Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

U Type Instruction description doesn't need left shift by 12 #27

Open
iammadab opened this issue Dec 3, 2024 · 1 comment
Open

U Type Instruction description doesn't need left shift by 12 #27

iammadab opened this issue Dec 3, 2024 · 1 comment

Comments

@iammadab
Copy link

iammadab commented Dec 3, 2024

image

Given the image above the u-type immediate takes instruction[31:12] and places it at immediate[31:12], this implies the lowest 12 bits are already zeros (implicit left shift by 12).

This means lui and auipc descriptions can be reduced to the following:

// lui
rd = imm

// auipc
rd = PC + imm

instead of:
image

Another potential solution is to change the immediate builder description

instruction[31:12] -> immediate[19:0]
@jameslzhu
Copy link
Owner

I believe this ambiguity is the result of the immediate being encoded differently in the asm syntax and the instruction, in GNU / LLVM implementations. This was previously discussed here: riscv/riscv-isa-manual#452

The assembly syntax doesn't match the notation used in the ISA spec. This is unfortunate, but changing it would be too disruptive at this point. Concretely, lui rd, 0x1 writes the value 0x1000 to rd.
[...]
I was referring to syntax used by the GNU/LLVM assemblers, which the spec should also strive to match.

Also, the immediate builder description is copied directly from the ISA manual source. I would prefer not to change that (instruction[31:12] -> immediate[19:0]).

Would it be better to rename the C description immediate to ui_const to distinguish it from imm? The MIT RISCV course resolves this ambiguity this way.

// lui rd, ui_const
rd = ui_const << 12

// auipc rd, ui_const
rd = PC + (ui_const << 12)

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Assignees
No one assigned
Labels
None yet
Projects
None yet
Milestone
No milestone
Development

No branches or pull requests
2 participants
@jameslzhu @iammadab