AMD-K6 Datasheet, PDF (30/346 Pages) Advanced Micro Devices – AMD-K6 Processor

AMD-K6^® Processor Data Sheet
Preliminary Information
20695H/0—March 1998
Tag
Address
Cache Line 1 Byte 31 Predecode Bits Byte 30 Predecode Bits ........ ........ Byte 0 Predecode Bits MESI Bits
Cache Line 2 Byte 31 Predecode Bits Byte 30 Predecode Bits ........ ........ Byte 0 Predecode Bits MESI Bits
Figure 2. Cache Sector Organization
Prefetching
Predecode Bits
Two forms of cache misses and associated cache fills can take
place—a sector replacement and a cache line replacement. In
the case of a sector replacement, the miss is due to a tag
mismatch, in which case the required cache line is filled from
external memory, and the cache line within the sector that was
not required is marked as invalid. In the case of a cache line
replacement, the address matches the tag, but the requested
cache line is marked as invalid. The required cache line is filled
from external memory, and the cache line within the sector that
is not required remains in the same cache state.
The AMD-K6 processor performs cache prefetching for sector
replacements only—as opposed to cache line replacements.
This cache prefetching results in the filling of the required
cache line first, and a prefetch of the second cache line.
Furthermore, the prefetch of the cache line that is not required
is initiated only in the forward direction—that is, only if the
requested cache line is the first cache line within the sector.
From the perspective of the external bus, the two cache-line
fills typically appear as two 32-byte burst read cycles occurring
back-to-back or, if allowed, as pipelined cycles.
Decoding x86 instructions is particularly difficult because the
instructions are variable-length and can be from 1 to 15 bytes
long. Predecode logic supplies the predecode bits that are
associated with each instruction byte. The predecode bits
indicate the number of bytes to the start of the next x86
instruction. The predecode bits are stored in an extended
instruction cache alongside each x86 instruction byte as shown
in Figure 2 on page 12. The predecode bits are passed with the
instruction bytes to the decoders where they assist with parallel
x86 instruction decoding.
12
Internal Architecture
Chapter 2