ARM R4 Manuale Utente
Cycle Timings and Interlock Behavior
ARM DDI 0363E
Copyright © 2009 ARM Limited. All rights reserved.
14-8
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14.3.3
Example interlocks
Most data processing instructions are single-cycle and can be executed back-to-back without
interlock cycles, even if there are data dependencies between them. The exceptions to this are
when shifts are used.
interlock cycles, even if there are data dependencies between them. The exceptions to this are
when shifts are used.
Shifter
The registers that the shifter requires are Early Regs and require an additional cycle of result
availability before use. For example, the following sequence introduces a 1-cycle interlock, and
takes three cycles to execute:
availability before use. For example, the following sequence introduces a 1-cycle interlock, and
takes three cycles to execute:
ADD R1,R2,R3
ADD R4,R5,R1 LSL #1
ADD R4,R5,R1 LSL #1
The second source register, which is not shifted, does not incur an extra data dependency check.
Therefore, the following sequence takes two cycles to execute:
Therefore, the following sequence takes two cycles to execute:
ADD R1,R2,R3
ADD R4,R1,R9 LSL #1
ADD R4,R1,R9 LSL #1
Register controlled shifts
The register containing the shift distance is an Early Reg. For example, the following sequence
takes three cycles to execute:
takes three cycles to execute:
ADD R1, R2, R3
ADD R4, R2, R4, LSL R1
ADD R4, R2, R4, LSL R1