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BRK

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Basic Info
Addressing Mode Opcode Length Speed
Stack (Interrupt) 00 2 bytes 8 cycles*
Flags Affected
N V M X / B D I Z C
65c816 native mode . . . . 0 1 . .
6502 emulation mode . . . 1 0 1 . .

BRK (Break) is a 65x instruction designed to trigger a software interrupt. The byte following the opcode is called the signature byte. The state of the interrupt disable flag has no effect on the behavior of BRK although it will be set after BRK runs. In other words, BRK causes an NMI.

Control is routed to the BRK handler, whose address is stored at the BRK vector:

  • In native mode, this vector is at $00:FFE6.
  • In emulation mode, this vector is at $FFFE.

The PBR is cleared, but in native mode its previous value is pushed to the stack.

Syntax

BRK
BRK sig

Cycle Skipped

BRK takes one fewer cycle in emulation mode as it doesn't need to push the program counter bank register to the stack.

If the signature byte was omitted from the assembler source, then it ends up serving double duty as the opcode of the next instruction. In this case the interrupt handler must decrement the return address on the stack so that the eventual RTI does not cause derailment.

brk.png

See Also

External Links