Motorola MC68020 - Instruction Timings from Timing Tables

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MOTOROLA M68020 USER’S MANUAL 8- 11

NOTE

This CC time is a maximum since the times given for the

MULU.L and DIVS.L are maximums.

The MOVE instruction timing tables include all necessary timing for extension word fetch,

address calculation, and operand fetch.

The instruction timing tables are used to calculate a best-case and worst-case bounds for

some target instruction stream. Calculating exact timing from the timing tables is

impossible because the tables cannot anticipate how the combination of factors will

influence every particular sequence of instructions. This is illustrated by comparing the

observed instruction timing from the prior four examples with instruction timing derived

from the instruction timing tables.

Table 8-2 lists the original instruction stream and the corresponding clock timing from the

appropriate timing tables for the best case, cache-only case, and worst case.

Table 8-2. Instruction Timings from Timing Tables

Instruction Best Case Cache Case Worst Case

#1) MOVE.L

#2) ADD.L

#3) MOVE.L

#4) ADD.L

D4,(A1)+

D4,D5

(A1),–(A2)

D5,D6

Total 10 15 21

Table 8-3 summarizes the observed instruction timings for the same instruction stream as

executed according to the assumptions of the four examples. For each example, Table 8-

3 shows which entry (BC/CC/WC) from the timing tables corresponds to the observed

timing for each of the four instructions. Some of the observed instruction timings cannot be

found in the timing tables and appear in Table 8-3 within parentheses in the most

appropriate column. These timings occur when instruction execution overlap dynamically

alters what would otherwise be a BC, CC, or WC timing.

Table 8-3. Observed Instruction Timings

Example 1 Example 2 Example 3 Example 4

Instruction BC CC WC BC CC WC BC CC WC BC CC WC