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Status Flag In Assembly8086

I am taking an undergraduate class in assembly right now. I have some discrepancies with how the TA graded my assignment. I was only testing the status of the carry, zero, sign, and overflow flags.

Here is one of the status flag problems I got wrong:

mov ah, 255
mov al, -1
cmp ah, al

My Solutions: CF=0, ZF=1, SF=0, OF=0

His solutions: CF=0, ZF=0, SF=1, OF=1

Can someone please help me verify which is correct? I don't think mine are wrong but if they are can you explain what answer is correct?

Solution

  • Here's what I get running this on VC 2013 (caveat, 32 bit inline assembly in C++)

    Before the 3 instructions, the flags are:

    OV = 0 UP = 0 EI = 1 PL = 0 ZR = 0 AC = 0 PE = 0 CY = 0

    After the 3 instructions

    OV = 0 UP = 0 EI = 1 PL = 0 ZR = 1 AC = 0 PE = 1 CY = 0

    Where the flags registers are as defined here

    Interpreted as Parity Set, Carry not set, zero is set, sign is not set, overflow is not set.

    Which seems to corroborate YOUR solution. The disassembly I get:

    __asm {
    mov ah, 255
00FC13CE  mov         ah,0FFh  
    mov al, -1
00FC13D0  mov         al,0FFh  
    cmp ah, al
00FC13D2  cmp         ah,al  
}

    My 8086 is terribly rusty, but FWIW:

    • It is unlikely that the Sign Flag would be set given there are no arithmetic, shift or logical operations. See @Jonathon's answer.
    • mov is done on the literals -1 (255) and 255 which wouldn't trigger overflow (no flags are set).
    • The zero flag should be set given the equality of 255 and -1 in an 8 bit register.

    So I'm guessing the TA might have copy pasted the wrong answer somewhere?