How is data width determined for load/store instructions in Rocket Core?

Question

I'm working on a project where we need to modify a Rocket-chip core with new instructions. We're wondering: how does data width for load/store instructions is determined ?

For instance, LB/LH/LU have the same parameters set https://github.com/chipsalliance/rocket-chip/blob/master/src/main/scala/rocket/IDecode.scala#L88-L90
LB/LH/LU have different encoding (for instance https://github.com/chipsalliance/rocket-chip/blob/master/src/main/scala/rocket/Instructions.scala#L277).

I assume the instruction decoder will determine the data width thanks to the funct3 field but I do not see where this operation is performed in the Rocket decoder.

Any idea?

Looking at the Github repository (links above) + grep

JohnDoe · Accepted Answer · 2023-05-31T12:41:01.383

0

Found the answer by myself.

The instruction in binary format is read here: https://github.com/chipsalliance/rocket-chip/blob/v1.6/src/main/scala/rocket/RocketCore.scala#L470
https://github.com/chipsalliance/rocket-chip/blob/v1.6/src/main/scala/rocket/RocketCore.scala#L470. Bits 13 and 12 of the instruction are equal to the lowest bits of the func3 field which are different for each load width.
The highest bit of the func3 field is needed for unsigned operations.

edited May 31 '23 at 12:41

answered May 31 '23 at 12:40

JohnDoe

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How is data width determined for load/store instructions in Rocket Core?

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