65816 Instruction Set: Addressing modes

Introduction

The address bus is 24 bits: a bank byte and a 16-bit offset. Most modes give only the 16-bit offset and take the bank from a register: the data bank register DB for data, the program bank register PB for jumps and branches, and bank 0 for the direct page, the stack and the vectors. The long modes carry the bank in the instruction. When an offset plus an index passes $FFFF it carries into the bank, except in program-counter arithmetic, which wraps inside the bank.

The modes

“Bytes” is the length of an instruction that uses the mode, native mode with M and X set. A 16-bit memory operand (M or X clear) is read from the address and the next one, low byte first.

Mode	Syntax	Bytes	The address
Implied	`clc`	1	The operand is a register or flag the opcode names.
Accumulator	`asl a`	1	The accumulator is the operand.
Immediate	`lda #$12`	2, or 3	The operand is in the instruction. The length is 3 for LDA, ADC, SBC, AND, ORA, EOR, CMP, BIT when M is clear and for LDX, LDY, CPX, CPY when X is clear; REP, SEP, COP, BRK and WDM always have one byte.
Direct page	`lda $12`	2	Bank 0, address D + the operand byte.
Direct page indexed with X	`lda $12,x`	2	Bank 0, D + operand + X.
Direct page indexed with Y	`ldx $12,y`	2	Bank 0, D + operand + Y (LDX and STX only).
Direct page indirect	`lda ($12)`	2	The 16-bit pointer at bank 0, D + operand, in the data bank.
Direct page indexed indirect	`lda ($12,x)`	2	The pointer at bank 0, D + operand + X, in the data bank.
Direct page indirect indexed	`lda ($12),y`	2	The pointer at bank 0, D + operand, in the data bank, plus Y.
Direct page indirect long	`lda [$12]`	2	A 24-bit pointer at bank 0, D + operand: the bank comes from the third byte.
Direct page indirect long indexed	`lda [$12],y`	2	The 24-bit pointer plus Y.
Absolute	`lda $1234`	3	The data bank, the 16-bit operand. For JMP and JSR the program bank.
Absolute indexed with X	`lda $1234,x`	3	DB:operand + X.
Absolute indexed with Y	`lda $1234,y`	3	DB:operand + Y.
Absolute long	`lda $123456`	4	The 24-bit operand.
Absolute long indexed with X	`lda $123456,x`	4	The 24-bit operand + X.
Absolute indirect	`jmp ($1234)`	3	JMP only: the 16-bit pointer at bank 0, offset operand; the jump stays in the program bank.
Absolute indexed indirect	`jmp ($1234,x)`	3	JMP and JSR: the pointer at PB:operand + X, in the program bank.
Absolute indirect long	`jml [$1234]`	3	JML only: a 24-bit pointer at bank 0, offset operand.
Stack relative	`lda $03,s`	2	Bank 0, S + the operand byte: a local variable on the stack.
Stack relative indirect indexed	`lda ($03,s),y`	2	The pointer at S + operand, in the data bank, plus Y.
Relative	`bne $8010`	2	PC after the instruction + the signed operand byte, wrapping inside the program bank.
Relative long	`brl $8010`	3	PC after the instruction + the signed 16-bit operand (BRL and PER).
Block move	`mvn $7e,$7f`	3	Two bank bytes: the instruction’s second byte is the destination bank, the third the source bank; the assembler writes the source first.

pea $1234 (absolute) and pei ($12) (direct page indirect) use these modes’ syntax, but push the address itself (PEA) or the 16-bit word the direct page holds (PEI) and read no data from it; see PEA.

Cycle tags

On the instruction pages each form is mode opcode, bytes, cycles. The cycles are the data sheet’s base count; a tag after it names an extra cost that depends on the state:

Tag	Adds	When
`+M`	1 cycle	M is clear: a 16-bit accumulator or memory operand (also PHA and PLA).
`+2M`	2 cycles	M is clear, for a read-modify-write instruction on memory: it reads and writes the extra byte.
`+X`	1 cycle	X is clear: a 16-bit index register operand (LDX, LDY, STX, STY, CPX, CPY, PHX, PHY, PLX, PLY).
`+DL`	1 cycle	The low byte of the direct page register is not zero (every direct page mode, and PEI).
`+P`	1 cycle	The index crosses a page boundary, or X is clear (16-bit index). A store always takes this cycle, so its base count already includes it.
`+T`	1 cycle	A conditional branch is taken.
`+E`	1 cycle	In emulation mode only: a taken branch (BRA always) crosses a page boundary.

BRK and COP take one cycle fewer in emulation mode (7, not 8), and RTI (6, not 7). A block move takes 7 cycles for each byte it moves, and runs until C reaches $FFFF after the byte, so it moves C + 1 bytes. REP and SEP take 3 cycles, XBA 3, WAI and STP 3 to start, other register instructions 2.

Not described

Known, not yet covered

Reference: how each mode wraps at the end of the bank or the direct page (a direct page pointer at $FF, a stack pointer at $0000), which the data sheet describes in its notes on address wrapping. The explorer does not model memory.

65816 Instruction Set – Addressing modes

Introduction

The modes

Cycle tags

Not described

Known, not yet covered

Links

Reference