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(nasm.info)Section B.4.127

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B.4.127. `IRET', `IRETW', `IRETD': Return from Interrupt
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     IRET                          ; CF                   [8086]
     IRETW                         ; o16 CF               [8086]
     IRETD                         ; o32 CF               [386]

   `IRET' returns from an interrupt (hardware or software) by means of
popping `IP' (or `EIP'), `CS' and the flags off the stack and then
continuing execution from the new `CS:IP'.

   `IRETW' pops `IP', `CS' and the flags as 2 bytes each, taking 6
bytes off the stack in total. `IRETD' pops `EIP' as 4 bytes, pops a
further 4 bytes of which the top two are discarded and the bottom two
go into `CS', and pops the flags as 4 bytes as well, taking 12 bytes
off the stack.

   `IRET' is a shorthand for either `IRETW' or `IRETD', depending on
the default `BITS' setting at the time.