0

I want to write a simple code (or algorithm) to set/clear overflow flag.can you help me?

ela he
  • 19
  • 3
  • 1
    http://stackoverflow.com/questions/1406783/flags-registers-can-we-read-or-write-them-directly – ABuckau Apr 27 '16 at 12:57

2 Answers2

2

Many instructions clear OF as a side effect, for example test. As such you can write something like test eax, eax and that will clear OF, although it will affect other flags. You didn't specify whether that's allowed.

To set OF you can perform some operation that will set it. For example, add 1 to the biggest positive number: mov al, 0x7f; add al, 1

Alternatively, you can make a copy of the flags on the stack using pushf, modify it to your liking and then get it back using popf.

Jester
  • 56,577
  • 4
  • 81
  • 125
  • You should consider deleting your answer here and moving it to the duplicate. Or flagging this question and asking a moderator to merge all of its answers into the duplicate. – Cody Gray - on strike Dec 09 '16 at 13:29
1

To set the Overflow flag(OF) - Bit 11 of EFLAGS use (in MASM syntax)

pushfd                   ; mov EFLAGS to stack
bts dword ptr [esp], 11  ; set OF flag
popfd                    ; write it back to status register

and, respectively, use the following to reset the OF flag:

pushfd                   ; mov EFLAGS to stack
btr dword ptr [esp], 11  ; reset OF flag
popfd                    ; write it back to status register
zx485
  • 28,498
  • 28
  • 50
  • 59
  • 3
    `or` or `and` with a memory destination is fewer uops than `bts [mem], imm` on Intel CPUs. Even with an immediate bit count, `bts` and friends with a memory destination aren't as efficient, so generally avoid them with memory operands other than when you need an atomic (`lock`) test-and-set. – Peter Cordes Apr 27 '16 at 14:35
  • `pushf` pushes that `FLAGS` register. To push `EFLAGS`, better use `pushfd`. – Nubok Mar 26 '18 at 14:04
  • 1
    ["The PUSHF (push flags) and PUSHFD (push flags double) mnemonics reference the same opcode...."](http://www.felixcloutier.com/x86/PUSHF:PUSHFD:PUSHFQ.html). I changed the answer anyway to improve the emphasis. – zx485 Mar 27 '18 at 00:30
  • 2
    @Nubok: even if it did only push FLAGS, `OF` is in the low 16 bits of EFLAGS, i.e. FLAGS, so this code works. `btr` and `popf` are both quite slow, and `btr dword ptr[]` isn't guaranteed to load/store a whole dword anyway, so you could get a store-forwarding stall either way. `or` or `and mem, imm32` to set or clear might take an extra 2 or 3 code bytes, though. But if you want min code size to set `OF` without clobbering regs, `push eax` / `xor eax,eax` / `sub al, -128` (2 bytes) / `pop eax` should do it in 6B total, vs. 7B for this answer which also avoids clobbering any other flags. – Peter Cordes Mar 27 '18 at 02:44
  • @zx485 Based on your source you it indeed seems to be a common convention that these instructions are treated as synonyms to each other. I had the other convention that is mentioned there in mind: "The PUSHF instruction is intended for use when the operand-size attribute is 16 and the PUSHFD instruction for when the operand-size attribute is 32. Some assemblers may force the operand size to 16 when PUSHF is used and to 32 when PUSHFD is used." – Nubok Mar 27 '18 at 11:01