Instruction Execution Timing; Timing Estimation Factors; Instruction Cache And Prefetch - Motorola MC68020 User Manual

SECTION 8

INSTRUCTION EXECUTION TIMING

This section describes the instruction execution and operations (table searches, etc.) of
the MC68020/EC020 in terms of external clock cycles. It provides accurate execution and
operation timing guidelines but not exact timings for every possible circumstance. This
approach is used since exact execution time for an instruction or operation is highly
dependent on memory speeds and other variables. The timing numbers presented in this
section allow the assembly language programmer or compiler writer to predict timings
needed to evaluate the performance of the MC68020/EC020.
In this section, instruction and operation times are shown in clock cycles, which eliminates
clock frequency dependencies.

8.1 TIMING ESTIMATION FACTORS

The advanced architecture of the MC68020/EC020 makes exact instruction timing
calculations difficult due to the effects of:
1. An On-Chip Instruction Cache and Instruction Prefetch
2. Operand Misalignment
3. Bus Controller/Sequence Concurrency
4. Instruction Execution Overlap
These factors make MC68020/EC020 instruction set timing difficult to calculate on a single
instruction basis since instructions vary in execution time from one context to another. A
detailed explanation of each of these factors follows.

8.1.1 Instruction Cache and Prefetch

The on-chip cache of the MC68020/EC020 is an instruction-only cache. Its purpose is to
increase execution efficiency by providing a quick store for instructions.
Instruction prefetches that hit in the cache will occur with no delay in instruction execution.
Instruction prefetches that miss in the cache will cause an external memory cycle to be
performed, which may overlap with internal instruction execution. Thus, while the
execution unit of the microprocessor is busy, the bus controller prefetches the next
instruction from external memory. Both cases are illustrated in later examples.
MOTOROLA M68020 USER'S MANUAL 8- 1