65816 Instruction Set – Jumps, branches and interrupts

JMP, JML, JSR, JSL, the returns, the branches, BRK and COP

Introduction

The program counter is 16 bits inside the 8-bit program bank register. The ordinary jumps, calls and branches stay in the bank; the long forms (jml, jsl, rtl) change the bank too. Branch offsets are relative to the next instruction and wrap inside the bank. A subroutine entered with jsr must return with rts and one entered with jsl with rtl: the two push different numbers of bytes. A target is shown with its 24-bit address in the disassembly.

The instructions

Each form is written mode opcode, bytes, cycles, then the cycle tags.

InstructionFormsWhat it doesFlags
jmpabs 4C, 3, 3
(abs) 6C, 3, 5
(abs,X) 7C, 3, 6		Jump within the program bank. jmp (abs) reads its pointer from bank 0; jmp (abs,X) reads it from the program bank.–
jmllong 5C, 4, 4
[abs] DC, 3, 6		Jump to a 24-bit address, changing the program bank. jml [abs] reads a 3-byte pointer from bank 0. Assemblers also write these as jmp with a long or bracketed operand.–
jsrabs 20, 3, 6
(abs,X) FC, 3, 8		Push the address of the last byte of the instruction (2 bytes) and jump within the bank. Return with rts.–
jsllong 22, 4, 8Push the program bank and then the address of the last byte (3 bytes) and jump to the 24-bit address. Return with rtl. Also written jsr with a long operand.–
rtsimp 60, 1, 6Pull 2 bytes, add one and continue there, in the same bank.–
rtlimp 6B, 1, 6Pull 3 bytes: the address plus one and the program bank.–
rtiimp 40, 1, 7 (6 in emulation mode)Return from an interrupt: pull P, then PC, then (in native mode only) the program bank. M and X come from the pulled P, and setting X clears the high bytes of X and Y.N V M X D I Z C
brkimm8 00, 2, 8 (7 in emulation mode)Software interrupt. The second byte is a signature the processor skips, so the return address is two bytes after the opcode. Pushes the program bank (native only), PC and P, clears D, sets I and goes through the BRK vector (the IRQ vector in emulation mode, with the B bit of the pushed P set). Written brk #$00 in the disassembly.I D
copimm8 02, 2, 8 (7 in emulation mode)Co-processor enable: like BRK, through the COP vector. A program uses the signature byte to say which service it wants.I D
bplrel 10, 2, 2 +T +EBranch if N is clear.–
bmirel 30, 2, 2 +T +EBranch if N is set.–
bvcrel 50, 2, 2 +T +EBranch if V is clear.–
bvsrel 70, 2, 2 +T +EBranch if V is set.–
bccrel 90, 2, 2 +T +EBranch if C is clear (also written blt).–
bcsrel B0, 2, 2 +T +EBranch if C is set (also written bge).–
bnerel D0, 2, 2 +T +EBranch if Z is clear.–
beqrel F0, 2, 2 +T +EBranch if Z is set.–
brarel 80, 2, 3 +EBranch always (a signed 8-bit offset).–
brlrel long 82, 3, 4Branch always with a signed 16-bit offset: reaches anywhere in the program bank.–

Not described

Known, not yet covered

  • Reference: the ABORT interrupt (a pin of the W65C816S; the Super Nintendo does not use it) and the interaction of WAI with interrupts, as on the overview.
  • Viewer: the disassembly names the target of jsr (abs,X) and jmp (abs) only by the pointer’s address, not by what is in the table.

Links

Reference