Lesson 30 · FX1E - add to the index - Build a CHIP-8 Emulator

FX1E - add to the index

The index register needs to move through memory, and FX1E advances it by a register. Today you implement it, noting that unlike the arithmetic opcodes it leaves VF alone.

The goal

Implement FX1E so it adds VX to the index register I without affecting VF.

Start here - the target

TO DO

Scenario: FX1E advances the index register

GivenI holds 0x300, V0 holds 0x05, and VF holds 1, about to execute 0xF01E

WhenStep runs it

ThenI becomes 0x305 and VF is still 1 - FX1E does not touch the flag

Andfrom I = 0x0FFF, V0 = 0x02, executing 0xF01E gives I = 0x1001 (I is 16-bit, so it does not wrap at the 12-bit address boundary)

Background

FX1E adds VX to the index register I. It is the tool for stepping I through a table or an array in memory: point I at the start with ANNN, then bump it along with FX1E. Because I is 16 bits wide, the sum has plenty of room - adding to an I near the top of normal program memory just produces a larger 16-bit value, it does not wrap at the 12-bit address edge.

The one thing to pin is what it does not do: FX1E leaves VF unchanged. It sits in the arithmetic-looking 0xF family and adds two values, so it is tempting to give it a carry flag like 8XY4 - but standard CHIP-8 does not, and a program may keep meaningful data in VF across it. The spec deliberately sets VF = 1 beforehand and checks it survives, the same discipline you applied to 7XNN.

Make it work

// in the 0xF000 arm:
case 0x1E:
  v.i += uint16(v.V[x]) // no flag effect on standard CHIP-8
  return nil

CheckpointDONE

FX1E advances the index register by a register value without touching VF. Commit and stop here.