Building Blocks for 64-bit AMD Opteron Clusters - Linux Clusters ...
Building Blocks for 64-bit AMD Opteron Clusters - Linux Clusters ...
Building Blocks for 64-bit AMD Opteron Clusters - Linux Clusters ...
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REX Prefix Byte<br />
•Optional REX prefix specifies <strong>64</strong>-<strong>bit</strong> operation size override<br />
and 3 additional register encoding <strong>bit</strong>s<br />
– Extra registers encoded without altering existing instruction <strong>for</strong>mat<br />
– REX is actually a family of 16 prefixes (40-4F)<br />
– <strong>64</strong>-<strong>bit</strong> mode Average instruction length increased by 0.4 bytes<br />
Instruction<br />
Prefixes<br />
Optional<br />
Instruction<br />
REX<br />
Prefixes<br />
Prefix Byte<br />
OPCODE<br />
M O D R M<br />
S I B<br />
Displacement<br />
Immediate<br />
0 1 0 0 W R X B<br />
MODRM r/m, SIB base, or opcode reg field extension<br />
SIB index field extension<br />
MODRM reg field extension<br />
Operand Size: 0 = 32-<strong>bit</strong>, 1 = <strong>64</strong>-<strong>bit</strong><br />
11
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