• Table of Contents

Building a Datapath

The components of a datapath include:

1. Register File
2. Instruction Memory and Data Memory
3. Arithmetic Logic Unit (ALU)
4. Program Counter
5. Multiplexers

As a review, let’s go over the different instruction formats. Remember that all instructions are 32-bit.

Figure 6.1 Basic instruction format.

• R-type
• I-type
• J-type

Register File

The register file is a collection of readable/writeable registers. It contains the 32 MIPS general-purpose registers.

• It has two outputs, $D_s$ and $D_t$, to read data. These are the contents of $r_s$ and $r_t$ respectively.
• It has one central input, $D_d$. It only writes to the register if $r_d\_w$ is high. It is guarded with a mux with multiple inputs:
  • Output from the ALU, $\textrm{ALU\_out}$.
  • Output from data memory, $\textrm{data\_mem}$.
  • $PC + 4$ to save return location during jal.
  • ALU’s negative output, upper zero extended, $\{31\ 0's, neg\}$.

Figure 6.2 Register file.

Instruction Memory