Undocumented "REAL" real mode x86 NOP opcodes

From	"Mike Gonta" <mikegonta@gmail.com>
Newsgroups	alt.lang.asm, alt.os.assembly, alt.os.development
Subject	Undocumented "REAL" real mode x86 NOP opcodes
Date	2017-03-26 10:52 -0400
Organization	Aioe.org NNTP Server
Message-ID	<ob8kja$rff$1@gioia.aioe.org> (permalink)

Cross-posted to 3 groups.

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Undocumented "REAL" real mode x86 NOP opcodes.
There are a series of two byte opcodes (opcode byte plus MOD-REG-R/M
byte) that have a MOD-R/M for a 32 bit register indirect addressing mode.
For example, one of these (of many) is ADD [EAX], AL and the encoding
is 0x00, 0x00. In real mode this instruction (on a 80386 or better)
requires an address override prefix byte (0x67) since there is no
instruction for ADD [AX], AL.
However, the encoding of 0x00, 0x00 is a perfectly valid instruction
in real mode, and in fact is a true "REAL" No OPeration.
Unlike XCHG AX, AX aka 0x90 which is a valid operation (but for the fact
that it has no effect and is thus classified as a NOP), the encoding of
0x00, 0x00 in real mode (without the address override prefix) not only
doesn't add AL to what AX (or EAX) is pointing to but does absolutely
nothing other than advance the program counter.
Of course, these undocumented "REAL" real mode x86 NOP opcodes are
(almost) totally useless.
I did however manage to save 3 bytes (which I didn't need) in my exFAT
boot sector boot loader.


Mike Gonta
look and see - many look but few see

http://mikegonta.com

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Undocumented "REAL" real mode x86 NOP opcodes "Mike Gonta" <mikegonta@gmail.com> - 2017-03-26 10:52 -0400
  Re: Undocumented "REAL" real mode x86 NOP opcodes "Mike Gonta" <mikegonta@gmail.com> - 2017-03-26 12:32 -0400
    Re: Undocumented "REAL" real mode x86 NOP opcodes "wolfgang kern" <nowhere@never.at> - 2017-03-26 21:00 +0200

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