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(nasm.info)Section 5.1

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5.1. `BITS': Specifying Target Processor Mode
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   The `BITS' directive specifies whether NASM should generate code
designed to run on a processor operating in 16-bit mode, or code
designed to run on a processor operating in 32-bit mode. The syntax is
`BITS 16' or `BITS 32'.

   In most cases, you should not need to use `BITS' explicitly. The
`aout', `coff', `elf' and `win32' object formats, which are designed
for use in 32-bit operating systems, all cause NASM to select 32-bit
mode by default. The `obj' object format allows you to specify each
segment you define as either `USE16' or `USE32', and NASM will set its
operating mode accordingly, so the use of the `BITS' directive is once
again unnecessary.

   The most likely reason for using the `BITS' directive is to write 32-
bit code in a flat binary file; this is because the `bin' output format
defaults to 16-bit mode in anticipation of it being used most
frequently to write DOS `.COM' programs, DOS `.SYS' device drivers and
boot loader software.

   You do _not_ need to specify `BITS 32' merely in order to use 32-
bit instructions in a 16-bit DOS program; if you do, the assembler will
generate incorrect code because it will be writing code targeted at a
32- bit platform, to be run on a 16-bit one.

   When NASM is in `BITS 16' state, instructions which use 32-bit data
are prefixed with an 0x66 byte, and those referring to 32-bit addresses
have an 0x67 prefix. In `BITS 32' state, the reverse is true: 32-bit
instructions require no prefixes, whereas instructions using 16-bit data
need an 0x66 and those working on 16-bit addresses need an 0x67.

   The `BITS' directive has an exactly equivalent primitive form,
`[BITS 16]' and `[BITS 32]'. The user-level form is a macro which has
no function other than to call the primitive form.

   Note that the space is neccessary, `BITS32' will _not_ work!

Section 5.1.1

`USE16' & `USE32': Aliases for BITS