is no clean
syntactic way to proceed apart from
An instruction is not required to use a prefix: prefixes such as
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 section 3.2.
Instruction operands may take a number of forms: they can be registers,
described simply by the register name (e.g.
: NASM does not use the
-style syntax in which register names must be
prefixed by a
sign), or they can be effective
addresses (see section 3.3), constants
(section 3.4) or expressions
For 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 appendix B. For example, you can code:
fadd st1 ; this sets st0 := st0 + st1 fadd st0,st1 ; so does this fadd st1,st0 ; this sets st1 := st1 + st0 fadd to st1 ; so does this
Almost any floating-point instruction that references memory must use
one of the prefixes
indicate what size of memory operand it refers to.
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
, their uninitialised counterparts
command, and the
and friends: Declaring Initialised Data
are used, much as in MASM, to declare
initialised data in the output file. They can be invoked in a wide range of
db 0x55 ; just the byte 0x55 db 0x55,0x56,0x57 ; three bytes in succession db 'a',0x55 ; character constants are OK db 'hello',13,10,'$' ; so are string constants dw 0x1234 ; 0x34 0x12 dw 'a' ; 0x61 0x00 (it's just a number) dw 'ab' ; 0x61 0x62 (character constant) dw 'abc' ; 0x61 0x62 0x63 0x00 (string) dd 0x12345678 ; 0x78 0x56 0x34 0x12 dd 1.234567e20 ; floating-point constant dq 1.234567e20 ; double-precision float dt 1.234567e20 ; extended-precision float
accept numeric constants or string constants as operands.
and friends: Declaring Uninitialised Data
are designed to be used in the BSS section
of a module: they declare uninitialised storage space. Each takes
a single operand, which is the number of bytes, words, doublewords or
whatever to reserve. As stated in
section 2.2.7, NASM does not
support the MASM/TASM syntax of reserving uninitialised space by writing
or similar things: this is what it does
instead. The operand to a
pseudo-instruction is a critical expression: see
buffer: resb 64 ; reserve 64 bytes wordvar: resw 1 ; reserve a word realarray resq 10 ; array of ten reals
: Including External Binary Files
is borrowed from the old Amiga
assembler DevPac: it includes a binary file verbatim into the output file.
This can be handy for (for example) including graphics and sound data
directly into a game executable file. It can be called in one of these
incbin "file.dat" ; include the whole file incbin "file.dat",1024 ; skip the first 1024 bytes incbin "file.dat",1024,512 ; skip the first 1024, and ; actually include at most 512
: Defining Constants
defines a symbol to a given constant
is used, the source line must
contain a label. The action of
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,
message db 'hello, world' msglen equ $-message
to be the constant 12.
may not then be redefined later. This is
not a preprocessor definition either: the value of
is evaluated once, using the
3.5 for an explanation of
) at the point of
definition, rather than being evaluated wherever it is referenced and using
the value of
at the point of reference. Note
that the operand to an
is also a critical
expression (section 3.8).
: Repeating Instructions or Data
prefix causes the instruction to be
assembled multiple times. This is partly present as NASM's equivalent of
syntax supported by MASM-compatible
assemblers, in that you can code
zerobuf: times 64 db 0
or similar things; but
is more versatile
than that. The argument to
is not just a
numeric constant, but a numeric expression, so you can do things
buffer: db 'hello, world' times 64-$+buffer db ' '
which will store exactly enough spaces to make the total length of
up to 64. Finally,
can be applied to ordinary instructions, so
you can code trivial unrolled loops in it:
times 100 movsb
Note that there is no effective difference between
, except that the latter will be
assembled about 100 times faster due to the internal structure of the
The operand to
, like that of
and those of
and friends, is a critical expression (section
Note also that
can't be applied to
macros: the reason for this is that
processed after the macro phase, which allows the argument to
to contain expressions such as
as above. To repeat more than one
line of code, or a complex macro, use the preprocessor
An effective address is any operand to an instruction which references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in square brackets. For example:
wordvar dw 123 mov ax,[wordvar] mov ax,[wordvar+1] mov ax,[es:wordvar+bx]
Anything not conforming to this simple system is not a valid memory
reference in NASM, for example
More complicated effective addresses, such as those involving more than one register, work in exactly the same way:
mov eax,[ebx*2+ecx+offset] mov ax,[bp+di+8]
NASM is capable of doing algebra on these effective addresses, so that things which don't necessarily look legal are perfectly all right:
mov eax,[ebx*5] ; assembles as [ebx*4+ebx] mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]
Some forms of effective address have more than one assembled form; in
most such cases NASM will generate the smallest form it can. For example,
there are distinct assembled forms for the 32-bit effective addresses
, and NASM will generally generate the
latter on the grounds that the former requires four bytes to store a zero
NASM has a hinting mechanism which will cause
to generate different opcodes; this is occasionally useful because
have different default segment registers.
However, you can force NASM to generate an effective address in a
particular form by the use of the keywords
. If you need
to be assembled using a double-word
offset field instead of the one byte NASM will normally generate, you can
. Similarly, you can force NASM
to use a byte offset for a small value which it hasn't seen on the first
pass (see section 3.8 for an example of such a
code fragment) by using
with a byte offset of zero, and
will code it with a double-word
offset of zero. The normal form,
, will be
coded with no offset field.
The form described in the previous paragraph is also useful if you are trying to access data in a 32-bit segment from within 16 bit code. For more information on this see the section on mixed-size addressing (section 9.2). In particular, if you need to access data with a known offset that is larger than will fit in a 16-bit value, if you don't specify that it is a dword offset, nasm will cause the high word of the offset to be lost.
Similarly, NASM will split
because that allows the offset field to
be absent and space to be saved; in fact, it will also split
. You can combat this behaviour
by the use of the
to be generated literally.
NASM understands four different types of constant: numeric, character, string and floating-point.
A numeric constant is simply a number. NASM allows you to specify
numbers in a variety of number bases, in a variety of ways: you can suffix
for hex, octal
and binary, or you can prefix
for hex in the
style of C, or you can prefix
for hex in the
style of Borland Pascal. Note, though, that the
prefix does double duty as a prefix on identifiers (see
section 3.1), so a hex number prefixed with a
sign must have a digit after the
rather than a letter.
mov ax,100 ; decimal mov ax,0a2h ; hex mov ax,$0a2 ; hex again: the 0 is required mov ax,0xa2 ; hex yet again mov ax,777q ; octal mov ax,777o ; octal again mov ax,10010011b ; binary
A character constant consists of up to four characters enclosed in either single or double quotes. The type of quote makes no difference to NASM, except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa.
A character constant with more than one character will be arranged with little-endian order in mind: if you code
then the constant generated is not
, so that if you were then to store
the value into memory, it would read
. This is also the sense of character
constants understood by the Pentium's
instruction (see section
String constants are only acceptable to some pseudo-instructions, namely
A string constant looks like a character constant, only longer. It is treated as a concatenation of maximum-size character constants for the conditions. So the following are equivalent:
db 'hello' ; string constant db 'h','e','l','l','o' ; equivalent character constants
And the following are also equivalent:
dd 'ninechars' ; doubleword string constant dd 'nine','char','s' ; becomes three doublewords db 'ninechars',0,0,0 ; and really looks like this
Note that when used as an operand to
is treated as a string
constant despite being short enough to be a character constant, because
would have the same effect as
, which would be silly. Similarly,
three-character or four-character constants are treated as strings when
they are operands to
Floating-point constants are acceptable only as arguments to
. They are expressed in the traditional form:
digits, then a period, then optionally more digits, then optionally an
followed by an exponent. The period is
mandatory, so that NASM can distinguish between
, which declares an integer constant, and
which declares a floating-point constant.
dd 1.2 ; an easy one dq 1.e10 ; 10,000,000,000 dq 1.e+10 ; synonymous with 1.e10 dq 1.e-10 ; 0.000 000 000 1 dt 3.141592653589793238462 ; pi
NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.
Expressions in NASM are similar in syntax to those in C.
NASM does not guarantee the size of the integers used to evaluate expressions at compile time: since NASM can compile and run on 64-bit systems quite happily, don't assume that expressions are evaluated in 32-bit registers and so try to make deliberate use of integer overflow. It might not always work. The only thing NASM will guarantee is what's guaranteed by ANSI C: you always have at least 32 bits to work in.
NASM supports two special tokens in expressions, allowing calculations
to involve the current assembly position: the
evaluates to the assembly position at the beginning of the line containing
the expression; so you can code an infinite loop using
the beginning of the current section; so you can tell how far into the
section you are by using
The arithmetic operators provided by NASM are listed here, in increasing order of precedence.
: Bitwise OR Operator
operator gives a bitwise OR, exactly as
performed by the
machine instruction. Bitwise
OR is the lowest-priority arithmetic operator supported by NASM.
: Bitwise XOR Operator
provides the bitwise XOR operation.
: Bitwise AND Operator
provides the bitwise AND operation.
: Bit Shift Operators
gives a bit-shift to the left, just
as it does in C. So
evaluates to 5
times 8, or 40.
gives a bit-shift to the
right; in NASM, such a shift is always unsigned, so that the bits
shifted in from the left-hand end are filled with zero rather than a
sign-extension of the previous highest bit.
: Addition and Subtraction Operators
operators do perfectly ordinary addition and subtraction.
: Multiplication and Division
is the multiplication operator.
is unsigned division and
is signed division. Similarly,
unsigned and signed modulo operators respectively.
NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.
character is used extensively by
the macro preprocessor, you should ensure that both the signed and unsigned
modulo operators are followed by white space wherever they appear.
The highest-priority operators in NASM's expression grammar are those
which only apply to one argument.
does nothing (it's provided for
computes the one's complement of its operand, and
provides the segment address of its operand
(explained in more detail in section 3.6).
When writing large 16-bit programs, which must be split into multiple
segments, it is often necessary to be able to refer to the segment part of
the address of a symbol. NASM supports the
operator to perform this function.
operator returns the
preferred segment base of a symbol, defined as the segment base
relative to which the offset of the symbol makes sense. So the code
mov ax,seg symbol mov es,ax mov bx,symbol
with a valid pointer to the
Things can be more complex than this: since 16-bit segments and groups
may overlap, you might occasionally want to refer to some symbol using a
different segment base from the preferred one. NASM lets you do this, by
the use of the
(With Reference To) keyword.
So you can do things like
mov ax,weird_seg ; weird_seg is a segment base mov es,ax mov bx,symbol wrt weird_seg
with a different, but
functionally equivalent, pointer to the symbol
NASM supports far (inter-segment) calls and jumps by means of the syntax
represent immediate values. So to call a far procedure, you could code
call (seg procedure):procedure call weird_seg:(procedure wrt weird_seg)
(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)
NASM supports the syntax
a synonym for the first of the above usages.
works identically to
in these examples.
To declare a far pointer to a data item in a data segment, you must code
dw symbol, seg symbol
NASM supports no convenient synonym for this, though you can always invent one using the macro processor.
: Inhibiting Optimization
When assembling with the optimizer set to level 2 or higher (see
section 2.1.16), NASM will use
size specifiers (
will give them the smallest possible size. The keyword
can be used to inhibit optimization and
force a particular operand to be emitted in the specified size. For
example, with the optimizer on, and in
push dword 33
is encoded in three bytes
push strict dword 33
is encoded in six bytes, with a full dword immediate operand
With the optimizer off, the same code (six bytes) is generated whether
keyword was used or not.
A limitation of NASM is that it is a two-pass assembler; unlike TASM and others, it will always do exactly two assembly passes. Therefore it is unable to cope with source files that are complex enough to require three or more passes.
The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,
times (label-$) db 0 label: db 'Where am I?'
The argument to
in this case could
equally legally evaluate to anything at all; NASM will reject this example
because it cannot tell the size of the
when it first sees it. It will just as firmly reject the slightly
times (label-$+1) db 0 label: db 'NOW where am I?'
in which any value for the
argument is by definition wrong!
NASM rejects these examples by means of a concept called a critical
expression, which is defined to be an expression whose value is
required to be computable in the first pass, and which must therefore
depend only on symbols defined before it. The argument to the
prefix is a critical expression; for the
same reason, the arguments to the
pseudo-instructions are also critical expressions.
Critical expressions can crop up in other contexts as well: consider the following code.
mov ax,symbol1 symbol1 equ symbol2 symbol2:
On the first pass, NASM cannot determine the value of
is defined to be equal to
which NASM hasn't seen yet. On the second
pass, therefore, when it encounters the line
, it is unable to generate the code
for it because it still doesn't know the value of
. On the next line, it would see the
again and be able to determine the value of
, but by then it would be too late.
NASM avoids this problem by defining the right-hand side of an
statement to be a critical expression, so the
would be rejected in the
There is a related issue involving forward references: consider this code fragment.
mov eax,[ebx+offset] offset equ 10
NASM, on pass one, must calculate the size of the instruction
without knowing the value of
. It has no way of knowing that
is small enough to fit into a one-byte
offset field and that it could therefore get away with generating a shorter
form of the effective-address encoding; for all it knows, in pass one,
could be a symbol in the code segment, and
it might need the full four-byte form. So it is forced to compute the size
of the instruction to accommodate a four-byte address part. In pass two,
having made this decision, it is now forced to honour it and keep the
instruction large, so the code generated in this case is not as small as it
could have been. This problem can be solved by defining
before using it, or by forcing byte size
in the effective address by coding
Note that use of the
switch (with n>=2)
makes some of the above no longer true (see
NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:
label1 ; some code .loop ; some more code jne .loop ret label2 ; some code .loop ; some more code jne .loop ret
In the above code fragment, each
instruction jumps to the line immediately before it, because the two
are kept separate by virtue
of each being associated with the previous non-local label.
This form of local label handling is borrowed from the old Amiga
assembler DevPac; however, NASM goes one step further, in allowing access
to local labels from other parts of the code. This is achieved by means of
defining a local label in terms of the previous non-local label:
the first definition of
above is really
defining a symbol called
, and the
second defines a symbol called
if you really needed to, you could write
label3 ; some more code ; and some more jmp label1.loop
Sometimes it is useful - in a macro, for instance - to be able to define
a label which can be referenced from anywhere but which doesn't interfere
with the normal local-label mechanism. Such a label can't be non-local
because it would interfere with subsequent definitions of, and references
to, local labels; and it can't be local because the macro that defined it
wouldn't know the label's full name. NASM therefore introduces a third type
of label, which is probably only useful in macro definitions: if a label
begins with the special prefix
, then it does
nothing to the local label mechanism. So you could code
label1: ; a non-local label .local: ; this is really label1.local ..@foo: ; this is a special symbol label2: ; another non-local label .local: ; this is really label2.local jmp ..@foo ; this will jump three lines up
NASM has the capacity to define other special symbols beginning with a
double period: for example,
is used to
specify the entry point in the
(see section 6.2.6).
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