65816 Instruction Set – Flags, modes and other instructions

CLC, SEC and the rest, REP, SEP, XCE, NOP, WDM, WAI and STP

Introduction

These change one flag, or several at once (REP, SEP), switch between the two modes (XCE), or do nothing in particular. REP, SEP and XCE are the instructions that change the width of the registers, and therefore the length of the instructions after them.

The instructions

Each form is written mode opcode, bytes, cycles.

InstructionFormsWhat it doesFlags
clcimp 18, 1, 2Clear carry. Before XCE it selects native mode.C
cldimp D8, 1, 2Clear decimal mode.D
cliimp 58, 1, 2Enable interrupts.I
clvimp B8, 1, 2Clear overflow.V
secimp 38, 1, 2Set carry. Before XCE it selects emulation mode.C
sedimp F8, 1, 2Set decimal mode.D
seiimp 78, 1, 2Disable interrupts.I
repimm8 C2, 2, 3Reset (clear) every status bit that is 1 in the operand: bit 7 N, 6 V, 5 M, 4 X, 3 D, 2 I, 1 Z, 0 C. rep #$30 makes the accumulator and the index registers 16 bits. In emulation mode M and X do not change.by the operand
sepimm8 E2, 2, 3Set every status bit that is 1 in the operand. sep #$30 makes both 8 bits (and clears the high bytes of X and Y).by the operand
xceimp FB, 1, 2Exchange the carry flag with the emulation flag. clc xce enters native mode, sec xce emulation mode. Entering emulation mode sets M and X, clears the high bytes of X and Y and forces the stack to page one.C E M X
nopimp EA, 1, 2Does nothing.–
wdmimm8 42, 2, 2Reserved by WDC for future expansion. It is two bytes, skips its signature byte and does nothing, like a two-byte NOP; the explorer marks it reserved.–
waiimp CB, 1, 3Wait for an interrupt: stops the processor until an interrupt arrives. With the interrupt flag set it continues at the next instruction when one is requested, without taking it.–
stpimp DB, 1, 3Stop the processor until a reset.–

Not described

Known, not yet covered

  • Reference: how WAI and STP interact with the RDY pin and with the interrupt inputs in detail, which the Super Nintendo does not use.

Links

Reference