Directives

The content here is taken and/or edited from the NASM Documentation.

SEGMENT / SECTION

The SECTION directive (SEGMENT is an exactly equivalent synonym) changes which section of the output file the code you write will be assembled into. In some object file formats, the number and names of sections are fixed; in others, the user may make up as many as they wish. Hence SECTION may sometimes give an error message, or may define a new section, if you try to switch to a section that does not (yet) exist.

Here is a table on sections that exist:

EXTERN

EXTERN is similar to the C keyword extern: it is used to declare a symbol which is not defined anywhere in the module being assembled, but is assumed to be defined in some other module and needs to be referred to by this one.

extern printf

GLOBAL

GLOBAL is the other end of EXTERN: if one module declares a symbol as EXTERN and refers to it, then in order to prevent linker errors, some other module must actually define the symbol and declare it as GLOBAL.

global main

segment .text
main:
  xor eax, eax
  ret

COMMON

The COMMON directive is used to declare common variables. A common variable is much like a global variable declared in the uninitialized data section, so that

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 intvar in all modules will point at the same piece of memory.

CPU

The CPU directive restricts assembly to those instructions which are available on the specified CPU.

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: