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Control

Control. New Instructions. addi $t0, $t1, 20 bne $t0, $t1, out jal out (see p. 372 for jump). Control. Simple combinational logic (truth tables). Our Simple Control Structure. All of the logic is combinational We wait for everything to settle down, and the right thing to be done

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Control

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  1. Control

  2. New Instructions • addi $t0, $t1, 20 • bne $t0, $t1, out • jal out (see p. 372 for jump)

  3. Control • Simple combinational logic (truth tables)

  4. Our Simple Control Structure • All of the logic is combinational • We wait for everything to settle down, and the right thing to be done • ALU might not produce “right answer” right away • we use write signals along with clock to determine when to write • Cycle time determined by length of the longest path We are ignoring some details like setup and hold times

  5. Single Cycle Implementation • Calculate cycle time assuming negligible delays except: • memory (2ns), ALU and adders (2ns), register file access (1ns)

  6. Single Cycle Implementation • Calculate cycle time assuming negligible delays except: • memory (2ns), ALU (2 ns), adder for PC+4 ( X ns), , adder for branch address computation (Y ns), register file access (1ns) • Consider: • X = 3, Y = 3 • X = 5, Y = 5 • X = 1, Y = 8

  7. Where we are headed • Single Cycle Problems: • what if we had a more complicated instruction like floating point? • wasteful of area • One Solution: • use a “smaller” cycle time • have different instructions take different numbers of cycles • a “multicycle” datapath:

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