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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.
%define
Single-line macros are defined using the
preprocessor directive. The definitions
work in a similar way to C; so you can do things like
%define ctrl 0x1F & %define param(a,b) ((a)+(a)*(b)) mov byte [param(2,ebx)], ctrl 'D'
which will expand to
mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
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
, even though the macro
wasn't defined at the time of definition of
.
Macros defined with
are case
sensitive: after
, only
will expand to
:
or
will not. By
using
instead of
(the `i' stands for `insensitive') you
can define all the case variants of a macro at once, so that
would cause
,
,
,
and so on all
to expand to
.
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
will expand once, becoming
, 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
will
become
whereas
will become
. However, if you define
%define foo bar
then no other definition of
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
is invoked, it
will be expanded according to the most recent definition. This is
particularly useful when defining single-line macros with
(see section
4.1.3).
You can pre-define single-line macros using the `-d' option on the NASM command line: see section 2.1.8.
%undef
Single-line macros can be removed with the
command. For example, the following
sequence:
%define foo bar %undef foo mov eax, foo
will expand to the instruction
,
since after
the macro
is no longer defined.
Macros that would otherwise be pre-defined can be undefined on the command-line using the `-u' option on the NASM command line: see section 2.1.9.
%assign
An alternative way to define single-line macros is by means of the
command (and its case
sensitivecase-insensitive counterpart
,
which differs from
in exactly the same
way that
differs from
).
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
directive is processed.
Like
, macros defined using
can be re-defined later, so you can do
things like
%assign i i+1
to increment the numeric value of a macro.
is useful for controlling the
termination of
preprocessor loops: see
section 4.4 for an example of this. Another use
for
is given in
section 7.4 and
section 8.1.
The expression passed to
is a critical
expression (see section 3.7), 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).
%macro
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
after the macro name in the
line defines the number of parameters the
macro
expects to receive. The use of
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
,
and so on.
Multi-line macros, like single-line macros, are case-sensitive, unless
you define them using the alternative directive
.
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
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
command-line option (see
section 2.1.12).
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
if the
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
. The names NASM invents are of the
form
, 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 3.8. 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
above will
work as expected: the text before the first comma,
, is used as the first macro
parameter and expanded when
is referred to,
and all the subsequent text is lumped into
and
placed after the
.
The greedy nature of the macro is indicated to NASM by the use of the
sign after the parameter count on the
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
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
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
macro defined in section 4.2.3) 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
would always be taken from the macro call.
, if not specified by the macro call, would
default to
, and
if not specified would default to
.
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
token (see section
4.2.5) 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
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 4.2.6.
%0
: Macro Parameter CounterFor a macro which can take a variable number of parameters, the
parameter reference
will return a numeric
constant giving the number of parameters passed to the macro. This can be
used as an argument to
(see
section 4.4) in order to iterate through all the
parameters of a macro. Examples are given in
section 4.2.6.
%rotate
: Rotating Macro ParametersUnix shell programmers will be familiar with the
shell command, which allows the arguments
passed to a shell script (referenced as
,
and so on) to be moved left by one place, so
that the argument previously referenced as
becomes available as
, and the argument
previously referenced as
is no longer
available at all.
NASM provides a similar mechanism, in the form of
. As its name suggests, it differs from
the Unix
in that no parameters are lost:
parameters rotated off the left end of the argument list reappear on the
right, and vice versa.
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
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:
%macro multipush 1-* %rep %0 push %1 %rotate 1 %endrep %endmacro
This macro invokes the
instruction on
each of its arguments in turn, from left to right. It begins by pushing its
first argument,
, then invokes
to move all the arguments one place to
the left, so that the original second argument is now available as
. Repeating this procedure as many times as
there were arguments (achieved by supplying
as
the argument to
) 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
macro.
It would be convenient, when using this macro, to have a
equivalent, which didn't require the
arguments to be given in reverse order. Ideally, you would write the
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
, and the macro would take care of
popping the registers in the opposite order from the one in which they were
pushed.
This can be done by the following definition:
%macro multipop 1-* %rep %0 %rotate -1 pop %1 %endrep %endmacro
This macro begins by rotating its arguments one place to the
right, so that the original last argument appears as
. This is then popped, and the arguments are
rotated right again, so the second-to-last argument becomes
. 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
%macro keytab_entry 2 keypos%1 equ $-keytab db %2 %endmacro keytab: keytab_entry F1,128+1 keytab_entry F2,128+2 keytab_entry Return,13
which would expand to
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
.
If you need to append a digit to a macro parameter, for example
defining labels
and
when passed the parameter
, you can't code
because that would be taken as the eleventh macro parameter. Instead, you
must code
, which will separate the first
(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
4.2.2) and context-local labels (section
4.6.2). In all cases, ambiguities in syntax can be resolved by
enclosing everything after the
sign and before
the literal text in braces: so
concatenates the text
to the end of the real
name of the macro-local label
. (This is
unnecessary, since the form NASM uses for the real names of macro-local
labels means that the two usages
and
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
by means of the alternative syntax
, 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
, which NASM will expand as the
inverse condition code. So the
macro defined in section 4.2.2 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
, which will cause the conditional-jump
instruction in the macro expansion to come out as
, or
which
will make the jump a
.
The
macro-parameter reference is quite
happy to interpret the arguments
and
as valid condition codes; however,
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
qualifier,
which you can include in a macro definition to inhibit the expansion of the
macro in the listing file. The
qualifier
comes directly after the number of parameters, like this:
%macro foo 1.nolist
Or like this:
%macro bar 1-5+.nolist a,b,c,d,e,f,g,h
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
clause is optional, as is the
clause. You can have more than one
clause as well.
%ifdef
: Testing Single-Line Macro ExistenceBeginning a conditional-assembly block with the line
will assemble the subsequent code
if, and only if, a single-line macro called
is defined. If not, then the
and
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
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
instead of
. You can also test for macro definitions
in
blocks by using
and
.
%ifctx
: Testing the Context StackThe conditional-assembly construct
will cause the subsequent code to
be assembled if and only if the top context on the preprocessor's context
stack has the name
. As with
, the inverse and
forms
,
and
are also supported.
For more details of the context stack, see
section 4.6. For a sample use of
, see section
4.6.5.
%if
: Testing Arbitrary Numeric ExpressionsThe conditional-assembly construct
will cause the subsequent code to be assembled if and only if the value of
the numeric expression
is non-zero. An
example of the use of this feature is in deciding when to break out of a
preprocessor loop: see
section 4.4 for a detailed example.
The expression given to
, and its
counterpart
, is a critical expression (see
section 3.7).
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.
%ifidn
and %ifidni
: Testing Exact Text IdentityThe construct
will cause
the subsequent code to be assembled if and only if
and
, after
expanding single-line macros, are identical pieces of text. Differences in
white space are not counted.
is similar to
, but is case-insensitive.
For example, the following macro pushes a register or number on the
stack, and allows you to treat
as a real
register:
%macro pushparam 1 %ifidni %1,ip call %%label %%label: %else push %1 %endif %endmacro
Like most other
constructs,
has a counterpart
, and negative forms
and
.
Similarly,
has counterparts
,
and
.
%ifid
, %ifnum
, %ifstr
: Testing Token TypesSome 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
,
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.
works similarly, but tests for the token
being a numeric constant;
tests for it
being a string.
For example, the
macro defined in
section 4.2.3 can be extended to take
advantage of
in the following fashion:
%macro writefile 2-3+ %ifstr %2 jmp %%endstr %if %0 = 3 %%str: db %2,%3 %else %%str: db %2 %endif %%endstr: mov dx,%%str mov cx,%%endstr-%%str %else mov dx,%2 mov cx,%3 %endif mov bx,%1 mov ah,0x40 int 0x21 %endmacro
Then the
macro can cope with being
called in either of the following two ways:
writefile [file], strpointer, length writefile [file], "hello", 13, 10
In the first,
is used as the
address of an already-declared string, and
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
inside the
: this is to detect whether the macro was
passed two arguments (so the string would be a single string constant, and
would be adequate) or more (in which case,
all but the first two would be lumped together into
, and
would
be required).
The usual
,
and
versions exist for each of
,
and
.
%error
: Reporting User-Defined ErrorsThe preprocessor directive
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.
%rep
NASM's
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:
.
The directives
and
(
takes a
numeric argument, which can be an expression;
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
instructions, incrementing every word of memory from
to
.
For more complex termination conditions, or to break out of a repeat
loop part way along, you can use the
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
. 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
directive:
%include "macros.mac"
will include the contents of the file
into the source file containing the
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
option.
The standard C idiom for preventing a file being included more than once
is just as applicable in NASM: if the file
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
will already be defined.
You can force a file to be included even if there is no
directive that explicitly includes it,
by using the
option on the NASM command line
(see section 2.1.7).
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
...
loop,
in which the expansion of the
macro would
need to be able to refer to a label which the
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
directive, and remove one using
. You can define labels that are local to a
particular context on the stack.
%push
and %pop
: Creating and Removing ContextsThe
directive is used to create a new
context and place it on the top of the context stack.
requires one argument, which is the name of
the context. For example:
%push foobar
This pushes a new context called
on the
stack. You can have several contexts on the stack with the same name: they
can still be distinguished.
The directive
, 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
defines a label which
is local to the particular macro call in which it is used, the usage
is used to define a label which is local to
the context on the top of the context stack. So the
and
example given above could be implemented by means of:
%macro repeat 0 %push repeat %$begin: %endmacro
%macro until 1 j%-1 %$begin %pop %endmacro
and invoked by means of, for example,
mov cx,string repeat add cx,3 scasb until e
which would scan every fourth byte of a string in search of the byte in
.
If you need to define, or access, labels local to the context
below the top one on the stack, you can use
, or
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 %$localmac 3
will define the single-line macro
to be local to the top context on the stack. Of course, after a subsequent
, it can then still be accessed by the name
.
%repl
: Renaming a ContextIf you need to change the name of the top context on the stack (in
order, for example, to have it respond differently to
), you can execute a
followed by a
; but this will have the side effect of
destroying all context-local labels and macros associated with the context
that was just popped.
NASM provides the directive
, which
replaces a context with a different name, without touching the
associated macros and labels. So you could replace the destructive code
%pop %push newname
with the non-destructive version
.
This example makes use of almost all the context-stack features,
including the conditional-assembly construct
, to implement a block IF statement as a
set of macros.
%macro if 1 %push if j%-1 %$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
and
macros given in
section 4.6.2, because it uses conditional
assembly to check that the macros are issued in the right order (for
example, not calling
before
) and issues a
if they're not.
In addition, the
macro has to be able to
cope with the two distinct cases of either directly following an
, or following an
. It achieves this, again, by using
conditional assembly to do different things depending on whether the
context on top of the stack is
or
.
The
macro has to preserve the context on
the stack, in order to have the
referred
to by the
macro be the same as the one defined
by the
macro, but has to change the
context's name so that
will know there was
an intervening
. It does this by the use of
.
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-
macros handle nesting quite
happily, by means of pushing another context, describing the inner
, on top of the one describing the outer
; thus
and
always refer to the last unmatched
or
.
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
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.
__NASM_MAJOR__
and __NASM_MINOR__
: NASM VersionThe single-line macros
and
expand to the major and minor
parts of the version number of NASM being used. So, under NASM 0.96 for
example,
would be defined to be 0
and
would be defined as 96.
__FILE__
and __LINE__
: File Name and Line NumberLike the C preprocessor, NASM allows the user to find out the file name
and line number containing the current instruction. The macro
expands to a string constant giving the
name of the current input file (which may change through the course of
assembly if
directives are used), and
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
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
, which is passed a line number in
and outputs something like `line 155: still
here'. You could then write a macro
%macro notdeadyet 0 push eax mov eax,__LINE__ call stillhere pop eax %endmacro
and then pepper your code with calls to
until you find the crash point.
STRUC
and ENDSTRUC
: Declaring Structure Data TypesThe 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
and
are
used to define a structure data type.
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
appended to it and is
then defined as an
giving the size of the
structure. Once
has been issued, you are
defining the structure, and should define fields using the
family of pseudo-instructions, and then
invoke
to finish the definition.
For example, to define a structure called
containing a longword, a word, a byte and
a string of bytes, you might code
struc mytype mt_long: resd 1 mt_word: resw 1 mt_byte: resb 1 mt_str: resb 32 endstruc
The above code defines six symbols:
as
0 (the offset from the beginning of a
structure to the longword field),
as 4,
as 6,
as
7,
as 39, and
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:
struc mytype .long: resd 1 .word: resw 1 .byte: resb 1 .str: resb 32 endstruc
This defines the offsets to the structure fields as
,
,
and
.
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
is not valid.
is a constant just like any other
constant, so the correct syntax is
or
.
ISTRUC
, AT
and IEND
: Declaring Instances of StructuresHaving 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
mechanism. To declare a structure of type
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
macro is to make use of
the
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
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
line completely, and start the structure field
on the next line:
at mt_str db 'hello, world' db 13,10,0
ALIGN
and ALIGNB
: Data AlignmentThe
and
macros provides a convenient way to align code or data on a word, longword,
paragraph or other boundary. (Some assemblers call this directive
.) The syntax of the
and
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
prefix to their second argument to perform
the alignment.
If the second argument is not specified, the default for
is
, and the
default for
is
. So if the second argument is specified,
the two macros are equivalent. Normally, you can just use
in code and data sections and
in BSS sections, and never need the second
argument except for special purposes.
and
,
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.
(or
with a second argument of
) can be used
within structure definitions:
struc mytype2 mt_byte: resb 1 alignb 2 mt_word: resw 1 alignb 4 mt_long: resd 1 mt_str: resb 32 endstruc
This will ensure that the structure members are sensibly aligned relative to the base of the structure.
A final caveat:
and
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
or
.