Compaq EV68A specifications External Cache and System Interface Unit, Onchip Caches

21264/EV68A Microarchitecture

The Fbox register file contains six reads ports and four write ports. Four read ports are used to source operands to the add and multiply pipelines, and two read ports are used to source data for store instructions. Two write ports are used to write results generated by the add and multiply pipelines, and two write ports are used to write results from floating-point load instructions.

2.1.4 External Cache and System Interface Unit

The interface for the system and external cache (Cbox) controls the Bcache and system ports. It contains the following structures:

•Victim address file (VAF)

•Victim data file (VDF)

•I/O write buffer (IOWB)

•Probe queue (PQ)

•Duplicate Dcache tag (DTAG)

2.1.4.1Victim Address File and Victim Data File

The victim address file (VAF) and victim data file (VDF) together form an 8-entry vic- tim buffer used for holding:

•Dcache blocks to be written to the Bcache

•Istream cache blocks from memory to be written to the Bcache

•Bcache blocks to be written to memory

•Cache blocks sent to the system in response to probe commands

2.1.4.2I/O Write Buffer

The I/O write buffer (IOWB) consists of four 64-byte entries and associated address and control logic used for buffering I/O write data between the store queue and the sys- tem port.

2.1.4.3 Probe Queue

The probe queue (PQ) is an 8-entry queue that holds pending system port cache probe commands and addresses.

2.1.4.4 Duplicate Dcache Tag Array

The duplicate Dcache tag (DTAG) array holds a duplicate copy of the Dcache tags and is used by the Cbox when processing Dcache fills, Icache fills, and system port probes.

2.1.5 Onchip Caches

The 21264/EV68A contains two onchip primary-level caches.

2.1.5.1 Instruction Cache

The instruction cache (Icache) is a 64KB virtual-addressed, 2-way set-predict cache. Set prediction is used to approximate the performance of a 2-set cache without slowing the cache access time. Each Icache block contains:

•16 Alpha instructions (64 bytes)

21264/EV68A Hardware Reference Manual Internal Architecture 2–11