Instruction Cycle Descriptions

Step Name

Action for R-type

Action for Mem Ref

Branch

Instruction fetch

IR=Memory[PC]

PC=PC+4

Instruction decode/

A=Reg[IR[25:21]]

register fetch

B=Reg[IR[20:16]]

Target=PC+(xtnd(IR[15:0])<ɚ)

Execution, addr comp

ALUout=A op B

ALUout=A+

if (A==B) then

or branch completion

xtnd(IR[15:0]])

PC=Target

Memory access or R-

Reg[IR[15:11]=

memdata=ALUout

type completion

ALUout

or

Mem[ALUout]=B

Write back

Reg[IR[20:16]]=

memdata

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