AMD-K6 Datasheet, PDF (33/346 Pages) Advanced Micro Devices – AMD-K6 Processor

20695H/0—March 1998
Preliminary Information
AMD-K6^® Processor Data Sheet
Chapter 2
The AMD-K6 processor uses a combination of decoders to
convert x86 instructions into RISC86 operations. The hardware
consists of three sets of decoders—two parallel short decoders,
one long decoder, and one vectoring decoder. The parallel short
decoders translate the most commonly-used x86 instructions
( moves, shifts, branches, ALU, MMX, FPU) into zero, one, or
two RISC86 operations each. The short decoders only operate
on x86 instructions that are up to seven bytes long. In addition,
they are designed to decode up to two x86 instructions per
clock. The commonly-used x86 instructions that are greater
than seven bytes but not more than 11 bytes long, and
semi-commonly-used x86 instructions that are up to seven bytes
long are handled by the long decoder.
The long decoder only performs one decode per clock and
generates up to four RISC86 operations. All other translations
(complex instructions, serializing conditions, interrupts and
exceptions, etc.) are handled by a combination of the vector
decoder and RISC86 operation sequences fetched from an
on-chip ROM. For complex operations, the vector decoder logic
provides the first set of RISC86 operations and a vector (initial
ROM address) to a sequence of further RISC86 operations. The
same types of RISC86 operations are fetched from the ROM as
those that are generated by the hardware decoders.
Note: Although all three sets of decoders are simultaneously fed a
copy of the instruction buffer contents, only one of the three
types of decoders is used during any one decode clock.
The decoders or the RISC86 sequencer always generate a group
of four RISC86 operations. For decodes that cannot fill the entire
group with four RISC86 operations, RISC86 NOP operations are
placed in the empty locations of the grouping. For example, a
long-decoded x86 instruction that converts to only three RISC86
operations is padded with a single RISC86 NOP operation and
then passed to the scheduler. Up to six groups or 24 RISC86
operations can be placed in the scheduler at a time.
All of the common, and a few of the uncommon, floating-point
instructions (also known as ESC instructions) are hardware
decoded as short decodes. This decode generates a RISC86
floating-point operation and, optionally, an associated
floating-point load or store operation. Floating-point or ESC
instruction decode is only allowed in the first short decoder, but
non-ESC instructions, excluding MMX instructions, can be
Internal Architecture
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