Assembly x86 registers signed or unsigned

by

The CPU does not know nor does it care. Instead, bits in the Flags register are set on certain operations, and how your program acts on those flags depends on what the source code told it to.

E.g.,

mov eax, 0FFFFFFFFh
test eax, eax
js isNegative

vs.

mov eax, 0FFFFFFFFh
test eax, eax
jb isNegative

The first jumps to ‘IsNegative’ because test sets the Sign Flag here. The second does not, because test resets the Carry Flag to 0 and jb only jumps if it is 1.

More Related Contents:

  1. Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?
  2. How do AX, AH, AL map onto EAX?
  3. What is the “FS”/”GS” register intended for?
  4. Why is there not a register that contains the higher bytes of EAX?
  5. What do the E and R prefixes stand for in the names of Intel 32-bit and 64-bit registers?
  6. Why is imul used for multiplying unsigned numbers?
  7. What are the names of the new X86_64 processors registers?
  8. How to determine if the registers are loaded right to left or vice versa
  9. Why isn’t the instruction pointer a normal register usable with MOV or ADD?
  10. x86 Calculating AX given AH and AL?
  11. Why are x86 registers named the way they are?
  12. What is the purpose of segment registers in x86 protected mode?
  13. Why can I access lower dword/word/byte in a register but not higher?
  14. How to know if a register is a “general purpose register”?
  15. Micro fusion and addressing modes
  16. Custom bootloader booted via USB drive produces incorrect output on some computers
  17. rbp not allowed as SIB base?
  18. Bomb lab phase 5 – 6 char string, movzbl load, and $0xf, %ecx, and index an array with that?
  19. execve shellcode writing segmentation fault
  20. double condition checking in assembly
  21. Printing hex values in x86 assembly
  22. What does the dollar sign ($) mean in x86 assembly when calculating string lengths like “$ – label”? [duplicate]
  23. Why not store function parameters in XMM vector registers?
  24. Weird performance effects from nearby dependent stores in a pointer-chasing loop on IvyBridge. Adding an extra load speeds it up?
  25. What is the default register state when program launches (asm, linux)?
  26. Difference between MOVDQA and MOVAPS x86 instructions?
  27. Where is VPERMB in AVX2?
  28. How do GNU assembler x86 instruction suffixes like “.s” in “mov.s” work?
  29. Purpose of ESI & EDI registers?
  30. What is register %eiz?

Leave a Comment