Instruction Pipeline; Prefetch Controller; Block Diagram Of Independent Resources - Motorola MC68340 User Manual

The bus controller and microsequencer operate concurrently. The bus controller can
perform a read or write or schedule a prefetch while the microsequencer controls EA
calculation or sets condition codes.
The microsequencer can also request a bus cycle that the bus controller cannot perform
immediately. When this happens, the bus cycle is queued, and the bus controller runs the
cycle when the current cycle is complete.
MICROSEQUENCER AND CONTROL
CONTROL STORE
CONTROL LOGIC
ADDRESS
BUS
Figure 5-30. Block Diagram of Independent Resources
5.7.1.3.1 Prefetch Controller. The instruction prefetch controller receives an initial
request from the microsequencer to initiate prefetching at a given address. Subsequent
prefetches are initiated by the prefetch controller whenever a pipeline stage is invalidated,
either through instruction completion or through use of extension words. Prefetch occurs
as soon as the bus is free of operand accesses previously requested by the
microsequencer. Additional state information permits the controller to inhibit prefetch
requests when a change in instruction flow (e.g., a jump or branch instruction) is
anticipated.
In a typical program, 10 to 25 percent of the instructions cause a change of flow. Each
time a change occurs, the instruction pipeline must be flushed and refilled from the new
instruction stream. If instruction prefetches, rather than operand accesses, were given
5-90
Freescale Semiconductor, Inc.

INSTRUCTION PIPELINE

STAGE
C
EXECUTION UNIT
PROGRAM
COUNTER
SECTION
WRITE-PENDING
BUFFER
MICROBUS
CONTROLLER
BUS CONTROL
SIGNALS
MC68340 USER'S MANUAL
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STAGE
B
DATA
SECTION
PREFETCH
CONTROLLER
DATA
BUS
MOTOROLA