Lesson 47 · The joypad - Build a Game Boy Emulator

The joypad

Today you implement the joypad register, the quirky active-low, matrixed input scheme that packs eight buttons into four data bits selected by row. This is the last hardware register your emulator needs before everything runs together.

The goal

Implement the 0xFF00 joypad register with its active-low, matrixed button rows.

Start here - the target

TO DO

Scenario: Reading a pressed direction

Giventhe Right button is held and 0x20 is written to 0xFF00 (selecting the direction pad)

When0xFF00 is read back

Thenbit 0 reads 0 (pressed is low) and bit 4 reads 0 (directions selected)

Andwriting 0x10 instead (selecting buttons) makes bit 0 read 1, since Right is not a button

Background

Input comes through a single register, P1/JOYP at 0xFF00, and it is delightfully backwards in two ways. First, it is active-low: a bit reads 0 when its button is pressed, 1 when released. Second, eight buttons share only four data bits through a matrix: you write bit 4 or bit 5 to select whether the low nibble reports the direction pad or the action buttons, then read the result.

So to check the D-pad, select it (bit 4 low) and read: Right is bit 0, Left bit 1, Up bit 2, Down bit 3. Select the buttons instead (bit 5 low) and the same four data bits now mean A, B, Select, Start. Model the pressed state internally and let the select bits choose which half to expose. This is the last hardware register your emulator needs - next, everything runs together.

Make it work

func (j *Joypad) Read() uint8 {
    var low uint8 = 0x0F // nothing pressed = all high
    if j.selectDir { low &^= j.dirBits }   // pressed bits pulled to 0
    if j.selectBtn { low &^= j.btnBits }
    return j.sel | low
}