CS 366: Using a 2-way Associative Write-through Cache
(Excerpted from Mano, pp. 628-631)
CPU Address to use is: $ 0F3F 4024. Instruction has a 15-bit tag, 13-bit Index, 2-bit word select, 2-bit byte select
0
F
3
F
4
0
2
4
In Binary: 0000 1111 0011 1111 0100 0000 0010 0100
Regrouping to match instruction format [Fig 12-9a]
_______Tag______ ______Index____ Word Byte
0000 1111 0011 111 1 0100 0000 0010
01
00
Regrouping Tag:
_000 0111 1001 1111
And in Hex:
$0 7 9 F
$1 4 0 2
$1
3 Cases we will look at:
1. Read Hit
a) The index value $1402 references the two tags at that address in the cache.
b) Those tags are passed through the Match Logic. Assume tag 0 matches, which sets Match 0.
c) Match 0 is ANDed together with the valid 0 bit for index $1402, along with the Read control
line. Output of this enables three-state buffer 0.
d) Concurrently with step 3, the 2 word_select bits ($1 in this case) causes word 1 to be selected
from the 4 possibilities for both data memories. These two words are sent to their respective
three-state buffers.
e) The hit/miss line is set, indicating a hit.
The CPU can internally then use the 2 byte-select bits to pull out one of the 4 bytes from the
32-bit word that has been retrieved.
2. Read Miss
a) The index value $1402 references the two tags at that address in the cache. Again they pass
through the match logic, only this time there is no match, so the hit/miss line is set to 0,
indicating a miss. This indicates the word must be fetched from Main Memory.
b) The cache control selects the cache entry to be replaced (some algorithm?). Four 32-bit words
(tot. 128 bits) read from memory are applied simultaneously to the cache inputs. While this is
happening, the 4-to-1 Mux selects the desired word out of the 4 and sends it to the CPU data
bus using three-state buffer 3.
3. Write
a) The 32-bit word write from the CPU (through three-state buffer 2) is fanned out into 4 words
(128 bits), since we don’t know to which of the 4 entries in the cache line it corresponds.
b) The CPU address (e.g. index $1402) is used to write back only the selected word to memory.
If there is a cache hit, it is written to the cache as well. (Match logic for write enabling correct
data bank is not shown.)