32-bit forms of 16-bit instructions
32-bit forms of 16-bit instructionsLast update: 2000-09-11
There are several instructions defined originally as 32-bit that can be
executed as 16-bit ones by means of OPSIZE prefix. The same trick can be applied
to some 16-bit instructions that can be forced to become 32-bit.
BSWAPDefined as 32-bit, the instruction swaps all bytes in a 32-bit
register. When forced to 16-bit, the instruction takes whole 32-bit register,
swaps the bytes and writes only lower 16-bits, so 0x11223344 becomes 0x11222211.
SMSWThe instruction (introduced in 80286) stores the MSW register into
a specified destination, usually a register. Starting with 386, 16-bit MSW
became a part of 32-bit CR0, and this instruction is obsoleted by MOV reg, CR0.
MOV reg, CR0 in turn is not allowed in protected mode from user privilege level.
SMSW EAX can be used to retrieve the content of CR0 from any privilege level on
most x86 CPUs (this excludes Cyrix chips).
PUSH sregThe instruction is supposed to push the content of a segment
(selector) register, yero extended to 32 bits if executed in 32-bit mode. (to be
continued).
Copyright 2000 by G. Mazur
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