[PDF] 6502 sbc carry flag

The operation of the carry flag in the 6502 architecture is like an active low borrow flag. In other words, when the carry is 0, a borrow has occurred. The operation of the SBC instruction is: A = A + ~M + C.
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  • What is subtract with carry in 6502?

    The 6502 has a SBC operation (subtract with carry) that subtracts two numbers and also subtracts the borrow bit. If the (unsigned) operation results in a borrow (is negative), then the borrow bit is set. However, there is no explicit borrow flag - instead the complement of the carry flag is used.

  • What triggers the carry flag?

    1. The carry flag is set if the addition of two numbers causes a carry out of the most significant (leftmost) bits added. 1111 + 0001 = 0000 (carry flag is turned on) 2. The carry (borrow) flag is also set if the subtraction of two numbers requires a borrow into the most significant (leftmost) bits subtracted.

  • How do you calculate carry flag?

    The carry flag is set according to this addition, and subtract with carry computes a+not(b)+C, while subtract without carry acts as if the carry bit were set. The result is that the carry bit is set if a?b, and clear if a<b.
  • Assuming intel x86 assembler: Both ADD and ADC will set the Carry Flag on high-order bit carry or borrow and it will be cleared otherwise. Using ADC when the CF is 1 and there is no overflow, will result in CF=0.
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