ADC
This is an article about the GB-Z80 CPU.
Opcodes:
Arithmetic and logical: ADD - ADD (16-bit) including LD HL,SP+nn - ADC - AND - CP - CPL - DAA - DEC - DEC and INC (16-bit) - INC - SBC - SUB - OR - XOR
Conditional: CALL - JP - JR - RET
Load: LD (8-bit) - LD (16-bit)
Extended Set: BIT - RES - RL - RLC - RR - RRC - SET - SLA - SRA - SRL - SWAP
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)
An alternative to this is:
half-carry = ((A & operand) | ((A ^ operand) & ~(A + operand + carry))) & $08
where 'operand' is the register, or immediate byte specified by the instruction.
ADC A, r
Machine Cycle Count: 1
r is a CPU register as specified in the table below.
| # | Register (in place of r) |
| 8F | A |
| 88 | B |
| 89 | C |
| 8A | D |
| 8B | E |
| 8C | H |
| 8D | L |
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.
