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(nasm.info)Section 6.3.1

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6.3.1. `win32' Extensions to the `SECTION' Directive
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   Like the `obj' format, `win32' allows you to specify additional
information on the `SECTION' directive line, to control the type and
properties of sections you declare. Section types and properties are
generated automatically by NASM for the standard section names `.text',
`.data' and `.bss', but may still be overridden by these qualifiers.

   The available qualifiers are:

   * `code', or equivalently `text', defines the section to be a code
     section. This marks the section as readable and executable, but not
     writable, and also indicates to the linker that the type of the
     section is code.

   * `data' and `bss' define the section to be a data section,
     analogously to `code'. Data sections are marked as readable and
     writable, but not executable. `data' declares an initialised data
     section, whereas `bss' declares an uninitialised data section.

   * `rdata' declares an initialised data section that is readable but
     not writable. Microsoft compilers use this section to place
     constants in it.

   * `info' defines the section to be an informational section, which is
     not included in the executable file by the linker, but may (for
     example) pass information _to_ the linker. For example, declaring
     an `info'-type section called `.drectve' causes the linker to
     interpret the contents of the section as command-line options.

   * `align=', used with a trailing number as in `obj', gives the
     alignment requirements of the section. The maximum you may specify
     is 64: the Win32 object file format contains no means to request a
     greater section alignment than this. If alignment is not
     explicitly specified, the defaults are 16-byte alignment for code
     sections, 8-byte alignment for rdata sections and 4-byte alignment
     for data (and BSS) sections. Informational sections get a default
     alignment of 1 byte (no alignment), though the value does not
     matter.

   The defaults assumed by NASM if you do not specify the above
qualifiers are:

     section .text    code  align=16
     section .data    data  align=4
     section .rdata   rdata align=8
     section .bss     bss   align=4

   Any other section name is treated by default like `.text'.