Condition Code

Apr 3, 2023

CPU Instruction for flow control

All 1-bit flags, not set directly

• CF: Carry Flag (for unsigned)
• SF: Sign Flag (for signed)
• ZF: Zero Flag
• OF: Overflow Flag (for signed)

Used for compare two values
cmpq SRC2, SRC1 : Reversed order
cmpq b,a like computing a-b without setting destination

• CF set if carry out from most significant bit (used for unsigned comparisons)
• ZF set if a == b
• SF set if (a-b) < 0 (as signed)(negative)
• OF set if two’s-complement (signed) overflow (a>0 && b<0 && (a-b)<0) || (a<0 && b>0 && (a-b)>0)

testq Src2, Src1
testq b,a like computing a&b without setting destination

Sets condition codes based on value of Src1 & Src2
Useful to have one of the operands be a mask

• ZF set when a&b == 0
• SF set when a&b < 0

Use set to access condition code