What does the “rep stos” x86 assembly instruction sequence do?

by

For ecx repetitions, stores the contents of eax into where edi points to, incrementing or decrementing edi (depending on the direction flag) by 4 bytes each time. Normally, this is used for a memset-type operation.

Usually, that instruction is simply written rep stosd. Experienced assembly coders know all the details mentioned above just by seeing that. 🙂

ETA for completeness (thanks PhiS): Each iteration, ecx is decremented by 1, and the loop stops when it reaches zero. For stos, the only thing you will observe is that ecx is cleared at the end. But, for scas or the like, where the repz/repnz prefixes are used, ecx can be greater than zero if the operation stopped before exhausting ecx bytes/words/whatevers.

Before you ask, scas is used for implementing strchr-type operations. 😛

More Related Contents:

  1. How do I print an integer in Assembly Level Programming without printf from the c library?
  2. How do AX, AH, AL map onto EAX?
  3. How to represent hex value such as FFFFFFBB in x86 assembly programming?
  4. Basic use of immediates vs. square brackets in YASM/NASM x86 assembly
  5. What does `rep ret` mean?
  6. Why is there not a register that contains the higher bytes of EAX?
  7. What is the function of the push / pop instructions used on registers in x86 assembly?
  8. Why isn’t my root directory being loaded? (FAT12)
  9. How to make the kernel for my bootloader?
  10. Assembly (x86): db ‘string’,0 does not get executed unless there’s a jump instruction
  11. about assembly CF(Carry) and OF(Overflow) flag
  12. Why is imul used for multiplying unsigned numbers?
  13. Assembly Language – How to do Modulo?
  14. What does “DS:[40207A]” mean in assembly?
  15. Does it matter where the ret instruction is called in a procedure in x86 assembly
  16. Does cmpxchg write destination cache line on failure? If not, is it better than xchg for spinlock?
  17. What is the point of SSE2 instructions such as orpd?
  18. What’s the purpose of the rotate instructions (ROL, RCL on x86)?
  19. Why flush the pipeline for Memory Order Violation caused by other logical processors?
  20. Using ymm registers as a “memory-like” storage location
  21. Why isn’t the instruction pointer a normal register usable with MOV or ADD?
  22. How to tell the length of an x86 instruction?
  23. Optimize for fast multiplication but slow addition: FMA and doubledouble
  24. How to force NASM to encode [1 + rax*2] as disp32 + index*2 instead of disp8 + base + index?
  25. How to tell the length of an x86 instruction?
  26. Arithmetic identities and EFLAGS
  27. SAR command in X86 assembly with one parameter
  28. Addressing Modes in Assembly Language (IA-32 NASM)
  29. What’s difference between number with $ or without $ symbol in at&t assembly syntax?
  30. Is processor can do memory and arithmetic operation at the same time?

Leave a Comment