Difference between revisions of "ADC"

Revision as of 02:04, 25 October 2010

Add With Carry

This opcode group adds the contents of the specified byte-valued input and the register F carry flag (Flag computed as a 1 or a 0) to the contents of register A, to be stored in register A. The formula can be seen as A=A+r+CY. Since register A is an 8-bit register, you must make sure it wraps around appropriately to only contain an 8-bit value in the range of 0-0xFF (0-255).

The ADC operation code groups are known as follows:
1) ADC A, r
2) ADC A, n
3) ADC A, (HL)

Register F Flag Computation

The subtract flag bit is always reset to 0 for any ADC operation. The zero flag bit is set to 1 if the operation sets register A to 0, otherwise the zero flag is reset to 0. The carry flag bit is set to 1 if the operation causes a mathematical carry from bit 7. The half-carry flag bit is set to 1 if the operation causes a mathematical carry from bit 3.

The half-carry flag can be tricky sometimes to set properly. For this operation it can be recommended that you use some form of the equation below.

half-carry = ((lower nibble of CPU register A) + (lower nibble of the input register) + (carry flag (if set, put 0x1)) > 0xF) ? (boolean true) : (boolean false)

ADC A, r

Machine Cycle Count: 1
r is a CPU register as specified in the table below.

ADC A, n

Machine Cycle Count: 2
OP Code #CE

n is the value of the byte of memory stored at a certain location, where the 16-bit address of the memory is the program counter. You must increment the program counter after the operation and wrap to a 16-bit length as appropriate.

ADC A, (HL)

Machine Cycle Count: 2
OP Code #8E

(HL) is the value of the byte of memory stored at a certain location, where the address is the 16-bit value of registers H and L, being that register H is the upper byte, and register L is the lower byte.