Datapath and control

Vocabulary

English	Meaning
datapath	Processor components and connections that store, carry and operate on instruction data.
ALU/ˌeɪ el ˈjuː/	The part of a processor that performs arithmetic and logical operations.
control/kənˈtrəʊl/	The switch settings per instruction: RegWrite, MemRead, MemWrite, MemToReg, ALUSrc, Branch — pure combinational logic on opcode and funct.

One clock, one instruction

The datapath is the plumbing: PC feeds instruction memory, the register file reads two sources, the ALU computes, data memory reads or writes, the result writes back, and the PC takes PC + 4 or a branch target.
Control is the switch settings per instruction: RegWrite, MemRead, MemWrite, MemToReg, ALUSrc, Branch — pure combinational logic on opcode and funct.

{exercise:uni-cs0602-path}

Practice

The single-cycle clock must wait for —

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All work fits in one clock, so the clock waits for the longest instruction: lw's path. With our parameters (IM 200, RF 100, ALU 200, DM 250, wiring 150 ps): $200 + 100 + 200 + 250 + 150 = 900$ ps per cycle.
100 instructions take $90\,000$ ps. Speeding up add would not help — the machine runs at its slowest instruction's pace.

{exercise:uni-cs0602-cycle}

Practice

With IM 200, RF 100, ALU 200, DM 250, wiring 150 ps, the lw cycle is —

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lw R4, 8(R1): RegWrite 1, MemRead 1, MemToReg 1, ALUSrc 1 (immediate to ALU), Branch 0, MemWrite 0.
sw R5, 12(R1): RegWrite 0, MemWrite 1, MemRead 0, MemToReg = x — a don't care: nothing writes back, so the bit changes nothing observable. Don't-cares are the designer's freedom.

{exercise:uni-cs0602-signals}

Practice

For lw, the ALUSrc control bit selects —

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