AMD K5 User Manual download pdf (Page 19)

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

PRELIMINARY INFORMATION

a given memory location returns valid data. Each cache line is

assigned one of the four protocol states to identify the status of

the information stored in the cache. The writeback cache

design updates memory only when necessary. This keeps the

system bus free for use by other devices and improves the

overall system performance.

4.7 Branch Prediction

A branch occurs on average once every seven x86 instructions.

When a branch is encountered, the processor predicts which

direction the instruction flow will follow. The AMD-K5 proces-

sor adds branch prediction information to each instruction

cache line in the form of a predicted address tag that indicates

the target address of the first branch that is predicted to be

taken in the cache line. The processor’s dynamic branch pre-

diction mechanism allows for 1024 branch targets and a 75%

branch prediction accuracy. Combined with a minimal 3-cycle

mispredict penalty, the branch prediction mechanism opti-

mizes the processor’s speculative execution of x86 software,

such as the Microsoft Windows operating system and associ-

ated applications.

The dynamic branch prediction of the processor enables

instructions to be fetched and fed into the processor’s execu-

tion core, eliminating many pipeline bubbles and contributing

to the superior performance of the AMD-K5 processor.

4.8 Unique x86 Instruction Conversion and Decoding

The logical instruction flow within the AMD-K5 processor con-

tinues as up to 32-bytes of predecoded x86 instructions are

fetched from the byte queue of the instruction cache and for-

warded in order to the decoder.

The processor's decoder converts complex x86 instructions into

relatively simple, fast-executing ROPs that are of fixed length

and easy to process. Simultaneously, the operands needed to

perform the ROPs’ operations are fetched from the register

file or from the reorder buffer.

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AMD K5 User Manual Page 19