NASM contains a powerful macro processor, which supports conditional
assembly, multi-level file inclusion, two forms of macro (single-line and
multi-line), and a `context stack' mechanism for extra macro power.
Preprocessor directives all begin with a % sign.
The preprocessor collapses all lines which end with a backslash (\)
character into a single line. Thus:
When the expansion of a single-line macro contains tokens which invoke
another macro, the expansion is performed at invocation time, not at
definition time. Thus the code
%define a(x) 1+b(x)
%define b(x) 2*x
mov ax,a(8)
will evaluate in the expected way to
mov ax,1+2*8, even though the macro
b wasn't defined at the time of definition of
a.
Macros defined with %define are case
sensitive: after %define foo bar, only
foo will expand to bar:
Foo or FOO will not. By
using %idefine instead of
%define (the `i' stands for `insensitive') you
can define all the case variants of a macro at once, so that
%idefine foo bar would cause
foo, Foo,
FOO, fOO and so on all
to expand to bar.
There is a mechanism which detects when a macro call has occurred as a
result of a previous expansion of the same macro, to guard against circular
references and infinite loops. If this happens, the preprocessor will only
expand the first occurrence of the macro. Hence, if you code
%define a(x) 1+a(x)
mov ax,a(3)
the macro a(3) will expand once, becoming
1+a(3), and will then expand no further. This
behaviour can be useful: see section
8.1 for an example of its use.
You can overload single-line macros: if you write
%define foo(x) 1+x
%define foo(x,y) 1+x*y
the preprocessor will be able to handle both types of macro call, by
counting the parameters you pass; so foo(3) will
become 1+3 whereas
foo(ebx,2) will become
1+ebx*2. However, if you define
%define foo bar
then no other definition of foo will be
accepted: a macro with no parameters prohibits the definition of the same
name as a macro with parameters, and vice versa.
This doesn't prevent single-line macros being redefined: you
can perfectly well define a macro with
%define foo bar
and then re-define it later in the same source file with
%define foo baz
Then everywhere the macro foo is invoked, it
will be expanded according to the most recent definition. This is
particularly useful when defining single-line macros with
%assign (see section
4.1.5).
You can pre-define single-line macros using the `-d' option on the NASM
command line: see section
2.1.12.
To have a reference to an embedded single-line macro resolved at the
time that it is embedded, as opposed to when the calling macro is expanded,
you need a different mechanism to the one offered by
%define. The solution is to use
%xdefine, or it's case-insensitive counterpart
%xidefine.
Suppose you have the following code:
%define isTrue 1
%define isFalse isTrue
%define isTrue 0
val1: db isFalse
%define isTrue 1
val2: db isFalse
In this case, val1 is equal to 0, and
val2 is equal to 1. This is because, when a
single-line macro is defined using %define, it is
expanded only when it is called. As isFalse
expands to isTrue, the expansion will be the
current value of isTrue. The first time it is
called that is 0, and the second time it is 1.
If you wanted isFalse to expand to the value
assigned to the embedded macro isTrue at the time
that isFalse was defined, you need to change the
above code to use %xdefine.
%xdefine isTrue 1
%xdefine isFalse isTrue
%xdefine isTrue 0
val1: db isFalse
%xdefine isTrue 1
val2: db isFalse
Now, each time that isFalse is called, it
expands to 1, as that is what the embedded macro
isTrue expanded to at the time that
isFalse was defined.
Individual tokens in single line macros can be concatenated, to produce
longer tokens for later processing. This can be useful if there are several
similar macros that perform similar functions.
As an example, consider the following:
%define BDASTART 400h ; Start of BIOS data area
struc tBIOSDA ; its structure
.COM1addr RESW 1
.COM2addr RESW 1
; ..and so on
endstruc
Now, if we need to access the elements of tBIOSDA in different places,
we can end up with:
An alternative way to define single-line macros is by means of the
%assign command (and its case-insensitive
counterpart %iassign, which differs from
%assign in exactly the same way that
%idefine differs from
%define).
%assign is used to define single-line macros
which take no parameters and have a numeric value. This value can be
specified in the form of an expression, and it will be evaluated once, when
the %assign directive is processed.
Like %define, macros defined using
%assign can be re-defined later, so you can do
things like
%assign i i+1
to increment the numeric value of a macro.
%assign is useful for controlling the
termination of %rep preprocessor loops: see
section 4.5 for an example of this. Another use
for %assign is given in
section 7.4 and
section 8.1.
The expression passed to %assign is a critical
expression (see section 3.8), and
must also evaluate to a pure number (rather than a relocatable reference
such as a code or data address, or anything involving a register).
It's often useful to be able to handle strings in macros. NASM supports
two simple string handling macro operators from which more complex
operations can be constructed.
The %strlen macro is like
%assign macro in that it creates (or redefines) a
numeric value to a macro. The difference is that with
%strlen, the numeric value is the length of a
string. An example of the use of this would be:
%strlen charcnt 'my string'
In this example, charcnt would receive the
value 8, just as if an %assign had been used. In
this example, 'my string' was a literal string
but it could also have been a single-line macro that expands to a string,
as in the following example:
Individual letters in strings can be extracted using
%substr. An example of its use is probably more
useful than the description:
%substr mychar 'xyz' 1 ; equivalent to %define mychar 'x'
%substr mychar 'xyz' 2 ; equivalent to %define mychar 'y'
%substr mychar 'xyz' 3 ; equivalent to %define mychar 'z'
In this example, mychar gets the value of 'y'. As with
%strlen (see section
4.2.1), the first parameter is the single-line macro to be created and
the second is the string. The third parameter specifies which character is
to be selected. Note that the first index is 1, not 0 and the last index is
equal to the value that %strlen would assign
given the same string. Index values out of range result in an empty string.
Multi-line macros are much more like the type of macro seen in MASM and
TASM: a multi-line macro definition in NASM looks something like this.
%macro prologue 1
push ebp
mov ebp,esp
sub esp,%1
%endmacro
This defines a C-like function prologue as a macro: so you would invoke
the macro with a call such as
myfunc: prologue 12
which would expand to the three lines of code
myfunc: push ebp
mov ebp,esp
sub esp,12
The number 1 after the macro name in the
%macro line defines the number of parameters the
macro prologue expects to receive. The use of
%1 inside the macro definition refers to the
first parameter to the macro call. With a macro taking more than one
parameter, subsequent parameters would be referred to as
%2, %3 and so on.
Multi-line macros, like single-line macros, are case-sensitive, unless
you define them using the alternative directive
%imacro.
If you need to pass a comma as part of a parameter to a
multi-line macro, you can do that by enclosing the entire parameter in
braces. So you could code things like
%macro silly 2
%2: db %1
%endmacro
silly 'a', letter_a ; letter_a: db 'a'
silly 'ab', string_ab ; string_ab: db 'ab'
silly {13,10}, crlf ; crlf: db 13,10
As with single-line macros, multi-line macros can be overloaded by
defining the same macro name several times with different numbers of
parameters. This time, no exception is made for macros with no parameters
at all. So you could define
%macro prologue 0
push ebp
mov ebp,esp
%endmacro
to define an alternative form of the function prologue which allocates
no local stack space.
Sometimes, however, you might want to `overload' a machine instruction;
for example, you might want to define
%macro push 2
push %1
push %2
%endmacro
so that you could code
push ebx ; this line is not a macro call
push eax,ecx ; but this one is
Ordinarily, NASM will give a warning for the first of the above two
lines, since push is now defined to be a macro,
and is being invoked with a number of parameters for which no definition
has been given. The correct code will still be generated, but the assembler
will give a warning. This warning can be disabled by the use of the
-w-macro-params command-line option (see
section 2.1.18).
NASM allows you to define labels within a multi-line macro definition in
such a way as to make them local to the macro call: so calling the same
macro multiple times will use a different label each time. You do this by
prefixing %% to the label name. So you can invent
an instruction which executes a RET if the
Z flag is set by doing this:
%macro retz 0
jnz %%skip
ret
%%skip:
%endmacro
You can call this macro as many times as you want, and every time you
call it NASM will make up a different `real' name to substitute for the
label %%skip. The names NASM invents are of the
form ..@2345.skip, where the number 2345 changes
with every macro call. The ..@ prefix prevents
macro-local labels from interfering with the local label mechanism, as
described in section 3.9. You
should avoid defining your own labels in this form (the
..@ prefix, then a number, then another period)
in case they interfere with macro-local labels.
Occasionally it is useful to define a macro which lumps its entire
command line into one parameter definition, possibly after extracting one
or two smaller parameters from the front. An example might be a macro to
write a text string to a file in MS-DOS, where you might want to be able to
write
writefile [filehandle],"hello, world",13,10
NASM allows you to define the last parameter of a macro to be
greedy, meaning that if you invoke the macro with more parameters
than it expects, all the spare parameters get lumped into the last defined
one along with the separating commas. So if you code:
%macro writefile 2+
jmp %%endstr
%%str: db %2
%%endstr:
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
then the example call to writefile above will
work as expected: the text before the first comma,
[filehandle], is used as the first macro
parameter and expanded when %1 is referred to,
and all the subsequent text is lumped into %2 and
placed after the db.
The greedy nature of the macro is indicated to NASM by the use of the
+ sign after the parameter count on the
%macro line.
If you define a greedy macro, you are effectively telling NASM how it
should expand the macro given any number of parameters from the
actual number specified up to infinity; in this case, for example, NASM now
knows what to do when it sees a call to writefile
with 2, 3, 4 or more parameters. NASM will take this into account when
overloading macros, and will not allow you to define another form of
writefile taking 4 parameters (for example).
Of course, the above macro could have been implemented as a non-greedy
macro, in which case the call to it would have had to look like
writefile [filehandle], {"hello, world",13,10}
NASM provides both mechanisms for putting commas in macro parameters,
and you choose which one you prefer for each macro definition.
See section 5.2.1 for a better
way to write the above macro.
NASM also allows you to define a multi-line macro with a range
of allowable parameter counts. If you do this, you can specify defaults for
omitted parameters. So, for example:
%macro die 0-1 "Painful program death has occurred."
writefile 2,%1
mov ax,0x4c01
int 0x21
%endmacro
This macro (which makes use of the writefile
macro defined in section 4.3.3) can be called
with an explicit error message, which it will display on the error output
stream before exiting, or it can be called with no parameters, in which
case it will use the default error message supplied in the macro
definition.
In general, you supply a minimum and maximum number of parameters for a
macro of this type; the minimum number of parameters are then required in
the macro call, and then you provide defaults for the optional ones. So if
a macro definition began with the line
%macro foobar 1-3 eax,[ebx+2]
then it could be called with between one and three parameters, and
%1 would always be taken from the macro call.
%2, if not specified by the macro call, would
default to eax, and %3
if not specified would default to [ebx+2].
You may omit parameter defaults from the macro definition, in which case
the parameter default is taken to be blank. This can be useful for macros
which can take a variable number of parameters, since the
%0 token (see section
4.3.5) allows you to determine how many parameters were really passed
to the macro call.
This defaulting mechanism can be combined with the greedy-parameter
mechanism; so the die macro above could be made
more powerful, and more useful, by changing the first line of the
definition to
%macro die 0-1+ "Painful program death has occurred.",13,10
The maximum parameter count can be infinite, denoted by
*. In this case, of course, it is impossible to
provide a full set of default parameters. Examples of this usage
are shown in section 4.3.6.
For a macro which can take a variable number of parameters, the
parameter reference %0 will return a numeric
constant giving the number of parameters passed to the macro. This can be
used as an argument to %rep (see
section 4.5) in order to iterate through all the
parameters of a macro. Examples are given in
section 4.3.6.
Unix shell programmers will be familiar with the
shift shell command, which allows the arguments
passed to a shell script (referenced as $1,
$2 and so on) to be moved left by one place, so
that the argument previously referenced as $2
becomes available as $1, and the argument
previously referenced as $1 is no longer
available at all.
NASM provides a similar mechanism, in the form of
%rotate. As its name suggests, it differs from
the Unix shift in that no parameters are lost:
parameters rotated off the left end of the argument list reappear on the
right, and vice versa.
%rotate is invoked with a single numeric
argument (which may be an expression). The macro parameters are rotated to
the left by that many places. If the argument to
%rotate is negative, the macro parameters are
rotated to the right.
So a pair of macros to save and restore a set of registers might work as
follows:
This macro invokes the PUSH instruction on
each of its arguments in turn, from left to right. It begins by pushing its
first argument, %1, then invokes
%rotate to move all the arguments one place to
the left, so that the original second argument is now available as
%1. Repeating this procedure as many times as
there were arguments (achieved by supplying %0 as
the argument to %rep) causes each argument in
turn to be pushed.
Note also the use of * as the maximum
parameter count, indicating that there is no upper limit on the number of
parameters you may supply to the multipush macro.
It would be convenient, when using this macro, to have a
POP equivalent, which didn't require the
arguments to be given in reverse order. Ideally, you would write the
multipush macro call, then cut-and-paste the line
to where the pop needed to be done, and change the name of the called macro
to multipop, and the macro would take care of
popping the registers in the opposite order from the one in which they were
pushed.
This macro begins by rotating its arguments one place to the
right, so that the original last argument appears as
%1. This is then popped, and the arguments are
rotated right again, so the second-to-last argument becomes
%1. Thus the arguments are iterated through in
reverse order.
NASM can concatenate macro parameters on to other text surrounding them.
This allows you to declare a family of symbols, for example, in a macro
definition. If, for example, you wanted to generate a table of key codes
along with offsets into the table, you could code something like
keytab:
keyposF1 equ $-keytab
db 128+1
keyposF2 equ $-keytab
db 128+2
keyposReturn equ $-keytab
db 13
You can just as easily concatenate text on to the other end of a macro
parameter, by writing %1foo.
If you need to append a digit to a macro parameter, for example
defining labels foo1 and
foo2 when passed the parameter
foo, you can't code %11
because that would be taken as the eleventh macro parameter. Instead, you
must code %{1}1, which will separate the first
1 (giving the number of the macro parameter) from
the second (literal text to be concatenated to the parameter).
This concatenation can also be applied to other preprocessor in-line
objects, such as macro-local labels (section
4.3.2) and context-local labels (section
4.7.2). In all cases, ambiguities in syntax can be resolved by
enclosing everything after the % sign and before
the literal text in braces: so %{%foo}bar
concatenates the text bar to the end of the real
name of the macro-local label %%foo. (This is
unnecessary, since the form NASM uses for the real names of macro-local
labels means that the two usages %{%foo}bar and
%%foobar would both expand to the same thing
anyway; nevertheless, the capability is there.)
NASM can give special treatment to a macro parameter which contains a
condition code. For a start, you can refer to the macro parameter
%1 by means of the alternative syntax
%+1, which informs NASM that this macro parameter
is supposed to contain a condition code, and will cause the preprocessor to
report an error message if the macro is called with a parameter which is
not a valid condition code.
Far more usefully, though, you can refer to the macro parameter by means
of %-1, which NASM will expand as the
inverse condition code. So the retz
macro defined in section 4.3.2 can be replaced
by a general conditional-return macro like this:
%macro retc 1
j%-1 %%skip
ret
%%skip:
%endmacro
This macro can now be invoked using calls like
retc ne, which will cause the conditional-jump
instruction in the macro expansion to come out as
JE, or retc po which
will make the jump a JPE.
The %+1 macro-parameter reference is quite
happy to interpret the arguments CXZ and
ECXZ as valid condition codes; however,
%-1 will report an error if passed either of
these, because no inverse condition code exists.
When NASM is generating a listing file from your program, it will
generally expand multi-line macros by means of writing the macro call and
then listing each line of the expansion. This allows you to see which
instructions in the macro expansion are generating what code; however, for
some macros this clutters the listing up unnecessarily.
NASM therefore provides the .nolist qualifier,
which you can include in a macro definition to inhibit the expansion of the
macro in the listing file. The .nolist qualifier
comes directly after the number of parameters, like this:
Similarly to the C preprocessor, NASM allows sections of a source file
to be assembled only if certain conditions are met. The general syntax of
this feature looks like this:
%if<condition>
; some code which only appears if <condition> is met
%elif<condition2>
; only appears if <condition> is not met but <condition2> is
%else
; this appears if neither <condition> nor <condition2> was met
%endif
The %else clause is optional, as is the
%elif clause. You can have more than one
%elif clause as well.
Beginning a conditional-assembly block with the line
%ifdef MACRO will assemble the subsequent code
if, and only if, a single-line macro called MACRO
is defined. If not, then the %elif and
%else blocks (if any) will be processed instead.
For example, when debugging a program, you might want to write code such
as
; perform some function
%ifdef DEBUG
writefile 2,"Function performed successfully",13,10
%endif
; go and do something else
Then you could use the command-line option
-dDEBUG to create a version of the program which
produced debugging messages, and remove the option to generate the final
release version of the program.
You can test for a macro not being defined by using
%ifndef instead of
%ifdef. You can also test for macro definitions
in %elif blocks by using
%elifdef and %elifndef.
The %ifmacro directive operates in the same
way as the %ifdef directive, except that it
checks for the existence of a multi-line macro.
For example, you may be working with a large project and not have
control over the macros in a library. You may want to create a macro with
one name if it doesn't already exist, and another name if one with that
name does exist.
The %ifmacro is considered true if defining a
macro with the given name and number of arguments would cause a definitions
conflict. For example:
%ifmacro MyMacro 1-3
%error "MyMacro 1-3" causes a conflict with an existing macro.
%else
%macro MyMacro 1-3
; insert code to define the macro
%endmacro
%endif
This will create the macro "MyMacro 1-3" if no macro already exists
which would conflict with it, and emits a warning if there would be a
definition conflict.
You can test for the macro not existing by using the
%ifnmacro instead of
%ifmacro. Additional tests can be performed in
%elif blocks by using
%elifmacro and
%elifnmacro.
The conditional-assembly construct
%ifctx ctxname will cause the subsequent code to
be assembled if and only if the top context on the preprocessor's context
stack has the name ctxname. As with
%ifdef, the inverse and
%elif forms %ifnctx,
%elifctx and %elifnctx
are also supported.
For more details of the context stack, see
section 4.7. For a sample use of
%ifctx, see section
4.7.5.
The conditional-assembly construct %if expr
will cause the subsequent code to be assembled if and only if the value of
the numeric expression expr is non-zero. An
example of the use of this feature is in deciding when to break out of a
%rep preprocessor loop: see
section 4.5 for a detailed example.
The expression given to %if, and its
counterpart %elif, is a critical expression (see
section 3.8).
%if extends the normal NASM expression syntax,
by providing a set of relational operators which are not normally available
in expressions. The operators =,
<, >,
<=, >= and
<> test equality, less-than, greater-than,
less-or-equal, greater-or-equal and not-equal respectively. The C-like
forms == and != are
supported as alternative forms of = and
<>. In addition, low-priority logical
operators &&,
^^ and || are provided,
supplying logical AND, logical XOR and logical OR. These work like the C
logical operators (although C has no logical XOR), in that they always
return either 0 or 1, and treat any non-zero input as 1 (so that
^^, for example, returns 1 if exactly one of its
inputs is zero, and 0 otherwise). The relational operators also return 1
for true and 0 for false.
The construct %ifidn text1,text2 will cause
the subsequent code to be assembled if and only if
text1 and text2, after
expanding single-line macros, are identical pieces of text. Differences in
white space are not counted.
%ifidni is similar to
%ifidn, but is case-insensitive.
For example, the following macro pushes a register or number on the
stack, and allows you to treat IP as a real
register:
Like most other %if constructs,
%ifidn has a counterpart
%elifidn, and negative forms
%ifnidn and %elifnidn.
Similarly, %ifidni has counterparts
%elifidni, %ifnidni and
%elifnidni.
Some macros will want to perform different tasks depending on whether
they are passed a number, a string, or an identifier. For example, a string
output macro might want to be able to cope with being passed either a
string constant or a pointer to an existing string.
The conditional assembly construct %ifid,
taking one parameter (which may be blank), assembles the subsequent code if
and only if the first token in the parameter exists and is an identifier.
%ifnum works similarly, but tests for the token
being a numeric constant; %ifstr tests for it
being a string.
For example, the writefile macro defined in
section 4.3.3 can be extended to take
advantage of %ifstr in the following fashion:
In the first, strpointer is used as the
address of an already-declared string, and length
is used as its length; in the second, a string is given to the macro, which
therefore declares it itself and works out the address and length for
itself.
Note the use of %if inside the
%ifstr: this is to detect whether the macro was
passed two arguments (so the string would be a single string constant, and
db %2 would be adequate) or more (in which case,
all but the first two would be lumped together into
%3, and db %2,%3 would
be required).
The usual %elifXXX,
%ifnXXX and %elifnXXX
versions exist for each of %ifid,
%ifnum and %ifstr.
The preprocessor directive %error will cause
NASM to report an error if it occurs in assembled code. So if other users
are going to try to assemble your source files, you can ensure that they
define the right macros by means of code like this:
%ifdef SOME_MACRO
; do some setup
%elifdef SOME_OTHER_MACRO
; do some different setup
%else
%error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.
%endif
Then any user who fails to understand the way your code is supposed to
be assembled will be quickly warned of their mistake, rather than having to
wait until the program crashes on being run and then not knowing what went
wrong.
NASM's TIMES prefix, though useful, cannot be
used to invoke a multi-line macro multiple times, because it is processed
by NASM after macros have already been expanded. Therefore NASM provides
another form of loop, this time at the preprocessor level:
%rep.
The directives %rep and
%endrep (%rep takes a
numeric argument, which can be an expression;
%endrep takes no arguments) can be used to
enclose a chunk of code, which is then replicated as many times as
specified by the preprocessor:
%assign i 0
%rep 64
inc word [table+2*i]
%assign i i+1
%endrep
This will generate a sequence of 64 INC
instructions, incrementing every word of memory from
[table] to [table+126].
For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the %exitrep
directive to terminate the loop, like this:
fibonacci:
%assign i 0
%assign j 1
%rep 100
%if j > 65535
%exitrep
%endif
dw j
%assign k j+i
%assign i j
%assign j k
%endrep
fib_number equ ($-fibonacci)/2
This produces a list of all the Fibonacci numbers that will fit in 16
bits. Note that a maximum repeat count must still be given to
%rep. This is to prevent the possibility of NASM
getting into an infinite loop in the preprocessor, which (on multitasking
or multi-user systems) would typically cause all the system memory to be
gradually used up and other applications to start crashing.
Using, once again, a very similar syntax to the C preprocessor, NASM's
preprocessor lets you include other source files into your code. This is
done by the use of the %include directive:
%include "macros.mac"
will include the contents of the file
macros.mac into the source file containing the
%include directive.
Include files are searched for in the current directory (the directory
you're in when you run NASM, as opposed to the location of the NASM
executable or the location of the source file), plus any directories
specified on the NASM command line using the -i
option.
The standard C idiom for preventing a file being included more than once
is just as applicable in NASM: if the file
macros.mac has the form
%ifndef MACROS_MAC
%define MACROS_MAC
; now define some macros
%endif
then including the file more than once will not cause errors, because
the second time the file is included nothing will happen because the macro
MACROS_MAC will already be defined.
You can force a file to be included even if there is no
%include directive that explicitly includes it,
by using the -p option on the NASM command line
(see section 2.1.11).
Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a
REPEAT ... UNTIL loop,
in which the expansion of the REPEAT macro would
need to be able to refer to a label which the
UNTIL macro had defined. However, for such a
macro you would also want to be able to nest these loops.
NASM provides this level of power by means of a context stack.
The preprocessor maintains a stack of contexts, each of which is
characterised by a name. You add a new context to the stack using the
%push directive, and remove one using
%pop. You can define labels that are local to a
particular context on the stack.
The %push directive is used to create a new
context and place it on the top of the context stack.
%push requires one argument, which is the name of
the context. For example:
%push foobar
This pushes a new context called foobar on the
stack. You can have several contexts on the stack with the same name: they
can still be distinguished.
The directive %pop, requiring no arguments,
removes the top context from the context stack and destroys it, along with
any labels associated with it.
Just as the usage %%foo defines a label which
is local to the particular macro call in which it is used, the usage
%$foo is used to define a label which is local to
the context on the top of the context stack. So the
REPEAT and UNTIL
example given above could be implemented by means of:
which would scan every fourth byte of a string in search of the byte in
AL.
If you need to define, or access, labels local to the context
below the top one on the stack, you can use
%$$foo, or %$$$foo for
the context below that, and so on.
NASM also allows you to define single-line macros which are local to a
particular context, in just the same way:
%define %$localmac 3
will define the single-line macro %$localmac
to be local to the top context on the stack. Of course, after a subsequent
%push, it can then still be accessed by the name
%$$localmac.
If you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to
%ifctx), you can execute a
%pop followed by a
%push; but this will have the side effect of
destroying all context-local labels and macros associated with the context
that was just popped.
NASM provides the directive %repl, which
replaces a context with a different name, without touching the
associated macros and labels. So you could replace the destructive code
This example makes use of almost all the context-stack features,
including the conditional-assembly construct
%ifctx, to implement a block IF statement as a
set of macros.
%macro if 1
%push if
j%-1 %$ifnot
%endmacro
%macro else 0
%ifctx if
%repl else
jmp %$ifend
%$ifnot:
%else
%error "expected `if' before `else'"
%endif
%endmacro
%macro endif 0
%ifctx if
%$ifnot:
%pop
%elifctx else
%$ifend:
%pop
%else
%error "expected `if' or `else' before `endif'"
%endif
%endmacro
This code is more robust than the REPEAT and
UNTIL macros given in
section 4.7.2, because it uses conditional
assembly to check that the macros are issued in the right order (for
example, not calling endif before
if) and issues a %error
if they're not.
In addition, the endif macro has to be able to
cope with the two distinct cases of either directly following an
if, or following an
else. It achieves this, again, by using
conditional assembly to do different things depending on whether the
context on top of the stack is if or
else.
The else macro has to preserve the context on
the stack, in order to have the %$ifnot referred
to by the if macro be the same as the one defined
by the endif macro, but has to change the
context's name so that endif will know there was
an intervening else. It does this by the use of
%repl.
A sample usage of these macros might look like:
cmp ax,bx
if ae
cmp bx,cx
if ae
mov ax,cx
else
mov ax,bx
endif
else
cmp ax,cx
if ae
mov ax,cx
endif
endif
The block-IF macros handle nesting quite
happily, by means of pushing another context, describing the inner
if, on top of the one describing the outer
if; thus else and
endif always refer to the last unmatched
if or else.
NASM defines a set of standard macros, which are already defined when it
starts to process any source file. If you really need a program to be
assembled with no pre-defined macros, you can use the
%clear directive to empty the preprocessor of
everything.
Most user-level assembler directives (see
chapter 5) are implemented as macros which
invoke primitive directives; these are described in
chapter 5. The rest of the standard macro set
is described here.
The single-line macros __NASM_MAJOR__,
__NASM_MINOR__,
__NASM_SUBMINOR__ and
___NASM_PATCHLEVEL__ expand to the major, minor,
subminor and patch level parts of the version number of NASM being used.
So, under NASM 0.98.32p1 for example,
__NASM_MAJOR__ would be defined to be 0,
__NASM_MINOR__ would be defined as 98,
__NASM_SUBMINOR__ would be defined to 32, and
___NASM_PATCHLEVEL__ would be defined as 1.
The single-line macro __NASM_VERSION_ID__
expands to a dword integer representing the full version number of the
version of nasm being used. The value is the equivalent to
__NASM_MAJOR__,
__NASM_MINOR__,
__NASM_SUBMINOR__ and
___NASM_PATCHLEVEL__ concatenated to produce a
single doubleword. Hence, for 0.98.32p1, the returned number would be
equivalent to:
dd 0x00622001
or
db 1,32,98,0
Note that the above lines are generate exactly the same code, the second
line is used just to give an indication of the order that the separate
values will be present in memory.
Like the C preprocessor, NASM allows the user to find out the file name
and line number containing the current instruction. The macro
__FILE__ expands to a string constant giving the
name of the current input file (which may change through the course of
assembly if %include directives are used), and
__LINE__ expands to a numeric constant giving the
current line number in the input file.
These macros could be used, for example, to communicate debugging
information to a macro, since invoking __LINE__
inside a macro definition (either single-line or multi-line) will return
the line number of the macro call, rather than
definition. So to determine where in a piece of code a crash is
occurring, for example, one could write a routine
stillhere, which is passed a line number in
EAX and outputs something like `line 155: still
here'. You could then write a macro
The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that data
structures can be implemented as a set of macros. The macros
STRUC and ENDSTRUC are
used to define a structure data type.
STRUC takes one parameter, which is the name
of the data type. This name is defined as a symbol with the value zero, and
also has the suffix _size appended to it and is
then defined as an EQU giving the size of the
structure. Once STRUC has been issued, you are
defining the structure, and should define fields using the
RESB family of pseudo-instructions, and then
invoke ENDSTRUC to finish the definition.
For example, to define a structure called
mytype containing a longword, a word, a byte and
a string of bytes, you might code
The above code defines six symbols: mt_long as
0 (the offset from the beginning of a mytype
structure to the longword field), mt_word as 4,
mt_byte as 6, mt_str as
7, mytype_size as 39, and
mytype itself as zero.
The reason why the structure type name is defined at zero is a side
effect of allowing structures to work with the local label mechanism: if
your structure members tend to have the same names in more than one
structure, you can define the above structure like this:
This defines the offsets to the structure fields as
mytype.long,
mytype.word,
mytype.byte and
mytype.str.
NASM, since it has no intrinsic structure support, does not
support any form of period notation to refer to the elements of a structure
once you have one (except the above local-label notation), so code such as
mov ax,[mystruc.mt_word] is not valid.
mt_word is a constant just like any other
constant, so the correct syntax is
mov ax,[mystruc+mt_word] or
mov ax,[mystruc+mytype.word].
Having defined a structure type, the next thing you typically want to do
is to declare instances of that structure in your data segment. NASM
provides an easy way to do this in the ISTRUC
mechanism. To declare a structure of type mytype
in a program, you code something like this:
mystruc:
istruc mytype
at mt_long, dd 123456
at mt_word, dw 1024
at mt_byte, db 'x'
at mt_str, db 'hello, world', 13, 10, 0
iend
The function of the AT macro is to make use of
the TIMES prefix to advance the assembly position
to the correct point for the specified structure field, and then to declare
the specified data. Therefore the structure fields must be declared in the
same order as they were specified in the structure definition.
If the data to go in a structure field requires more than one source
line to specify, the remaining source lines can easily come after the
AT line. For example:
at mt_str, db 123,134,145,156,167,178,189
db 190,100,0
Depending on personal taste, you can also omit the code part of the
AT line completely, and start the structure field
on the next line:
The ALIGN and ALIGNB
macros provides a convenient way to align code or data on a word, longword,
paragraph or other boundary. (Some assemblers call this directive
EVEN.) The syntax of the
ALIGN and ALIGNB macros
is
align 4 ; align on 4-byte boundary
align 16 ; align on 16-byte boundary
align 8,db 0 ; pad with 0s rather than NOPs
align 4,resb 1 ; align to 4 in the BSS
alignb 4 ; equivalent to previous line
Both macros require their first argument to be a power of two; they both
compute the number of additional bytes required to bring the length of the
current section up to a multiple of that power of two, and then apply the
TIMES prefix to their second argument to perform
the alignment.
If the second argument is not specified, the default for
ALIGN is NOP, and the
default for ALIGNB is
RESB 1. So if the second argument is specified,
the two macros are equivalent. Normally, you can just use
ALIGN in code and data sections and
ALIGNB in BSS sections, and never need the second
argument except for special purposes.
ALIGN and ALIGNB,
being simple macros, perform no error checking: they cannot warn you if
their first argument fails to be a power of two, or if their second
argument generates more than one byte of code. In each of these cases they
will silently do the wrong thing.
ALIGNB (or ALIGN
with a second argument of RESB 1) can be used
within structure definitions:
This will ensure that the structure members are sensibly aligned
relative to the base of the structure.
A final caveat: ALIGN and
ALIGNB work relative to the beginning of the
section, not the beginning of the address space in the final
executable. Aligning to a 16-byte boundary when the section you're in is
only guaranteed to be aligned to a 4-byte boundary, for example, is a waste
of effort. Again, NASM does not check that the section's alignment
characteristics are sensible for the use of ALIGN
or ALIGNB.
The following preprocessor directives may only be used when TASM
compatibility is turned on using the -t command
line switch (This switch is described in
section 2.1.17.)
The %arg directive is used to simplify the
handling of parameters passed on the stack. Stack based parameter passing
is used by many high level languages, including C, C++ and Pascal.
While NASM comes with macros which attempt to duplicate this
functionality (see section
7.4.5), the syntax is not particularly convenient to use and is not
TASM compatible. Here is an example which shows the use of
%arg without any external macros:
some_function:
%push mycontext ; save the current context
%stacksize large ; tell NASM to use bp
%arg i:word, j_ptr:word
mov ax,[i]
mov bx,[j_ptr]
add ax,[bx]
ret
%pop ; restore original context
This is similar to the procedure defined in
section 7.4.5 and adds the value
in i to the value pointed to by j_ptr and returns the sum in the ax
register. See section 4.7.1 for an explanation
of push and pop and the
use of context stacks.
The %stacksize directive is used in
conjunction with the %arg (see
section 4.9.1) and the
%local (see section
4.9.3) directives. It tells NASM the default size to use for subsequent
%arg and %local
directives. The %stacksize directive takes one
required argument which is one of flat,
large or small.
%stacksize flat
This form causes NASM to use stack-based parameter addressing relative
to ebp and it assumes that a near form of call
was used to get to this label (i.e. that eip is
on the stack).
%stacksize large
This form uses bp to do stack-based parameter
addressing and assumes that a far form of call was used to get to this
address (i.e. that ip and
cs are on the stack).
%stacksize small
This form also uses bp to address stack
parameters, but it is different from large
because it also assumes that the old value of bp is pushed onto the stack
(i.e. it expects an ENTER instruction). In other
words, it expects that bp,
ip and cs are on the
top of the stack, underneath any local space which may have been allocated
by ENTER. This form is probably most useful when
used in combination with the %local directive
(see section 4.9.3).
The %local directive is used to simplify the
use of local temporary stack variables allocated in a stack frame.
Automatic local variables in C are an example of this kind of variable. The
%local directive is most useful when used with
the %stacksize (see
section 4.9.2 and is also compatible with the
%arg directive (see
section 4.9.1). It allows simplified reference
to variables on the stack which have been allocated typically by using the
ENTER instruction (see
section B.4.65 for a description
of that instruction). An example of its use is the following:
silly_swap:
%push mycontext ; save the current context
%stacksize small ; tell NASM to use bp
%assign %$localsize 0 ; see text for explanation
%local old_ax:word, old_dx:word
enter %$localsize,0 ; see text for explanation
mov [old_ax],ax ; swap ax & bx
mov [old_dx],dx ; and swap dx & cx
mov ax,bx
mov dx,cx
mov bx,[old_ax]
mov cx,[old_dx]
leave ; restore old bp
ret ;
%pop ; restore original context
The %$localsize variable is used internally by
the %local directive and must be defined
within the current context before the %local
directive may be used. Failure to do so will result in one expression
syntax error for each %local variable declared.
It then may be used in the construction of an appropriately sized ENTER
instruction as shown in the example.
The %line directive is used to notify NASM
that the input line corresponds to a specific line number in another file.
Typically this other file would be an original source file, with the
current NASM input being the output of a pre-processor. The
%line directive allows NASM to output messages
which indicate the line number of the original source file, instead of the
file that is being read by NASM.
This preprocessor directive is not generally of use to programmers, by
may be of interest to preprocessor authors. The usage of the
%line preprocessor directive is as follows:
%line nnn[+mmm] [filename]
In this directive, nnn indentifies the line of
the original source file which this line corresponds to.
mmm is an optional parameter which specifies a
line increment value; each line of the input file read in is considered to
correspond to mmm lines of the original source
file. Finally, filename is an optional parameter
which specifies the file name of the original source file.
After reading a %line preprocessor directive,
NASM will report all file name and line numbers relative to the values
specified therein.
The %!<env> directive makes it possible
to read the value of an environment variable at assembly time. This could,
for example, be used to store the contents of an environment variable into
a string, which could be used at some other point in your code.
For example, suppose that you have an environment variable
FOO, and you want the contents of
FOO to be embedded in your program. You could do
that as follows:
At the time of writing, this will generate an "unterminated string"
warning at the time of defining "quote", and it will add a space before and
after the string that is read in. I was unable to find a simple workaround
(although a workaround can be created using a multi-line macro), so I
believe that you will need to either learn how to create more complex
macros, or allow for the extra spaces if you make use of this feature in
that way.