Features

This section describes details of the 602 implementation of the PowerPC architecture. Major features of the

602 are as follows:

• High-performance, superscalar microprocessor

— As many as two instructions are fetched from the instruction queue per clock

— One instruction can be issued and one retired per clock

— As many as four instructions in execution per clock

— Single-cycle execution for most instructions

• Four independent execution units and two register files

— BPU performs architecturally-defined static branch prediction

— A 32-bit IU

— Fully IEEE 754-compliant FPU for single-precision operations

— Emulation support for double-precision operations

— An implementation of the non-IEEE floating-point mode

— Thirty-two 32-bit general-purpose registers (GPRs) for integer operands

— Thirty-two 32-bit floating-point registers (FPRs) for single-precision operands

— LSU for data transfer between data cache and GPRs and FPRs

• Instruction pipelining and split cache organization

— Zero-cycle branch capability (branch folding)

— Programmable static branch prediction on unresolved conditional branches

— BPU that performs CR lookahead operations

— Instruction fetch unit capable of fetching two instructions per clock from the instruction cache

(one of which is a branch instruction)

— A four-entry instruction queue that provides lookahead capability

— Independent pipelines with feed-forwarding that reduces data dependencies in hardware

— 4-Kbyte data cache—two-way set-associative, physically addressed; LRU replacement

algorithm

— 4-Kbyte instruction cache—two-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 managament features

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

segment size

— A 32-entry, two-way set-associative ITLB

— A 32-entry, two-way set-associative DTLB

— Four-entry data and instruction BAT arrays providing 128-Kbyte to 256-Mbyte blocks

— Software table search operations and updates supported through fast trap mechanism

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

— Optional configuration of the TLBs that offers protection for up to 4-Mbytes of memory per

TLB, but no effective address translation

• Facilities for enhanced system performance

— A 64-bit (address and data multiplexed) external data bus with burst transfers

— Support for injected snoops by other devices during ownership of bus tenure

— Ability to broadcast a line-fill address, during the address tenure of a writeback transaction on

the bus

• Integrated power management

— Low-power 3.3-volt design

— Internal processor/bus clock multiplier that provides 2/1 and 3/1 ratios

— Three static power-saving modes—doze, nap, and sleep

— Automatic dynamic power reduction when internal functional units are idle

• Data bus externally selectable as either 32 or 64 bits

• In-system testability and debugging features through JTAG port

• Three power saving modes

— Doze—All the functional units of the 602 are disabled except for the time base/decrementer

registers and the bus snooping logic. When the processor is in doze mode, an external

asynchronous interrupt, a system management interrupt, a decrementer exception, a hard or soft

reset, or machine check brings the 602 into the full-power state. The 602 in doze mode

maintains the PLL in a fully-powered state and locked to the system external clock input

(SYSCLK) so a transition to the full-power state takes only a few processor clock cycles.

— Nap—The nap mode further reduces power consumption by disabling bus snooping, leaving

only the time base register and the PLL in a powered state. The 602 returns to the full-power

state upon receipt of an external asynchronous interrupt, a system management interrupt, a

decrementer exception, a hard or soft reset, or a machine check input (MCP). A return to fullpower

state from a nap state takes only a few processor clock cycles.

— Sleep—Sleep mode reduces power consumption to a minimum by disabling all internal

functional units, after which external system logic may disable the PLL and SYSCLK.

Returning the 602 to the full-power state requires the enabling of the PLL and SYSCLK,

followed by the assertion of an external asynchronous interrupt, a system management

interrupt, a hard or soft reset, or a machine check input (MCP) signal after the time required to

relock the PLL