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ThaumicMekanism / venus

RISC-V instruction set simulator built for education
https://ThaumicMekanism.github.io/venus/
MIT License
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Circular shift behavior wrong #17

Closed MaxBanister closed 4 years ago

MaxBanister commented 4 years ago

It seems that for R-type shift instructions, if rs2 is greater than 31, the shift will actually be circular (not consistently but that's the general behavior).

Minimal example:

li x2, 0xBEEF; li x3, 0x128; sra x2, x2, x3

ThaumicMekanism commented 4 years ago

It’s behavior is correct. Is uses the lowest five bits in the rs2 register. From the RISC-V spec:

“SLL, SRL, and SRA perform logical left, logical right, and arithmetic right shifts on the value in register rs1 by the shift amount held in the lower 5 bits of register rs2.”