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Questions - eflags

Why is the Overflow-Flag only set when single shifts are used?

In the x86 intel reference manual it says: "The overflow flag is set only if the single-shift forms of the instruction are used. [...]" But when I have the following scenario: xor eax, eax...
test-img

stht55

assembly

x86

cpu-architecture

bit-shift

eflags

Votes: 0

Answers: 1

Latest Answer

OF=undefined for shift counts other than 1; results in practice depend on your CPU. See below for my theory of how it's set on my Intel CPU. This design decision makes some sense, letting the hardw...
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Peter Cordes

How can x86 arithmetic in 8 bit mode and update the flags?

I want simulate 8051 using the x86 instruction. In 32 bits mode, I have to rotate left to get the carry flag. it is more steps to get the overflow flag. Do you know how to AL, BL, CL, DL, to do arithm...
test-img

qjpdzjcb

assembly

x86

8051

8-bit

eflags

Votes: 0

Answers: 1

Latest Answer

Thank you very much. "ADD AL, CL" work. I am not quite familiar with x86 ISA, because of 8bit 16bit 32bit 64bit and its compatibility. I try ADD instruction to see the flags changing all day...
test-img

qjpdzjcb

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