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3- 2 M68040 USER'S MANUAL MOTOROLA
indexing into the on-chip instruction and data caches. The MMU MDIS signal dynamically
disables address translation for emulation and diagnostic support.
Figure 3-1 illustrates the MMUs contained in the two memory units, one for instructions
(supporting instruction prefetches) and one for data (supporting all other accesses). Each
unit contains an MMU, main cache, and snoop controller. The corresponding MMUs
contain two transparent translation registers, which identify blocks of memory that can be
accessed without translation. The MMUs also contain control logic and corresponding
address translation caches (ATCs) in which recently used logical-to-physical address
translations are stored. The data memory unit contains a data write and data read buffer,
and the instruction memory unit contains an instruction line read buffer. These buffers
temporarily hold data until an opportune moment arises to write the data to external
memory or read the operand/instruction into the integer unit.
INSTRUCTION
FETCH
DECODE
EA
CALCULATE
EXECUTE
WRITE-
BACK
EA
FETCH
INTEGER
UNIT
CONVERT
EXECUTE
WRITE-
BACK
INSTRUCTION
ATC
INSTRUCTION
MMU/CACHE/SNOOP
CONTROLLER
BUS
CONTROL
SIGNALS
DATA
BUS
ADDRESS
BUS
DATA
ATC
DATA
MMU/CACHE/SNOOP
CONTROLLER
OPERAND DATA BUS
INSTRUCTION DATA BUS
INSTRUCTION
CACHE
DATA
CACHE
FLOATING-
POINT
UNIT
DATA MEMORY UNIT
INSTRUCTION MEMORY UNIT B
U
S
C
O
N
T
R
O
L
L
E
R
INSTRUCTION
ADDRESS
DATA
ADDRESS
Figure 3-1. Memory Management Unit
The principal MMU function is to translate logical addresses to physical addresses using
translation tables stored in memory. As the MMU receives a logical address from the
integer unit, it searches its ATC for the corresponding physical address using the upper
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