This is one of the ELF section stack manipulation directives. The others are
.section (see Section),
.subsection (see SubSection),
.pushsection (see PushSection), and
This directive swaps the current section (and subsection) with most recently
referenced section/subsection pair prior to this one. Multiple
.previous directives in a row will flip between two sections (and their
subsections). For example:
.section A .subsection 1 .word 0x1234 .subsection 2 .word 0x5678 .previous .word 0x9abc
Will place 0x1234 and 0x9abc into subsection 1 and 0x5678 into subsection 2 of section A. Whilst:
.section A .subsection 1 # Now in section A subsection 1 .word 0x1234 .section B .subsection 0 # Now in section B subsection 0 .word 0x5678 .subsection 1 # Now in section B subsection 1 .word 0x9abc .previous # Now in section B subsection 0 .word 0xdef0
Will place 0x1234 into section A, 0x5678 and 0xdef0 into subsection 0 of section B and 0x9abc into subsection 1 of section B.
In terms of the section stack, this directive swaps the current section with the top section on the section stack.