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NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM, is nevertheless forced to support a few directives. These are described in this chapter.
NASM's directives come in two types: user-level directives and primitive directives. Typically, each directive has a user-level form and a primitive form. In almost all cases, we recommend that users use the user-level forms of the directives, which are implemented as macros which call the primitive forms.
Primitive directives are enclosed in square brackets; user-level directives are not.
In addition to the universal directives described in this chapter, each object file format can optionally supply extra directives in order to control particular features of that file format. These format-specific directives are documented along with the formats that implement them, in chapter 7.
BITS The
In most cases, you should not need to use
The most likely reason for using the
You do not need to specify
When NASM is in
When NASM is in
The default address size is 64 bits; 32-bit addressing can be selected
with the 0x67 prefix. The default operand size is still 32 bits, however,
and the 0x66 prefix selects 16-bit operand size. The
When the
The
Note that the space is neccessary, e.g.
USE16 USE32 The `
DEFAULT The
Currently,
REL ABS This sets whether registerless instructions in 64-bit mode are
The special handling of
BND NOBND BND If
DEFAULT BND
call foo ; BND will be prefixed
nobnd call foo ; BND will NOT be prefixed
SECTION SEGMENT The
The Unix object formats, and the
__SECT__ The
SECTION .text
expands to the two lines
%define __SECT__ [SECTION .text]
[SECTION .text]
Users may find it useful to make use of this in their own macros. For
example, the
%macro writefile 2+
[section .data]
%%str: db %2
%%endstr:
__SECT__
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
This form of the macro, once passed a string to output, first switches
temporarily to the data section of the file, using the primitive form of
the
ABSOLUTE The
absolute 0x1A
kbuf_chr resw 1
kbuf_free resw 1
kbuf resw 16
This example describes a section of the PC BIOS data area, at segment
address 0x40: the above code defines
The user-level form of
org 100h ; it's a .COM program
jmp setup ; setup code comes last
; the resident part of the TSR goes here
setup:
; now write the code that installs the TSR here
absolute setup
runtimevar1 resw 1
runtimevar2 resd 20
tsr_end:
This defines some variables `on top of' the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol `tsr_end' can be used to calculate the total size of the part of the TSR that needs to be made resident.
EXTERN
The
extern _printf extern _sscanf,_fscanf
Some object-file formats provide extra features to the
extern _variable:wrt dgroup
The primitive form of
You can declare the same variable as
GLOBAL
The
global _main
_main:
; some code
global hashlookup:function, hashtable:data
Like
COMMON The
common intvar 4
is similar in function to
global intvar section .bss intvar resd 1
The difference is that if more than one module defines the same common
variable, then at link time those variables will be merged, and
references to
Like
common commvar 4:near ; works in OBJ common intarray 100:4 ; works in ELF: 4 byte aligned
Once again, like
CPU The
Options are:
CPU 8086 CPU 186 CPU 286 CPU 386 CPU 486 CPU 586 CPU PENTIUM CPU 686 CPU PPRO CPU P2 CPU P3 CPU KATMAI CPU P4 CPU WILLAMETTE CPU PRESCOTT CPU X64 CPU IA64 All options are case insensitive. All instructions will be selected only if they apply to the selected CPU or lower. By default, all instructions are available.
FLOAT By default, floating-point constants are rounded to nearest, and IEEE denormals are supported. The following options can be set to alter this behaviour:
FLOAT DAZ FLOAT NODAZ FLOAT NEAR FLOAT UP FLOAT DOWN FLOAT ZERO FLOAT DEFAULT The standard macros