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.