GWIXP425BCT datasheet(31/134 Pages) INTEL

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Intel® IXP42X Product Line of Network Processors and IXC1100 Control Plane Processor

Datasheet

March 2005

Document Number: 252479, Revision: 005

2.2.8

Fill Buffer (FB) and Pend Buffer (PB)

The four-entry fill buffer (FB) works with the core to hold non-cacheable loads until the bus

controller can act on them. The FB and the four-entry pend buffer (PB) work with the D-cache and

mini-data cache to provide “hit-under-miss” capability, allowing the core to seek other data in the

caches while “miss” data is being fetched from memory.

The FB can contain up to four unique “miss” addresses (logical), allowing four “misses” before the

core is stalled. The PB holds up to four addresses (logical) for additional “misses” to those

addresses that are already in the FB. A coprocessor register can specify draining of the fill and pend

(write) buffers.

2.2.9

Write Buffer (WB)

The write buffer (WB) holds data for storage to memory until the bus controller can act on it. The

WB is eight entries deep, where each entry holds 16 bytes. The WB is constantly enabled and

accepts data from the core, D-cache, or mini-data cache.

Coprocessor 15, Register 1 specifies whether WB coalescing is enabled or disabled. When

coalescing is disabled, stores to memory occur in program order regardless of the attribute bits

within the descriptors located in the DTLB. When coalescing is enabled, the attribute bits within

the descriptors located in the DTLB are examined to determine when coalescing is enabled for the

destination region of memory. When coalescing is enabled in both CP15, R1 and the DTLB, data

entering the WB can coalesce with any of the eight entries (16 bytes) and be stored to the

destination memory region, but possibly out of program order.

Stores to a memory region specified to be non-cacheable and non-bufferable by the attribute bits

within the descriptors located in the DTLB causes the core to stall until the store completes. A

coprocessor register can specify draining of the write buffer.

2.2.10

Multiply-Accumulate Coprocessor (CP0)

For efficient processing of high-quality, media-and-signal-processing algorithms, CP0 provides

40-bit accumulation of 16 x 16, dual-16 x 16 (SIMD), and 32 x 32 signed multiplies. Special MAR

and MRA instructions are implemented to move the 40-bit accumulator to two core-general

registers (MAR) and move two core-general registers to the 40-bit accumulator (MRA). The 40-bit

accumulator can be stored or loaded to or from D-cache, mini-data cache, or memory using two

STC or LDC instructions.

The 16 x 16 signed multiply-accumulates (MIAxy) multiply either the high/high, low/low, high/

low, or low/high 16 bits of a 32-bit core general register (multiplier) and another 32-bit core

general register (multiplicand) to produce a full, 32-bit product that is sign-extended to 40 bits and

added to the 40-bit accumulator.

Dual-signed, 16 x 16 (SIMD) multiply-accumulates (MIAPH) multiply the high/high and low/low

16-bits of a packed 32-bit, core-general register (multiplier) and another packed 32-bit, core-

general register (multiplicand) to produce two 16-bits products that are both sign-extended to

40 bits and added to the 40-bit accumulator.

The 32 x 32 signed multiply-accumulates (MIA) multiply a 32-bit, core-general register

(multiplier) and another 32-bit, core-general register (multiplicand) to produce a 64-bit product

where the 40 LSBs are added to the 40-bit accumulator. The 16 x 32 versions of the 32 x 32

multiply-accumulate instructions complete in a single cycle.

GWIXP425BCT Datasheet(PDF) 31 Page - Intel Corporation

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