Slide 7.7: Flags affected by addition and subtraction (cont.)

Flags Affected by Addition and Subtraction (Cont.)

`0FFh + 1`	`80h – 1`	`80h + 0FFh`
.data X SBYTE 0FFh .code add X, 1 call DumpRegs jc L1 call WaitMsg L1: movsx eax, X call WriteHex	.data X BYTE 80h .code sub X, 1 call DumpRegs jc L1 call WaitMsg L1: movsx eax, X call WriteHex	.data X BYTE 80h .code add X, 0FFh call DumpRegs jc L1 call WaitMsg L1: movsx eax, X call WriteHex
Output	Output	Output
CF= SF= ZF= OF= AF= PF= EAX=	CF= SF= ZF= OF= AF= PF= EAX=	CF= SF= ZF= OF= AF= PF= EAX=

Overflow occurs when two positive operands generate a negative sum or two negative operands generate a positive sum.
Overflow never occurs when the signs of two addition operands are different.
The NEG instruction can produce an invalid result if the destination operand can not be stored correctly.
Applying NEG to a nonzero operand always sets the Carry flag.

`–128 – 1`	`1 – 2`	`–(–128)`
.data X SBYTE -128 .code sub X, 1 call DumpRegs jc L1 call WaitMsg L1: movsx eax, X call WriteHex	.data X SBYTE 1 .code sub X, 2 call DumpRegs jc L1 call WaitMsg L1: movsx eax, X call WriteHex	.code sub eax, eax mov al, -128 neg al call DumpRegs jo L1 call WaitMsg L1: call WriteHex
Output	Output	Output
CF= SF= ZF= OF= AF= PF= EAX=	CF= SF= ZF= OF= AF= PF= EAX=	CF= SF= ZF= OF= AF= PF= EAX=