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Simple Implementation

Learn how to build the datapath and control units required for executing instructions efficiently. This guide covers using multiplexors, selecting operations, controlling data flow, and more. Understand the ALU's operations, control inputs, and how to compute ALU control inputs based on instruction type. Dive into the hardware description and truth tables for optimal control implementation. Explore a simple combinational logic structure ensuring correct execution timing.

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Simple Implementation

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  1. Simple Implementation • Include the functional units we need for each instruction Why do we need this stuff?

  2. Building the Datapath • Use multiplexors to stitch them together

  3. Control • Selecting the operations to perform (ALU, read/write, etc.) • Controlling the flow of data (multiplexor inputs) • Information comes from the 32 bits of the instruction • Example: add $8, $17, $18 Instruction Format: 000000 10001 10010 01000 00000 100000 op rs rt rd shamt funct • ALU's operation based on instruction type and function code

  4. Control • e.g., what should the ALU do with this instruction • Example: lw $1, 100($2) 35 2 1 100 op rs rt 16 bit offset • ALU control input000 AND 001 OR 010 add 110 subtract 111 set-on-less-than • Why is the code for subtract 110 and not 011?

  5. ALUOp computed from instruction type Control • Must describe hardware to compute 3-bit ALU control input • given instruction type 00 = lw, sw 01 = beq, 11 = arithmetic • function code for arithmetic • Describe it using a truth table (can turn into gates):

  6. Control

  7. Control • Simple combinational logic (truth tables)

  8. 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

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