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18522F/0—Jan1997 AMD-K5 Processor Data Sheet

PRELIMINARY INFORMATION

results, recover from mispredictions and exceptions, and hold

the relative speculative state.

The processor’s 16-entry reorder buffer stores results from x86

instructions that have been speculatively executed at the time

a branch was predicted. When ROPs are dispatched to one of

the processor’s six independent execution units, an entry at

the top of the reorder buffer is allocated for each ROP. Up to

four entries are allocated simultaneously. The reorder buffer

keeps track of the original instruction sequence and ensures

that results are retired in program order, writing the results of

the executed instruction to the register file. If a branch is

mispredicted, the results of the instructions along the mispre-

dicted path are invalidated in the reorder buffer before there

is any effect on the x86 registers or memory system.

4.10 Register File

A problem with the x86 architecture has been its limited num-

ber of general-purpose registers. Fewer registers means fre-

quent reuse of registers, which potentially leads to a reduction

in performance. The AMD-K5 CPU utilizes register renaming

and avoids this performance reduction.

Because the movement of values between registers and mem-

ory locations is unavoidable with the x86 instruction set, a key

advantage of the AMD-K5 CPU is its single-cycle load from the

data cache. This, in combination with the multiported register

file and renaming in the reorder buffer, gives near optimal

speculative performance within the constraints of the x86

instruction set.

4.11 The Right Combination—Compatibility and Performance

While each feature has a significant function, it is the combina-

tion of all features that is responsible for the AMD-K5 proces-

sor’s overall design and performance advantages.

Compatibility with the Microsoft Windows operating system

and the immense library of x86 software furthers these advan-

tages, and is the foundation of the AMD-K5 processor’s lead-

ing-edge solution.

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