Register file: writing

Reading from a register file is only part of the story

Writing to a register file

Control input called write enable (WE)

When WE = 1, then we want to write to the register file

Once we add R[2] to R[3], we need to save the result to destination register R[1]

Require 5 bits to indicate the destination register (DST addr)

Can come from IR: bits B15-11 are used for destination register (R-type)

Need 32 bits of data to write to the register (DST data)

Data comes from the output of the ALU

How the register file looks now:

Summary:

Inputs

SRC 1 Addr: index of the first source register (5 bits)

SRC 2 Addr: index of the second source register (5 bits)

DST Addr: index of the destination register (5 bits)

DST Data: data to be written to the destination register (32 bits)

WE: DST Data is written to the register at index DST Addr only when WE = 1.

Outputs

SRC 1 Data: contents of the first source register, as specified by SRC 1 Addr.

SRC 2 Data: contents of the second source register,

as specified by SRC 2 Addr.