Lesson 12 · 1NNN and BNNN - jumps - Build a CHIP-8 Emulator

1NNN and BNNN - jumps

A program is more than a straight line, and jumps are how it bends. Today you implement the unconditional jump 1NNN and its computed cousin BNNN, which jumps to an address plus V0.

The goal

Implement 1NNN (jump to NNN) and BNNN (jump to NNN plus V0).

Start here - the target

TO DO

Scenario: Both jumps redirect the program counter

Givena VM about to execute 0x1300

WhenStep runs it

ThenPC becomes 0x300 (not 0x302 - a jump replaces PC, it does not add to it)

Andwith V0 = 0x02, executing 0xB300 sets PC to 0x302 (0x300 + V0)

Anda jump to its own address - 0x120C fetched from 0x20C - leaves PC at 0x20C, the idle loop every ROM ends on

Background

1NNN is the plainest control-flow instruction: “jump to address NNN.” It sets PC directly to NNN. The subtlety is that Fetch already advanced PC by two, and a jump overwrites that - it does not add to it. So a program can loop forever by jumping to its own address, which is exactly how CHIP-8 ROMs idle once their work is done. That self-jump is the standard “halt” pattern you will see at the end of the capstone ROM.

BNNN is the same idea with an offset: “jump to NNN + V0.” It gives a program a computed jump - a jump table, effectively - by choosing the offset in V0 first. (This is one of CHIP-8’s ambiguous opcodes: some later interpreters read it as BXNN and offset by VX instead. This project pins the original BNNN + V0 behaviour, which is what classic ROMs expect.) Both jumps overwrite PC outright, so neither adds the usual two.

Make it work

case 0x1000:
  v.pc = op & 0x0FFF // nnn; overwrite PC, do not add the usual 2
  return nil
case 0xB000:
  v.pc = (op & 0x0FFF) + uint16(v.V[0])
  return nil

CheckpointDONE

1NNN jumps to a fixed address and BNNN jumps to an address offset by V0. Commit and stop here.