DEC

DEC (short for "DECrease") is the mnemonic for a machine language instruction which decrements the numerical value of the contents of the address specified, by one, and "wraps over" if the value goes below the numerical limits of a byte.

• If the byte in question is taken as an unsigned integer, DEC "counts down" from 255 thru 0/$FF thru $0. If a byte already containing the value 0/$0 is DECremented, it "wraps over" to the value 255/$FF.
• If the byte is taken as a signed integer, DEC will "count down" from +127 thru −128, or +$7F thru −$80. If a byte already holding the value −128/−$80 is DECremented, it "wraps over" to the value +127/$7F.

Addressing modes

Opcode		Addressing mode	Assembler format	Length in bytes	Number of cycles
Dec	Hex
206	CE	Absolute	DEC nnnn	3	6
222	DE	Absolute,X	DEC nnnn,X	3	7
198	C6	Zeropage	DEC nn	2	5
214	D6	Zeropage,X	DEC nn,X	2	6

DEC supports the four 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.

Although DEC does not support indexed-indirect and indirect-indexed addressing modes, DCP can be used to effectively implement these modes for DEC.

CPU flags

DEC affects 2 of the CPU's status flags:

• The negative status 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.