Uses
The carry flag is affected by the result of most arithmetic (and typically several bitwise) instructions and is also used as an input to many of them. Several of these instructions has two forms which either reads or ignores the carry. In assembly languages these instructions are represented by mnemonics such as ADD/SUB, ADC/SBC (ADD/SUB including carry), SHL/SHR (bit shifts), ROL/ROR (bit rotates), RCR/RCL (rotate through carry), and so on. The use of the carry flag in this manner enables multi-word add, subtract, shift, and rotate operations.
An example is what happens if one were to add 255 and 255 using 8-bit registers. The result should be 510 which is 1_1111_1110 in binary, requiring 9 bits. The 8 least significant bits always stored in the register would be 1111_1110 binary (254 decimal) but since there is carry out of bit 7 (the eight bit), the carry is set, indicating that the result needs 9 bits. The valid 9-bit result is the concatenation of the carry flag with the result. Note that in an 8-bit two's complement interpretation, this operation is −1 + −1 and yields the correct result of −2, with no overflow, even if the carry is ignored.
Another example may be an 8-bit register with the bit pattern 0101_0101 and the carry flag set; if we execute a rotate left through carry instruction, the result would be 1010_1011 with the carry flag cleared because the most significant bit (bit 7) was rotated into the carry while the carry was rotated into the least significant bit (bit 0).
While the first micro processors Intel 4004 (CLC) and Intel 8008 (FC) had specific operations to set the carry bit to 0, the later Intel 8080 and its compatible processors (such as the Z80) only had operations to set the carry bit to 1. For setting it to zero they use the logic instructions (AND, OR, XOR) which actually do not use the carry bit at all for their functions.
The carry flag is also often used following comparison instructions, which are typically implemented by subtractive operations, to allow a decision to be made about which of the two compared values is lower than (or greater or equal to) the other. Branch instructions which examine the carry flag are often represented by mnemonics such as BCC and BCS to branch if the carry is clear, or branch if the carry is set respectively. When used in this way the carry flag provides a mechanism for comparing the values as unsigned integers. This is in contrast to the overflow flag which provides a mechanism for comparing the values as signed integer values.
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