INC

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INC (short for "INCrease") is the mnemonic for a machine language instruction which increases the numerical value of the contents of the address specified by one, and "wraps over" when the numerical limits of a byte are exceeded.

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

Addressing modes[edit]

Opcode Addressing
mode
Assembler
format
Length
in bytes
Number of
cycles
Dec Hex
238 EE Absolute INC nnnn 3 6
254 FE Absolute,X INC nnnn,X 3 7
230 E6 Zeropage INC nn 2 5
246 F6 Zeropage,X INC nn,X 2 6

INC 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.

CPU flags[edit]

INC 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.