The carry flag implementation is not correct for subtract instructions

[Rahul-Kande]

The carry flag value is not as expected for some cases. For instance, with the following trigger input, since r1 > r3, carry flag should be reset, but it is actually being set.

Issue location: rtl/verilog/mor1kx_execute_alu.v

Triggering input: //set r1=00020000 and r3=00002000 l.sub r4,r1,r3 Expected output: Carry flag = 0

mor1kx output: Carry flag = 1

Please check this bug.

[rkrajnc]

I can't test this right now, so I'm just guessing, but it seems the problem might be operator precedence in this line: https://github.com/openrisc/mor1kx/blob/95eee0596a160ffdfd8ee6bc8b88268b2e49ec5e/rtl/verilog/mor1kx_execute_alu.v#L205

Besides, shouldn't the statement be using logical operators instead of bitwise?

[skristiansson]

I can't test this right now, so I'm just guessing, but it seems the problem might be operator precedence in this line:

https://github.com/openrisc/mor1kx/blob/95eee0596a160ffdfd8ee6bc8b88268b2e49ec5e/rtl/verilog/mor1kx_execute_alu.v#L205

No, the precedence is correct, for subtraction you want to do the (2 complement) calculation: a - b => a + ~b + 1 carry_in is used as the '1', thus it always need to be set for subtraction.

Besides, shouldn't the statement be using logical operators instead of bitwise?

They are all 1-bit signals, so bitwise and logical obtain the same result.

On Mon, Sep 6, 2021 at 12:40 PM Rok Krajnc @.***> wrote:

I can't test this right now, so I'm just guessing, but it seems the problem might be operator precedence in this line:

https://github.com/openrisc/mor1kx/blob/95eee0596a160ffdfd8ee6bc8b88268b2e49ec5e/rtl/verilog/mor1kx_execute_alu.v#L205

Besides, shouldn't the statement be using logical operators instead of bitwise?

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