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CSC258H1 (46)


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University of Toronto St. George
Computer Science
Steve Engels

o Datapath control hen input A to ALU is from the PC  These instructions are executed by turning various parts of the  If value = 1 then input A to ALU is from register file datapath on and off to direct the flow of data form the correct  ALUSrcB (2 wires) source to the correct destination  If value = 0 then input B to ALU is from register file o CONTROL UNIT – tells the processor to turn on these various  If value = 1 then input B to ALU is a constant value of 4 componenets at the correct times  If value = 2 then input B to ALU is from instruction register  Control unit takes in the opcode from the current instruction, and sends signals to the rest of the processor  If value = 3 then input B to ALU is from shifted instruction register  Within the control unit is a finite state machine that can occupy  RegWrite multiple clock cycles for a single instruction  The processor is writing to the register file  The control unit send out different signals on each clock  RegDst cycle, to make the overall operation happen  Which part of the instruction is providing the destination  The control unit sends signals (orange lines) to various processor address for a register write (rt vs. rd) componenets to enact all possible operations  If R-type instruction want rd; then RegDst = 1 o Control unit signals  PCWrite  If I-type instruction want rt; then RegDst = 0  Can be X if not writing to registers  Write the ALU output to the PC o ALUOut  Temporary stores data for a long operation  PCWriteCond  Ex. first clock cycle find the address  Write the ALU output to the PC only if the Zero condition has been met  Second cycle process the data and put it at the memory address in ALUOut  For branching, ALU comparison, usually test conditions  PCSource = 0 value directly from ALU, not synced to clock for registers; if condition meet  Zero high  May flipflop –e
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