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Processor Programming (Continued)

Cache Test Registers

The processor’s 16 KB on-chip cache is a four-way set

associative memory that is configured as write-back

cache. Each cache set contains 256 entries. Each entry

consists of a 20-bit tag address, a 16-byte data field, a

valid bit, and four dirty bits.

The 20-bit tag represents the high-order 20 bits of the

physical address. The 16-byte data represents the 16

bytes of data currently in memory at the physical address

represented by the tag. The valid bit indicates whether the

data bytes in the cache actually contain valid data. The

four dirty bits indicate if the data bytes in the cache have

been modified internally without updating external mem-

ory (write-back configuration). Each dirty bit indicates the

status for one double-word (4 bytes) within the 16-byte

data field.

Foreachlineinthe cache, thereare threeLRU bits that

indicate which of the four sets was most recently

accessed. A line is selected using bits [11:4] of the physi-

cal address. Figure 3-1 illustrates the CPU cache archi-

tecture.

The CPU contains three test registers (TR5-TR3) that

allow testing of its internal cache. Bit definitions for the

cache test registers are shown in Table 3-17. Using a 16-

byte cache fill buffer and a 16-byte cache flush buffer,

cache reads and writes may be performed.

Figure 3-1. CPU Cache Architecture

255

254

A11-A4

Line

Address

= Cache Entry (153 bits)

Tag Address (20 bits)

Data (128 bits)

Valid Status (1 bit)

Dirty Status (4 bits)

Set 0

Set 1

Set 2

Set 3

LRU

152 --- 0

2 --- 0

Table 3-16. Cache Test Registers

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10

8765

TR5 Register (R/W)

RSVD

Line Selection

Set/

DWORD

Control

Bits

TR4 Register (R/W)

Cache Tag Address

LRU Bits

Dirty Bits

TR3 Register (R/W)

Cache Data

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