MC68020 Motorola, MC68020 Datasheet - Page 217

• Manufacturer Part Number: MC68020
• Description: (MC68020 / MC68EC020) MICROPROCESSORS USERS MANUAL
• Manufacturer: Motorola

8.1.4 Instruction Execution Overlap
Overlap is the time, measured in clocks, when two instructions execute concurrently. In
Figure 8-1, instructions A and B execute concurrently, and the overlapped portion of
instruction B is absorbed in the instruction execution time of A (the previous instruction).
The overlap time is deducted from the execution time of instruction B. Similarly, there is an
overlap period between instruction B and instruction C, which reduces the attributed
execution time for C.
The execution time attributed to instructions A, B, and C (after considering the overlap) is
depicted in Figure 8-2.
It is possible that the execution time of an instruction will be absorbed by the overlap with
a previous instruction for a net execution time of zero clocks.
Because of this overlap, a NOP is required between a write to a peripheral to clear an
interrupt request and a subsequent MOVE to SR instruction to lower the interrupt mask
level. Otherwise, the MOVE to SR instruction may complete before the write is
accomplished, and a new interrupt exception will be generated for an old interrupt request.
MOTOROLA
Figure 8-2. Instruction Execution for Instruction Timing Purposes
INSTRUCTION A
INSTRUCTION A
Figure 8-1. Concurrent Instruction Execution
INSTRUCTION A)
(ABSORBED BY
OVERLAP
OVERLAP
PERIOD
M68020 USER’S MANUAL
INSTRUCTION B
INSTRUCTION B
INSTRUCTION B)
(ABSORBED BY
OVERLAP
OVERLAP
PERIOD
INSTRUCTION C
INSTRUCTION C
8- 3