Combining datapaths: R-type and memory

Version 1: execute instruction in 1 clock cycle

No datapath resource can be used more than once in a single instruction

If needed more than once, must be duplicated

Separate instruction, data memory

Some resources can be shared between different instruction types

Need to have multiple inputs

Control signal to select which to use: multiplexor

Arithmetic-logical (R-type) and memory access (load/store) are similar in some ways

Differences

Second operand input to ALU:

Register contents for R-type

Sign-extended immediate value (offset) for load/store

Value stored in destination register:

ALU output for R-type

Data memory value for load

To combine datapaths:

Select source of second ALU operand

Select source of data to write to register

Use 2 MUXes with control inputs

ALUsrc for ALU

MemtoReg for register write

Fig. 5.11