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PROGRAMMING WITH THE INTEL MMX TECHNOLOGY
8.1.5.Data Formats for MMX Registers
Values in MMX registers have the same format as a 64-bit quantity in memory. MMX
registers have two data access modes: 64-bit access mode and 32-bit access mode.
The 64-bit access mode is used for 64-bit memory access, 64-bit transfer between MMX
registers, all pack, logical and arithmetic instructions, and some unpack instructions.
The 32-bit access mode is used for 32-bit memory access, 32-bit transfer between integer regis-
ters and MMX registers, and some unpack instructions.
8.2.MMX INSTRUCTION SET
The MMX instruction set consists of 57 instructions, grouped into the following categories:
Data transfer instructions
Arithmetic instructions
Comparison instructions
Conversion instructions
Logical instructions
Shift instructions
Empty MMX state instruction (EMMS)
When operating on packed data within an MMX register, the data is cast by the type specified
by the instruction. For example, the PADDB (add packed bytes) instruction treats the packed
data in an MMX register as 8 packed bytes; whereas, the PADDW (add packed words)
instruction treats the packed data as 4 packed words. Refer to Section 9.3.6., Additional SIMD
Integer Instructions, in Chapter 9, Programming with the Streaming SIMD Extensions, for
additional SIMD integer instructions added with the Streaming SIMD Extensions.
