Instruction Execution Overlap - Motorola MC68020 User Manual

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.
INSTRUCTION A
The execution time attributed to instructions A, B, and C (after considering the overlap) is
depicted in Figure 8-2.
INSTRUCTION A
Figure 8-2. Instruction Execution for Instruction Timing Purposes
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
INSTRUCTION B
OVERLAP
Figure 8-1. Concurrent Instruction Execution
OVERLAP
PERIOD
(ABSORBED BY
INSTRUCTION A)
M68020 USER'S MANUAL
INSTRUCTION C
OVERLAP
INSTRUCTION B
OVERLAP
PERIOD
(ABSORBED BY
INSTRUCTION B)
INSTRUCTION C
8- 3