1- 4 M68040 USER’S MANUAL MOTOROLA
more common case of the branch taken, and both execution paths of the branch are
fetched and decoded to minimize refilling of the instruction pipeline.
DECODE
EA
CALCULATE
WRITE-
BACK
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
INSTRUCTION
FETCH
EXECUTE
EA
FETCH
Figure 1-1. Block Diagram
To improve memory management, the M68040 includes separate, independent paged
MMUs for instruction and data accesses. Each MMU stores recently used address
mappings in separate 64-entry address translation caches (ATCs). Each MMU also has
two transparent translation registers that define a one-to-one mapping for address space
segments ranging in size from 16 Mbytes to 4 Gbytes each.
Two memory units independently interface with the IU and FPU. Each unit consists of an
MMU, an ATC, a main cache, and a snoop controller. The MMUs perform memory
management on a demand-page basis. By translating logical-to-physical addresses using
translation tables stored in memory, the MMUs support virtual memory systems. Each
MMU stores recently used address mappings in an ATC, reducing the average translation
time.
Separate on-chip instruction and data caches operate independently and are accessed in
parallel with address translation. The caches improve the overall performance of the
system by reducing the number of bus transfers required by the processor to fetch
information from memory and by increasing the bus bandwidth available for alternate bus
