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:

mor1kx/rtl/verilog/mor1kx_execute_alu.v

Line 205 in 95eee05

assign carry_in = adder_do_sub_i | adder_do_carry_i & carry_i;

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? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#139 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAH3FMB7MWLJZCPURMQXVTLUASEABANCNFSM5DLSM7CA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.