B.2. Register interlock examples

Table B.2 shows register interlock examples using LDR and ADD instructions.

LDR instructions take one cycle, have a result latency of two, and require their base register as a Very Early Reg.

ADD instructions take one cycle and have a result latency of one.

Table B.2. Register interlock examples

Instruction sequence

Behavior
LDR R1, [R2]
ADD R6, R5, R4
Takes two cycles because there are no register dependencies.
ADD R1, R2, R3
ADD R9, R6, R1
Takes two cycles because ADD instructions have a result latency of one.
LDR R1, [R2]
ADD R6, R5, R1
Takes three cycles because of the result latency of R1.
ADD R2, R5, R6
LDR R1, [R2]
Takes four cycles because of the use of the result of R2 as a Very Early Reg.
LDR R1, [R2]
LDR R5, [R1]
Takes four cycles because of the result latency of R1, the use of the result of R1 as a Very Early Reg, and the use of an LDR to generate R1.

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