PowerPC 601 Microprocessor Features

This section describes details of the 601’s implementation of the PowerPC architecture. Major features of

the 601 are as follows:

• High-performance, superscalar microprocessor

— As many as three instructions in execution per clock (one to each of the three execution units)

— Single clock cycle execution for most instructions

— Pipelined FPU for all single-precision and most double-precision operations

• Three independent execution units and two register files

— BPU featuring static branch prediction

— A 32-bit IU

— Fully IEEE 754-compliant FPU for both single- and double-precision operations

— Thirty-two GPRs for integer operands

— Thirty-two FPRs for single- or double-precision operands

• High instruction and data throughput

— Zero-cycle branch capability

— Programmable static branch prediction on unresolved conditional branches

— Instruction unit capable of fetching eight instructions per clock from the cache

— An eight-entry instruction queue that provides look-ahead capability

— Interlocked pipelines with feed-forwarding that control data dependencies in hardware

— Unified 32-Kbyte cache—eight-way set-associative, physically addressed; LRU replacement

algorithm

— Cache write-back or write-through operation programmable on a per page or per block basis

— Memory unit with a two-element read queue and a three-element write queue

— Run-time reordering of loads and stores

— BPU that performs condition register (CR) look-ahead operations

— Address translation facilities for 4-Kbyte page size, variable block size, and 256-Mbyte

segment size

— A 256-entry, two-way set-associative UTLB

— Four-entry BAT array providing 128-Kbyte to 8-Mbyte blocks

— Four-entry, first-level ITLB

— Hardware table search (caused by UTLB misses) through hashed page tables

— 52-bit virtual address; 32-bit physical address

• Facilities for enhanced system performance

— Bus speed defined as selectable division of operating frequency

— A 64-bit split-transaction external data bus with burst transfers

— Support for address pipelining and limited out-of-order bus transactions

— Snooped copyback queues for cache block (sector) copyback operations

— Bus extensions for I/O controller interface operations

— Multiprocessing support features that include the following:

– Hardware enforced, four-state cache coherency protocol (MESI)

– Separate port into cache tags for bus snooping

• In-system testability and debugging features through boundary-scan capability