RRA

RRA is the mnemonic for the illegal opcode machine language instruction which rotates one bit in memory then add memory to accumulator with carry. Bit 0 is shifted out into the carry flag and Carry flag is shifted into bit 7 by the ROR. Then an ADC is performed with memory and all flags are set according to the ADC.

This illegal opcode is a combination of two operations with the same addressing mode: ROR, ADC

Addressing modes

Opcode		Addressing mode	Assembler format	Length in bytes	Number of cycles
Dec	Hex
103	67	Zeropage	RRA nn	2	5
119	77	Zeropage,X	RRA nn,X	2	6
111	6F	Absolute	RRA nnnn	3	6
127	7F	Absolute,X	RRA nnnn,X	3	7
123	7B	Absolute,Y	RRA nnnn,Y	3	7
99	63	Indexed-indirect	RRA (nn,X)	2	8
115	73	Indirect-indexed	RRA (nn),Y	2	8

RRA 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.

CPU flags

RRA affects 4 of the CPU's status flags:

• The negative flag is set if the result is negative, i.e. has it's most significant bit set (not in decimal mode).
• The overflow flag is set if the operation results in an overflow (not in decimal mode).
• The zero flag is set if the result is zero, or cleared if it is non-zero (not in decimal mode).
• The carry flag is set if the addition resulted in an outgoing carry.