Motorola MC68030 User Manual

On-Chip Cache Memories
MOTOROLA MC68030 USER’S MANUAL 6-5
When enabled, the instruction cache is used to store instruction prefetches (instruction
words and extension words) as they are requested by the CPU. Instruction prefetches are
normally requested from sequential memory addresses except when a change of program
flow occurs (e.g., a branch taken) or when an instruction is executed that can modify the
status register, in which cases the instruction pipe is automatically flushed and refilled. The
output signal REFILL
indicates this condition. For more information on the operation of this
signal, refer to Section 12 Applications Information .
In the instruction cache, each of the 16 lines has a tag consisting of the 24 most significant
logical address bits, the FC2 function code bit (used to distinguish between user and
supervisor accesses), and the four valid bits (one corresponding to each long word). Refer
to Figure 6-2 for the instruction cache organization. Address bits A7–A4 select one of 16
lines and its associated tag. The comparator compares the address and function code bits
in the selected tag with address bits A31–A8 and FC2 from the internal prefetch request to
determine if the requested word is in the cache. A cache hit occurs when there is a tag match
and the corresponding valid bit (selected by A3–A2) is set. On a cache hit, the word selected
by address bit A1 is supplied to the instruction pipe.
When the address and function code bits do not match or the requested entry is not valid, a
miss occurs. The bus controller initiates a long-word prefetch operation for the required
Figure 6-2. On-Chip Instruction Cache Organization
F F F
CCC 3 222211111111110000000000
210 1 32 01 987 65432109876543210
COMPARATOR
TAG
1 OF 16
SELECT
VALID
TAG REPLACE
INDEX
TAG
LINE HIT
DATA FROM INSTRUCTION
CACHE DATA BUS
CACHE CONTROL LOGIC
V V VV
ACCESS ADDRESS
DATA TO INSTRUCTION
CACHE HOLDING REGISTER
ENTRY HIT
A
LONG-WORD
SELECT
CACHE SIZE = 64 (LONG WORDS)
LINE SIZE = 4 (LONG WORDS)
SET SIZE = 1
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Motorola MC68030 Specifications

General

Brand	Motorola
Model	MC68030
Category	Computer Hardware
Language	English