DCP

DCP (short for "Decrease then ComPare") is a mnemonic for an illegal opcode machine language instruction.

This illegal opcode is a combination of two operations with the same addressing mode: DEC, followed by CMP

Function: {addr} = {addr} - 1; then compare A and {addr}

Addressing modes

Opcode		Addressing mode	Assembler format	Length in bytes	Number of cycles
Dec	Hex
207	CF	Absolute	DCP nnnn	3	6
223	DF	Absolute,X	DCP nnnn,X	3	7
219	DB	Absolute,Y	DCP nnnn,Y	3	7
199	C7	Zeropage	DCP nn	2	5
215	D7	Zeropage,X	DCP nn,X	2	6
195	C3	Indexed-indirect	DCP (nn,X)	2	8
211	D3	Indirect-indexed	DCP (nn),Y	2	8

DCP supports the 7 different addressing modes shown in the table at right. In the assembler formats listed, nn represents a single-byte (8-bit) figure, and nnnn is a two-byte (16-bit) address.

DCP can be used to effectively implement the indexed-indirect and indirect-indexed addressing modes that DEC lacks

CPU flags

DCP affects 3 of the CPU's status flags according to the CMP compare, after the decrement:

• The negative flag is set if the result is negative, i.e. has its most significant bit set.
• The zero flag is set if the result is zero, or cleared if it is non-zero.
• The carry flag is set or cleared depending on the result.

Examples

Decrementing a 16 bit pointer

DCP can be used in a simple trick to decrement a 16 bit pointer:

LDA #$FF
DCP pointer ; Decrease pointer low byte and compare result to A=#$FF
BNE +
DEC pointer+1 ; Decrease pointer high byte if underflow occurred
+

The standard implementation

LDA pointer
BNE +
DEC pointer+1
+
DEC pointer

consumes at least 1 more cycle and possibly 1 more byte in code size depending on the addressing mode to access pointer.

Links

• NMOS 6510 Unintended Opcodes (No More Secrets)