This is nasm.info, produced by Makeinfo version 3.12f from nasmdoc.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * NASM: (nasm). The Netwide Assembler for x86. END-INFO-DIR-ENTRY This file documents NASM, the Netwide Assembler: an assembler targetting the Intel x86 series of processors, with portable source. Copyright 1997 Simon Tatham All rights reserved. This document is redistributable under the licence given in the file "Licence" distributed in the NASM archive.  File: nasm.info, Node: Section 3.5, Next: Section 3.5.1, Prev: Section 3.4.4, Up: Chapter 3 3.5. Expressions **************** 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 `$' and `$$' tokens. `$' evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using `JMP $'. `$$' evaluates to 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. * Menu: * Section 3.5.1:: `|': Bitwise OR Operator * Section 3.5.2:: `^': Bitwise XOR Operator * Section 3.5.3:: `&': Bitwise AND Operator * Section 3.5.4:: `<<' and `>>': Bit Shift Operators * Section 3.5.5:: `+' and `-': Addition and Subtraction Operators * Section 3.5.6:: `*', `/', `//', `%' and `%%': Multiplication and Division * Section 3.5.7:: Unary Operators: `+', `-', `~' and `SEG'  File: nasm.info, Node: Section 3.5.1, Next: Section 3.5.2, Prev: Section 3.5, Up: Section 3.5 3.5.1. `|': Bitwise OR Operator ******************************* The `|' operator gives a bitwise OR, exactly as performed by the `OR' machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.  File: nasm.info, Node: Section 3.5.2, Next: Section 3.5.3, Prev: Section 3.5.1, Up: Section 3.5 3.5.2. `^': Bitwise XOR Operator ******************************** `^' provides the bitwise XOR operation.  File: nasm.info, Node: Section 3.5.3, Next: Section 3.5.4, Prev: Section 3.5.2, Up: Section 3.5 3.5.3. `&': Bitwise AND Operator ******************************** `&' provides the bitwise AND operation.  File: nasm.info, Node: Section 3.5.4, Next: Section 3.5.5, Prev: Section 3.5.3, Up: Section 3.5 3.5.4. `<<' and `>>': Bit Shift Operators ***************************************** `<<' gives a bit-shift to the left, just as it does in C. So `5<<3' 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.  File: nasm.info, Node: Section 3.5.5, Next: Section 3.5.6, Prev: Section 3.5.4, Up: Section 3.5 3.5.5. `+' and `-': Addition and Subtraction Operators ****************************************************** The `+' and `-' operators do perfectly ordinary addition and subtraction.  File: nasm.info, Node: Section 3.5.6, Next: Section 3.5.7, Prev: Section 3.5.5, Up: Section 3.5 3.5.6. `*', `/', `//', `%' and `%%': Multiplication and Division **************************************************************** `*' is the multiplication operator. `/' and `//' are both division operators: `/' is unsigned division and `//' is signed division. Similarly, `%' and `%%' provide unsigned and signed modulo operators respectively. NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator. Since the `%' 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.  File: nasm.info, Node: Section 3.5.7, Next: Section 3.6, Prev: Section 3.5.6, Up: Section 3.5 3.5.7. Unary Operators: `+', `-', `~' and `SEG' *********************************************** The highest-priority operators in NASM's expression grammar are those which only apply to one argument. `-' negates its operand, `+' does nothing (it's provided for symmetry with `-'), `~' computes the one's complement of its operand, and `SEG' provides the segment address of its operand (explained in more detail in *Note Section 3.6::).  File: nasm.info, Node: Section 3.6, Next: Section 3.7, Prev: Section 3.5.7, Up: Chapter 3 3.6. `SEG' and `WRT' ******************** 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 `SEG' operator to perform this function. The `SEG' 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 will load `ES:BX' with a valid pointer to the symbol `symbol'. 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 `WRT' (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 to load `ES:BX' with a different, but functionally equivalent, pointer to the symbol `symbol'. NASM supports far (inter-segment) calls and jumps by means of the syntax `call segment:offset', where `segment' and `offset' both represent immediate values. So to call a far procedure, you could code either of 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 `call far procedure' as a synonym for the first of the above usages. `JMP' works identically to `CALL' 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.  File: nasm.info, Node: Section 3.7, Next: Section 3.8, Prev: Section 3.6, Up: Chapter 3 3.7. Critical Expressions ************************* 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 `TIMES' in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the `TIMES' line when it first sees it. It will just as firmly reject the slightly paradoxical code times (label-$+1) db 0 label: db 'NOW where am I?' in which _any_ value for the `TIMES' 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 `TIMES' prefix is a critical expression; for the same reason, the arguments to the `RESB' family of 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 `symbol1', because `symbol1' is defined to be equal to `symbol2' which NASM hasn't seen yet. On the second pass, therefore, when it encounters the line `mov ax,symbol1', it is unable to generate the code for it because it still doesn't know the value of `symbol1'. On the next line, it would see the `EQU' again and be able to determine the value of `symbol1', but by then it would be too late. NASM avoids this problem by defining the right-hand side of an `EQU' statement to be a critical expression, so the definition of `symbol1' would be rejected in the first pass. 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 `mov eax,[ebx+offset]' without knowing the value of `offset'. It has no way of knowing that `offset' 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, `offset' 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 `offset' before using it, or by forcing byte size in the effective address by coding `[byte ebx+offset]'.  File: nasm.info, Node: Section 3.8, Next: Chapter 4, Prev: Section 3.7, Up: Chapter 3 3.8. Local Labels ***************** 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 `JNE' instruction jumps to the line immediately before it, because the two definitions of `.loop' 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 `.loop' above is really defining a symbol called `label1.loop', and the second defines a symbol called `label2.loop'. So, 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, `..start' is used to specify the entry point in the `obj' output format (see *Note Section 6.2.6::).  File: nasm.info, Node: Chapter 4, Next: Section 4.1, Prev: Section 3.8, Up: Top Chapter 4: The NASM Preprocessor ******************************** 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. * Menu: * Section 4.1:: Single-Line Macros * Section 4.2:: Multi-Line Macros: `%macro' * Section 4.3:: Conditional Assembly * Section 4.4:: Preprocessor Loops: `%rep' * Section 4.5:: Including Other Files * Section 4.6:: The Context Stack * Section 4.7:: Standard Macros  File: nasm.info, Node: Section 4.1, Next: Section 4.1.1, Prev: Chapter 4, Up: Chapter 4 4.1. Single-Line Macros *********************** * Menu: * Section 4.1.1:: The Normal Way: `%define' * Section 4.1.2:: Undefining macros: `%undef' * Section 4.1.3:: Preprocessor Variables: `%assign'  File: nasm.info, Node: Section 4.1.1, Next: Section 4.1.2, Prev: Section 4.1, Up: Section 4.1 4.1.1. The Normal Way: `%define' ******************************** Single-line macros are defined using the `%define' 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 `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 *Note 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 *Note Section 4.1.3::). You can pre-define single-line macros using the `-d' option on the NASM command line: see *Note Section 2.1.8::.  File: nasm.info, Node: Section 4.1.2, Next: Section 4.1.3, Prev: Section 4.1.1, Up: Section 4.1 4.1.2. Undefining macros: `%undef' ********************************** Single-line macros can be removed with the `%undef' command. For example, the following sequence: %define foo bar %undef foo mov eax, foo will expand to the instruction `mov eax, foo', since after `%undef' the macro `foo' 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 *Note Section 2.1.9::.  File: nasm.info, Node: Section 4.1.3, Next: Section 4.2, Prev: Section 4.1.2, Up: Section 4.1 4.1.3. Preprocessor Variables: `%assign' **************************************** An alternative way to define single-line macros is by means of the `%assign' command (and its case sensitivecase-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 *Note Section 4.4:: for an example of this. Another use for `%assign' is given in *Note Section 7.4:: and *Note Section 8.1::. The expression passed to `%assign' is a critical expression (see *Note 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).  File: nasm.info, Node: Section 4.2, Next: Section 4.2.1, Prev: Section 4.1.3, Up: Chapter 4 4.2. Multi-Line Macros: `%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 `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 * Menu: * Section 4.2.1:: Overloading Multi-Line Macros * Section 4.2.2:: Macro-Local Labels * Section 4.2.3:: Greedy Macro Parameters * Section 4.2.4:: Default Macro Parameters * Section 4.2.5:: `%0': Macro Parameter Counter * Section 4.2.6:: `%rotate': Rotating Macro Parameters * Section 4.2.7:: Concatenating Macro Parameters * Section 4.2.8:: Condition Codes as Macro Parameters * Section 4.2.9:: Disabling Listing Expansion  File: nasm.info, Node: Section 4.2.1, Next: Section 4.2.2, Prev: Section 4.2, Up: Section 4.2 4.2.1. Overloading Multi-Line Macros ************************************ 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 *Note Section 2.1.12::).  File: nasm.info, Node: Section 4.2.2, Next: Section 4.2.3, Prev: Section 4.2.1, Up: Section 4.2 4.2.2. Macro-Local Labels ************************* 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 *Note 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.  File: nasm.info, Node: Section 4.2.3, Next: Section 4.2.4, Prev: Section 4.2.2, Up: Section 4.2 4.2.3. Greedy Macro Parameters ****************************** 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 *Note Section 5.2.1:: for a better way to write the above macro.  File: nasm.info, Node: Section 4.2.4, Next: Section 4.2.5, Prev: Section 4.2.3, Up: Section 4.2 4.2.4. Default Macro Parameters ******************************* 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 *Note 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 `%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 *Note 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 `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 *Note Section 4.2.6::.  File: nasm.info, Node: Section 4.2.5, Next: Section 4.2.6, Prev: Section 4.2.4, Up: Section 4.2 4.2.5. `%0': Macro Parameter Counter ************************************ 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 *Note Section 4.4::) in order to iterate through all the parameters of a macro. Examples are given in *Note Section 4.2.6::.  File: nasm.info, Node: Section 4.2.6, Next: Section 4.2.7, Prev: Section 4.2.5, Up: Section 4.2 4.2.6. `%rotate': Rotating Macro Parameters ******************************************* 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: %macro multipush 1-* %rep %0 push %1 %rotate 1 %endrep %endmacro 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 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 `%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.  File: nasm.info, Node: Section 4.2.7, Next: Section 4.2.8, Prev: Section 4.2.6, Up: Section 4.2 4.2.7. Concatenating Macro Parameters ************************************* 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 `%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 (*Note Section 4.2.2::) and context-local labels (*Note 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 `%{%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.)  File: nasm.info, Node: Section 4.2.8, Next: Section 4.2.9, Prev: Section 4.2.7, Up: Section 4.2 4.2.8. Condition Codes as Macro Parameters ****************************************** 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 *Note 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 `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.  File: nasm.info, Node: Section 4.2.9, Next: Section 4.3, Prev: Section 4.2.8, Up: Section 4.2 4.2.9. Disabling Listing Expansion ********************************** 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: %macro foo 1.nolist Or like this: %macro bar 1-5+.nolist a,b,c,d,e,f,g,h  File: nasm.info, Node: Section 4.3, Next: Section 4.3.1, Prev: Section 4.2.9, Up: Chapter 4 4.3. Conditional Assembly ************************* 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 ; some code which only appears if is met %elif ; only appears if is not met but is %else ; this appears if neither nor was met %endif The `%else' clause is optional, as is the `%elif' clause. You can have more than one `%elif' clause as well. * Menu: * Section 4.3.1:: `%ifdef': Testing Single-Line Macro Existence * Section 4.3.2:: `%ifctx': Testing the Context Stack * Section 4.3.3:: `%if': Testing Arbitrary Numeric Expressions * Section 4.3.4:: `%ifidn' and `%ifidni': Testing Exact Text Identity * Section 4.3.5:: `%ifid', `%ifnum', `%ifstr': Testing Token Types * Section 4.3.6:: `%error': Reporting User-Defined Errors  File: nasm.info, Node: Section 4.3.1, Next: Section 4.3.2, Prev: Section 4.3, Up: Section 4.3 4.3.1. `%ifdef': Testing Single-Line Macro Existence **************************************************** 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'.  File: nasm.info, Node: Section 4.3.2, Next: Section 4.3.3, Prev: Section 4.3.1, Up: Section 4.3 4.3.2. `%ifctx': Testing the Context Stack ****************************************** 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 *Note Section 4.6::. For a sample use of `%ifctx', see *Note Section 4.6.5::.  File: nasm.info, Node: Section 4.3.3, Next: Section 4.3.4, Prev: Section 4.3.2, Up: Section 4.3 4.3.3. `%if': Testing Arbitrary Numeric Expressions *************************************************** 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 *Note Section 4.4:: for a detailed example. The expression given to `%if', and its counterpart `%elif', is a critical expression (see *Note Section 3.7::). `%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.  File: nasm.info, Node: Section 4.3.4, Next: Section 4.3.5, Prev: Section 4.3.3, Up: Section 4.3 4.3.4. `%ifidn' and `%ifidni': Testing Exact Text Identity ********************************************************** 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: %macro pushparam 1 %ifidni %1,ip call %%label %%label: %else push %1 %endif %endmacro Like most other `%if' constructs, `%ifidn' has a counterpart `%elifidn', and negative forms `%ifnidn' and `%elifnidn'. Similarly, `%ifidni' has counterparts `%elifidni', `%ifnidni' and `%elifnidni'.  File: nasm.info, Node: Section 4.3.5, Next: Section 4.3.6, Prev: Section 4.3.4, Up: Section 4.3 4.3.5. `%ifid', `%ifnum', `%ifstr': Testing Token Types ******************************************************* 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 *Note Section 4.2.3:: can be extended to take advantage of `%ifstr' 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 `writefile' 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, `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'.  File: nasm.info, Node: Section 4.3.6, Next: Section 4.4, Prev: Section 4.3.5, Up: Section 4.3 4.3.6. `%error': Reporting User-Defined Errors ********************************************** 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.  File: nasm.info, Node: Section 4.4, Next: Section 4.5, Prev: Section 4.3.6, Up: Chapter 4 4.4. Preprocessor Loops: `%rep' ******************************* 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.  File: nasm.info, Node: Section 4.5, Next: Section 4.6, Prev: Section 4.4, Up: Chapter 4 4.5. Including Other Files ************************** 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 *Note Section 2.1.7::).  File: nasm.info, Node: Section 4.6, Next: Section 4.6.1, Prev: Section 4.5, Up: Chapter 4 4.6. The Context Stack ********************** 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. * Menu: * Section 4.6.1:: `%push' and `%pop': Creating and Removing Contexts * Section 4.6.2:: Context-Local Labels * Section 4.6.3:: Context-Local Single-Line Macros * Section 4.6.4:: `%repl': Renaming a Context * Section 4.6.5:: Example Use of the Context Stack: Block IFs  File: nasm.info, Node: Section 4.6.1, Next: Section 4.6.2, Prev: Section 4.6, Up: Section 4.6 4.6.1. `%push' and `%pop': Creating and Removing Contexts ********************************************************* 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.  File: nasm.info, Node: Section 4.6.2, Next: Section 4.6.3, Prev: Section 4.6.1, Up: Section 4.6 4.6.2. Context-Local Labels *************************** 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: %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 `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 `%$$$foo' for the context below that, and so on.  File: nasm.info, Node: Section 4.6.3, Next: Section 4.6.4, Prev: Section 4.6.2, Up: Section 4.6 4.6.3. Context-Local Single-Line Macros *************************************** NASM also allows you to define single-line macros which are local to a particular context, in just the same way: %define %$localmac 3 will define the single-line macro `%$localmac' to be local to the top context on the stack. Of course, after a subsequent `%push', it can then still be accessed by the name `%$$localmac'.