65C02 Reference Manual

Table of Contents:

1. General Overview
2. Instruction Set
3. Addressing Modes
4. Instruction Encodings
5. Examples

The 65C02 has a 16 bit address space (64K) arranged in 256 pages each of which is 256 bytes long.

Page 0 ($0000-$00FF) has some special properties involving addressing modes and is very valuable memory.

Page 1 ($0100-$01FF) is the stack.

Bytes $FFFA-$FFFB are the NMIB interrupt vector.
Bytes $FFFC-$FFFD are the reset vector.
Bytes $FFFE-$FFFF are the IRQ/BRK vector.

The 65C02 has 3 registers, stack pointer, processor flags (P), and program counter. The registers are the Accumulator (A), X index, and Y index. Each is 8 bits wide. Most instructions leave results in the accumulator. The stack pointer is an 8 bit register that is used as an offset into the stack (page 1). It is auto incrementing and decrementing when used with the push and pull (pop) instructions. You can also directly access and modify it's value via the TSX and TXS instructions.

The P register contains the following one bit flags:

Negative (N)

Set when the result of an operation leaves the high bit set. Cleared otherwise.

Overflow (V)

Set when there is a borrow or carry out of an operation.

Break (B)

Set when a 'BRK' instruction is encountered.

Decimal (D)

When set all arithmatic is BCD (ie. 09+01=10). When clear all arithmatic is binary 2s complement (ie. 09+01=0A).

Interrupt Disable (I)

When set no interrupts can occur.

Zero (Z)

Set when the result of an operation is zero. Cleared otherwise.

Carry (C)

Set when there is a carry. (more here)

Notes:

1. The BIT instruction copies bit 6 to the V flag, and bit 7 to the N flag (except in immediate addressing mode where V & N are untouched.) The accumulator and the operand are ANDed and the Z flag is set appropriately.
2. The BCC & BCS instructions instructions are sometimes known as BLT (branch less than) and BGE (branch greater or equal), respectively.

Implied Addressing [Implied]

In the implied addressing mode, the address containing the operand is implicitly stated in the operation code of the intruction.

Accumulator Addressing [Accumulator]

This form of addressing is represented with a one byte instruction and implies an operation on the accumulator.

Immediate Addressing [Immediate]

With immediate addressing, the operand is contained in the second byte of the instruction; no further memory addressing is required.

Absolute Addressing [Absolute]

For absolute addressing, the second byte of the instruction specifies the eight low-order bits of the effective address, while the third byte specifies the eight high-order bits. Therefore, this addressing mode allows access to the total 64K bytes of addressable memory.

Zero Page Addressing [Zero Page]

Zero page addressing allows shorter code and execution times by only fetching the second byte of the instruction and assuming a zero high address byte. The careful use of zero page addressing can result in significant increase in code efficiency.

Absolute Indexed Addressing [Absolute,X or Absolute,Y]

Absolute indexed addressing is used in conjunction with X or Y index register and is referred to as "Absolute, X," and "Absolute, Y." The effective address is formed by adding the contents of X or Y to the address contained in the second and third bytes of the instruction. This mode allows the index register to contain the index or count value and the instruction to contain the base address. This type of indexing allows any location referencing and the index to modify multiple fields, resulting in reduced coding and instruction time.

Zero Page Indexed Addressing [Zero Page,X or Zero Page,Y]

Zero page indexed addressing is used in conjunction with the index register and is referred to as "Zero Page, X" or "Zero Page, Y." The effective address is calculated by adding the second byte to the contents of the indexed register. Since this is a form of "Zero Page" addressing, the content of the second byte references a location in page zero. Additionally, due to the "Zero Page" addressing nature of this mode, no carry is added to the high-order eight bits of memory, and crossing of page boundaries does not occur.

Relative Addressing [Relative]

Relative addressing is used only with branch instructions; it establishes a destination for the conditional branch. The second byte of the instruction becomes the operand which is an "Offset" added to the contents of the program counter when the program counter is set at the next instruction. The range of the offset is -128 to 127 bytes from the next instruction.

Zero Page Indexed Indirect Addressing [(Zero Page,X)]

With zero page indexed indirect addressing (usually referred to as indirect X) the second byte of the instruction is added to the contents of the X index register; the carry is discarded. The result of this addition points to a memory location on page zero whose contents is the low-order eight bits of the effective address. The next memory location in page zero contains the high-order eight bits of the effective address. Both memory locations specifying the high- and low-order bytes of the effective address must be in page zero.

Absolute Indexed Indirect Addressing [(Absolute,X)]

(Jump Instruction Only) With absolute indexed indirect addressing the contents of the second and third instruction bytes are added to the X register. The result of this addition, points to a memory location containing the low-order eight bits of the effective address. The next memory location contains the higher-order eight bits of the effective address.

Indexed Indirect Addressing [(Zero Page),Y]

This form of addressing is usually referred to as Indirect, Y. The second byte of the instruction points to a memory location in page zero. The contents of this memory location are added to the Y index register, the result being the low-order eight bits of the effective address. The carry from this addition is added to the contents of the next page zero memory location, the result being the high-order eight bits of the effective address.

Zero Page Indirect Addressing [(Zero Page)]

In the zero page indirect addressing mode, the second byte of the instruction points to a memory location on page zero containing the low-order byte of the effective address. The next location on page zero contains the high-order byte of the effective address.

Absolute Indirect Addressing [(Absolute)]

(Jump Instruction Only) The second byte of the instruction contains the low-order eight bits of a memory location. The high-order eight bits of that memory location is contained in the third byte of the instruction. The contents of the fully specified memory location is the low-order byte of the effective address. The next memory location contains the high-order byte of the effective address which is loaded into the 16 bit program counter.