Basically, this instruction set extension allow AMD to "patch" the only two deficiencies of SSE5 (compared to AVX): 256-bit vectors and 4-operand instructions, while preserving all SSE5's (many) strengths.
In fact, looking at the instruction encoding, AMD did not intend or attempt to follow AVX. For example, AVX's 2-byte prefix format was not used, while AVX's 3-byte prefix format was used to allow access to the 4th operand and the 256-bit YMM registers.
This is IMHO an aggressive move. It is a confidence call from AMD, saying whatever we're going to do in our next generation will be better than yours (Intel's).
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