UM0434 datasheet(212/390 Pages) STMICROELECTRONICS

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UM0434

Instruction pipeline and execution timing

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Back-to-back load/store instructions are executed in a pipelined fashion, provided that their

EA calculations are not dependent on their previous load instructions.

Figure 18 shows the

basic pipeline flow for two back-to-back load instructions. In this case, the second load does

not depend on its previous load data for its EA calculation. Notice that the memory access of

the first load instruction overlaps in time with the EA calculation of the second load

instruction.

Figure 18.

Back-to-Back load instructions

When a load is followed by a load or a store instruction that depends on the first load data

for EA calculation, a pipeline stall is incurred.

Figure 19 shows the instruction flow for a load

instruction followed by a dependent store instruction through EA calculation. The second

store instruction in this case is dependent on the first load instruction for its EA calculation.

Figure 19.

A load followed by a dependent store instruction

A store instruction that depends on its previous load for its stored data, does not stall the

pipeline.

8.3.5

Change-of-Flow instruction pipeline operation

A branch instruction takes either one or 2 cycles to execute. Simple change of flow

instructions require 2 cycles to refill the pipeline with the target instruction for taken

branches and branch and link instructions with no prediction.

For branch-type instructions, in some situations, this 2-cycle timing may be reduced by

performing the target fetch speculatively while the branch instruction is still being fetched

into the instruction buffer. The branch target address is obtained from the BTB. The resulting

branch timing reduces to a single clock when the target fetch is initiated early enough and

the branch is taken.

Figure 20 shows basic pipeline flow for branch instructions.

DEC / EA

MEM

First Load Instruction

Time Slot

Second Load Instruction

DEC / EA

MEM

IFETCH

DECODE

EXECUTE

First Load Instruction

Time Slot

Second Store Instruction

DECODE

EA Calc

MEM

IFETCH

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