6502 Instruction Set – Logic, shifts and bit tests

AND, OR, exclusive OR, shifts, rotates and BIT

Introduction

The logic instructions combine the accumulator with a memory byte. The shifts and rotates move the bits of the accumulator or of a memory byte one place, passing the bit that falls off through the carry flag. Shifting left doubles a number and shifting right halves it (unsigned).

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
andimm 29, 2, 2
zp 25, 2, 3
zp,X 35, 2, 4
abs 2D, 3, 4
abs,X 3D, 3, 4 +1 page
abs,Y 39, 3, 4 +1 page
(zp,X) 21, 2, 6
(zp),Y 31, 2, 5 +1 page
(zp) 32, 2, 5 (65C02)		A = A AND memory.N Z
oraimm 09, 2, 2
zp 05, 2, 3
zp,X 15, 2, 4
abs 0D, 3, 4
abs,X 1D, 3, 4 +1 page
abs,Y 19, 3, 4 +1 page
(zp,X) 01, 2, 6
(zp),Y 11, 2, 5 +1 page
(zp) 12, 2, 5 (65C02)		A = A OR memory.N Z
eorimm 49, 2, 2
zp 45, 2, 3
zp,X 55, 2, 4
abs 4D, 3, 4
abs,X 5D, 3, 4 +1 page
abs,Y 59, 3, 4 +1 page
(zp,X) 41, 2, 6
(zp),Y 51, 2, 5 +1 page
(zp) 52, 2, 5 (65C02)		A = A exclusive-OR memory.N Z
bitzp 24, 2, 3
abs 2C, 3, 4
imm 89, 2, 2 (65C02)
zp,X 34, 2, 4 (65C02)
abs,X 3C, 3, 4 +1 page (65C02)		Tests the bits of memory against A without changing either. Z is set when A AND memory is zero. N and V become copies of bits 7 and 6 of the memory byte, not of the result. The 65C02’s immediate form sets Z only.N V Z (Z only for imm)
aslA 0A, 1, 2
zp 06, 2, 5
zp,X 16, 2, 6
abs 0E, 3, 6
abs,X 1E, 3, 7 (6 +1 page on the 65C02)		Shifts left: bit 7 goes to C and a zero comes in at bit 0.N Z C
lsrA 4A, 1, 2
zp 46, 2, 5
zp,X 56, 2, 6
abs 4E, 3, 6
abs,X 5E, 3, 7 (6 +1 page on the 65C02)		Shifts right: bit 0 goes to C and a zero comes in at bit 7. N is always cleared.N Z C
rolA 2A, 1, 2
zp 26, 2, 5
zp,X 36, 2, 6
abs 2E, 3, 6
abs,X 3E, 3, 7 (6 +1 page on the 65C02)		Rotates left through the carry: bit 7 goes to C and the old C comes in at bit 0.N Z C
rorA 6A, 1, 2
zp 66, 2, 5
zp,X 76, 2, 6
abs 6E, 3, 6
abs,X 7E, 3, 7 (6 +1 page on the 65C02)		Rotates right through the carry: bit 0 goes to C and the old C comes in at bit 7.N Z C

Notes

  • Memory shifts are read-modify-write. The NMOS 6502 writes the unchanged byte back before the changed one, which can matter on a hardware register.
  • ROR on the earliest chips. The first production 6502s, before mid-1976, had no working ROR. Every later chip has it, and the explorer decodes it as an ordinary instruction.
  • Bit set and reset. The 6502 has no instruction to set or clear one bit of a byte; ORA and AND with a mask do it. The 65C02 adds TSB and TRB, and the Rockwell and WDC chips RMB and SMB.