mc68ec040v Freescale Semiconductor, Inc, mc68ec040v Datasheet - Page 297

• Manufacturer Part Number: mc68ec040v
• Description: M68040 User Manual
• Manufacturer: Freescale Semiconductor, Inc

The separation of calculation and execution in the <ea> calculate and execute stages
allows instruction reordering during compile time to take advantage of potential instruction
overlap. Figure 10-2 illustrates this overlap for an instruction requiring multiple clocks in
the execute stage and with an instruction with a long lead time. The execution time for
LEA (3
execution.
When the LEA (A) instruction precedes the ABCD (B) instruction, the execution stalls
during C4–C6 (equivalent to the LEA lead time) while the instruction completes in the
<ea> calculate and <ea> fetch stages. The resulting execution time for the LEA (A) and
ABCD (B) sequence is eight clocks.
However, if the LEA (C) instruction follows the ABCD (B) instruction, the LEA stalls in the
<ea> fetch instead, during C9–C11. The LEA then executes in a single clock in the
execution stage. The resulting execution time for the LEA (C) and ABCD (B) sequence is
five clocks.
10-6
L
+ 1) indicates that the instruction can be stalled three clocks without affecting
<ea> CALCULATE
NOTE: *Possible stalls in this stage.
WRITE-BACK
<ea> FETCH
EXECUTE
Figure 10-2. Instruction Overlap with Multiple Clocks
LABEL
P1
N1
N2
C1
P1
A
B
C
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C2
P1
A
TRAPF
LEA
ABCD
LEA
TRAPF
TRAPF
INSTRUCTION
C3
P1
A
A
M68040 USER’S MANUAL
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C4
$24(PC),A1
D0,D1
$24(PC),A1
A L
A
A
C5
A L
A
A
CALCULATE
C6
A L
B
A
<ea>
1
4
1
4
1
1
C7
C
B
A
C8
EXECUTE
C
C
B
3 L + 1
3 L + 1
1
3
1
1
C L1
C9
C
B
C10
C L2
C
B
C11
C L3
N1
C*
C12
N2
N1
C
C13
N2
N1
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