Processor Programming (Continued)
3.3.2.5 Cache Test Registers
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 DWORD (4 bytes) within the 16-byte data
field.
Three test registers are used in testing the processor’s on-
chip cache, TR3-TR5. Table 3-16 is a register map for the
Cache Test Registers with their bit definitions given in Table
3-17 on page 60. The test registers are accessed through
MOV instructions that can be executed only at privilege
level 0 (real mode is always privilege level 0).
For each line in the cache, there are three LRU 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. Using a 16-byte cache fill buffer and a 16-
byte cache flush buffer, cache reads and writes may be
performed.
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
Figure 3-1 illustrates the internal cache architecture.
Line
Address
Set 0
Set 1
Set 2
Set 3
LRU
255
D
E
C
O
D
E
254
A11-A4
.
.
0
.
.
.
.
.
.
.
.
.
.
152 --- 0
152 --- 0
152 --- 0
152 --- 0
2 --- 0
= Cache Entry (153 bits)
Tag Address (20 bits)
Data (128 bits)
Valid Status (1 bit)
Dirty Status (4 bits)
Figure 3-1. Cache Architecture
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
9
8
7
6
5
4
3
2
1
0
TR5 Register (R/W)
RSVD
Line Selection
Set/
DWORD
Control
Bits
TR4 Register - Cache (R/W)
Cache Tag Address
0
Cache
Dirty Bits
0
0
0
LRU Bits
TR3 Register - Cache (R/W)
Cache Data
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