CSCI341 Lecture 31, Control RECALL... The datapath is a - PowerPoint PPT Presentation

CSCI341 Lecture 31, Control

RECALL... • The “datapath” is a representation of the flow of information (data, instructions) through the CPU • Implemented as combination of circuitry and combinatorial & sequential chips • In order for machine code to cause the machine to “act” a certain way, there must be control circuitry (not magic).

CONTROL

OUR IMPLEMENTATION • Simplified instruction set: • lw, sw, beq, add/sub/AND/OR, j

ALU CONTROL ALU control Function 0000 and 0001 or 0010 add 0110 sub 0111 set on lt 1100 nor

ALU CONTROL ALU control Function “Hey ALU, 0010!” 0000 and 0001 or 0010 add “Ok. My output is equal to the sum of my inputs. 0110 sub Happy now?” 0111 set on lt 1100 nor

ALU CONTROL • lw and sw • ALU computes memory address via addition • beq • ALU computes address via subtraction • R-types • ALU performs one of five actions (eg, add ) per funct

ALU CONTROL (We’re not going to care about the logic specifically used to generate ALU control input from ALUOp and Funct)

ALU CONTROL Multiple levels of control decoding are used. Why?

ALU CONTROL

INSTRUCTION FORMAT TO CONTROL SIGNALS

IMPORTANT POINTS OF CONTROL

IMPORTANT POINTS OF CONTROL

CONTROL UNIT

INSTRUCTIONS TO CONTROL UNIT TO CONTROL SIGNALS

add $t1, $t2, $t3

lw $t1, offset($t2)

beq $t1, $t2, label

j somewhere

CONTROL OVERVIEW

SINGLE-CYCLE INSTRUCTION SET • Look at the PC, fetch the instruction from that address • “Read” one or two registers per the instruction • Carry out the instruction • Prepare PC for the next instruction

SINGLE-CYCLE INSTRUCTION SET What’s wrong with this?

HOMEWORK • Reading 27 • Project 7, “Mmm!” yes, this is what the reading is really trying to tell you