6502 Instruction Set – Data transfer and the stack

Loads, stores, register transfers and the stack

Introduction

These instructions move bytes between the registers and memory, and between the registers and the stack. Loads and register transfers set N and Z from the value moved; stores change no flags. A store never reads the byte it replaces. The modes are on the addressing modes page. Every instruction here is documented on the NMOS 6502 and the 65C02 unless the table says otherwise.

The instructions

Each form is written mode opcode, bytes, cycles; “+1 page” is one more cycle when an index crosses a page boundary.

InstructionFormsWhat it doesFlags
ldaimm A9, 2, 2
zp A5, 2, 3
zp,X B5, 2, 4
abs AD, 3, 4
abs,X BD, 3, 4 +1 page
abs,Y B9, 3, 4 +1 page
(zp,X) A1, 2, 6
(zp),Y B1, 2, 5 +1 page
(zp) B2, 2, 5 (65C02)		Loads the accumulator.N Z
ldximm A2, 2, 2
zp A6, 2, 3
zp,Y B6, 2, 4
abs AE, 3, 4
abs,Y BE, 3, 4 +1 page		Loads X.N Z
ldyimm A0, 2, 2
zp A4, 2, 3
zp,X B4, 2, 4
abs AC, 3, 4
abs,X BC, 3, 4 +1 page		Loads Y.N Z
stazp 85, 2, 3
zp,X 95, 2, 4
abs 8D, 3, 4
abs,X 9D, 3, 5
abs,Y 99, 3, 5
(zp,X) 81, 2, 6
(zp),Y 91, 2, 6
(zp) 92, 2, 5 (65C02)		Stores the accumulator. An indexed store always takes the extra cycle, so it takes the same time whether or not a page is crossed.–
stxzp 86, 2, 3
zp,Y 96, 2, 4
abs 8E, 3, 4		Stores X.–
styzp 84, 2, 3
zp,X 94, 2, 4
abs 8C, 3, 4		Stores Y.–
taximplied AA, 1, 2X = A.N Z
tayimplied A8, 1, 2Y = A.N Z
txaimplied 8A, 1, 2A = X.N Z
tyaimplied 98, 1, 2A = Y.N Z
tsximplied BA, 1, 2X = S, the stack pointer.N Z
txsimplied 9A, 1, 2S = X. The only transfer that changes no flags.–
phaimplied 48, 1, 3Pushes A: writes it at $0100 + S, then decrements S.–
phpimplied 08, 1, 3Pushes P with the B flag (bit 4) and the unused bit (bit 5) set in the pushed copy.–
plaimplied 68, 1, 4Increments S, then loads A from $0100 + S.N Z
plpimplied 28, 1, 4Increments S, then loads P from the stack. Bits 4 and 5 are ignored: they are not in the register.all of N V D I Z C

Added by the 65C02

The 65C02 adds more transfers on this theme:

InstructionFormsWhat it doesFlags
stzzp 64, 2, 3
zp,X 74, 2, 4
abs 9C, 3, 4
abs,X 9E, 3, 5		Stores zero.–
phximplied DA, 1, 3Pushes X.–
phyimplied 5A, 1, 3Pushes Y.–
plximplied FA, 1, 4Pulls X.N Z
plyimplied 7A, 1, 4Pulls Y.N Z

On the NMOS 6502 these opcodes are different: 64 and 74 are undocumented NOPs, 9C and 9E are the unstable SHY and SHX, DA, 5A, FA and 7A are one-byte NOPs, and B2 and 92 jam the processor.

Notes

  • The stack is page one. S points at the next free byte, so a push writes at $0100 + S and then decrements S; a pull increments S and then reads. S wraps within the page: after 256 pushes the first byte is overwritten.
  • The 6510’s port. On the Commodore 64 the addresses $00 and $01 are the processor’s own I/O port, so a load or store there reaches the port, not RAM.