Alternate Instruction Set

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2021-05-30 17:30:04

The Alternate Instruction Set (AIS) is a second 32-bit instruction set architecture found in some x86 CPUs made by VIA Technologies. On these VIA C3 processors, the second hidden processor mode is accessed by executing the x86 instruction ALTINST (0F 3F ). If AIS mode has been enabled, the processor will perform a JMP EAX[1] and begin executing AIS instructions at the address of the EAX register. Using AIS allows native access to the Centaur Technology-designed RISC core inside the processor.[2]

The manufacter describes the Alternate Instruction Set as "an extended set of integer, MMX, floating-point, and 3DNow! instructions along with additional registers and some more powerful instruction forms".[1] Every AIS instruction is prefixed with the 3-byte sequence 0x8D8400 followed by the 32-bit instruction; this prefix form for the AIS instructions makes them appear to be x86 Load Effective Address (LEA) instructions. In 2018 researcher Christoper Domas reported that the prefix 0x620405 (x86 BOUND) also worked.

A proposal made in 2002 to add AIS support to the Netwide Assembler (NASM) was partially declined in 2005, on the basis that NASM was an x86 assembler, and AIS is a separate instruction set.[3] An assembler is available from Domas's 2018 research.[4]