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21264/EV68A Hardware Reference Manual
Internal Architecture 2–11
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)
2.1.4.1 Victim 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
2.1.4.2 I/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)