Chapter 5: C790 Processor Core
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
5-3
PC Unit:
The 32-bit
Program Counter
(
PC
) holds the address of the instruction that is
being executed. It also contains a 64-entry
Branch Target Address Cache (BTAC)
which stores branch target addresses used for branch prediction (to eliminate branch
penalties).
Issue Logic and Staging Registers:
The issue logic controls the transfer of fetched
instructions to the appropriate execution pipes. It can issue a maximum of two
instructions per cycle, and any instructions remaining that were fetched but could not be
issued because of conflicts such as resource conflicts or hazards are held in staging
registers until they can be issued.
General Purpose Registers (GPRs):
The width of the GPRs is extended from MIPS
III’s 64 bits, to a width of 128 bits. The upper 64 bits of the GPRs are accessible using
the quad word Load/Store instructions, the quad word funnel shift instruction, and the
parallel (multimedia) instructions.
I0 and I1 Pipes:
The two integer pipelines I0 and I1 each contain a complete 64-bit ALU,
Shifter, and Multiply-Accumulate (MAC) unit. The I0 pipeline additionally contains a Shift
Amount (SA) register that is used for funnel shift operations, and the I1 pipeline contains
a leading zero counter. The two 64-bit data paths can be configured dynamically, on an
instruction-by-instruction basis, into a single 128-bit data-path when it is necessary to
execute 128-bit wide multimedia, shift, ALU or Multiply-Accumulate instructions. The two
64-bit data paths share a single 128-bit multimedia shifter during 128-bit wide shift
operations.
Load / Store (LS) Pipe:
The Load/Store (LS) pipe supports a single issue of Load and
Store instructions at widths ranging from one byte (8 bits), to one quad-word (128 bits).
Memory Management Unit (MMU):
The Memory Management Unit supports the
address translation functions of the C790. It contains a 48-entry fully associative JTLB, a
2-entry Instruction Translation Lookaside Buffer (ITLB), and a 4-entry Data Translation
Lookaside Buffer (DTLB).
Memory Caches:
The C790 includes an Instruction Cache and a Data Cache. For each
branch instruction present in the instruction cache, two bits of branch history information
are stored in a
Branch History Table
(BHT)
.
Response and Write-back Buffer:
The Write-back Buffer (WBB) is an 8-entry by 16-
byte (one quad-word) FIFO queues up stores prior to accessing the C790 bus. It
increases C790 performance by isolating the processor from the latencies of the C790
bus. It is also used during the gathering operation of uncached accelerated stores.
Sequential stores less than a quad-word in length are gathered in the WBB, thereby
improving bus bandwidth usage.
Uncached Accelerated Buffer (UCAB) :
The Uncached Accelerated Buffer (UCAB) is a
2-entry by 4 quad-word buffer. It caches 128 sequential bytes of data during an
uncached accelerated load miss. Subsequent loads from the uncached accelerated
address space get their data from this buffer if the address hits in the UCAB, thereby
eliminating bus latencies and providing higher performance.
Bus Interface Unit (BIU) :
The Bus Interface Unit (BIU) connects the core’s internal bus
to the C790 bus. It interfaces the core’s internal bus signals to the C790 Bus.
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