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My computer architecture books explains that

"Since writes to the register file are edge-triggered, our design can legally read and write the same register within a clock cycle: the read will get the value written in an earlier clock cycle, while the value written will be available to read in a subsequent clock cycle."

This makes some sense, and I somewhat understand what's going on with the register file. However, I don't understand when each event happens. Say we're reading from one of the 32 register files and writing to it in the same cycle. When would the register be read from? When would it be written to? I don't totally understand how events are triggered by the clock-edges, so it'd help to have that explained too. Thank you!

Maharlikans
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2 Answers2

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Reading the value of a register is asynchronous, whereas in the architecture you are working in your class, the registers are written sychronously (i.e. the writes are edge-triggered).

This means that you can read the current value of a register, apply some operation on it (e.g. add some immediate) and write the result at the next raising clock edge.

Suppose you want to issue an addiu $1, $1, 123, that is take the current value of $1, add 123 and store the result back in $1.

At the start of the clock cycle the control unit would instruct the register file to put the contents of $1 in one of the data buses that gets into the ALU. the control unit would also instruct to put the immediate 123 in the other data bus that also gets into the ALU. The addition which is just a combinatorial circuit implemented inside the ALU would compute the said addition and put the result in the data bus that connects the register file for storage. All of this is done before the raising edge of the clock happens and the result of the addition gets presented until the next raising edge. At some point the raising edge occurs and the result of the addition is now written back into register $1.

gusbro
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The register file is built from flip-flops. Each flip-flop has a store, an input, an output and a trigger. The output is always presenting the stored value, so can be read all the time. With a rising edge on the trigger, the input value moves into the store.

Conor OG
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