Why is the Carry Flag set during a subtraction when zero is the minuend?

by

Carry flag is carry or borrow out of the Most Significant bit (MSb):

CF (bit 0) Carry flag — Set if an arithmetic operation generates a carry or a borrow out of the mostsignificant bit of the result; cleared otherwise. This flag indicates an overflow condition for
unsigned-integer arithmetic. It is also used in multiple-precision arithmetic.

Don’t associate the CF with the sign bit, in a subtraction CF is set whenever the minuend, treated as unsigned, is less than the subtrahend, treated as unsigned.
This is equivalent to an overflow condition, for signed numbers the equivalent flag is OF.

For an (unnecessary?) visual clue, in this 4-5 operation, it is the second borrow, the red one, that set the CF

Borrow and CF

Not if you subtract from zero, it comes naturally that for any number, but zero itself, you’ll always have the CF set, as the subtrahend has at least one bit set.

Finally, some instructions let you change the sign bit without affecting the CF (see for example the logic operations or the behavior of neg).

More Related Contents:

  1. about assembly CF(Carry) and OF(Overflow) flag
  2. Division by subtraction [closed]
  3. Referencing the contents of a memory location. (x86 addressing modes)
  4. How to convert a binary integer number to a hex string?
  5. Which 2’s complement integer operations can be used without zeroing high bits in the inputs, if only the low part of the result is wanted?
  6. What is the stack engine in the Sandybridge microarchitecture?
  7. How do you use gcc to generate assembly code in Intel syntax?
  8. What do the E and R prefixes stand for in the names of Intel 32-bit and 64-bit registers?
  9. What is stack frame in assembly?
  10. Does the Intel Memory Model make SFENCE and LFENCE redundant?
  11. Assembly Language (x86): How to create a loop to calculate Fibonacci sequence
  12. MOV src, dest (or) MOV dest, src?
  13. problem in understanding mul & imul instructions of Assembly language
  14. x64 instruction encoding and the ModRM byte
  15. Random number in emu8086 without using DOS/BIOS calls?
  16. gas: too many memory reference
  17. Why do we need to disambiguate when adding an immediate value to a value at a memory address
  18. How to determine if ModR/M is needed through Opcodes?
  19. “enter” vs “push ebp; mov ebp, esp; sub esp, imm” and “leave” vs “mov esp, ebp; pop ebp”
  20. How to multiply a register by 37 using only 2 consecutive leal instructions in x86?
  21. What does “rep; nop;” mean in x86 assembly? Is it the same as the “pause” instruction?
  22. Convert string to int. x86 32 bit Assembler using Nasm
  23. Add 2 numbers and print the result using Assembly x86
  24. Multiplying arbitrarily long sequences of ASCII digits by a single-digit multiplier
  25. How to produce a minimal BIOS hello world boot sector with GCC that works from a USB stick on real hardware?
  26. Does Skylake need vzeroupper for turbo clocks to recover after a 512-bit instruction that only reads a ZMM register, writing a k mask?
  27. How to know if a register is a “general purpose register”?
  28. Assembly bubble sort swap
  29. x86 Assembly pushl/popl don’t work with “Error: suffix or operands invalid”
  30. how does push and pop work in assembly

Leave a Comment