Input [Code documentation]

input.asm

input.asm contains functions and variables that handles inputs.

Sections in input.asm:

• Input Variables, WRAM0
• Input Handler, ROM0

References and more info:

• A very informative video on how input works and is read on the Gameboy.
• Code reference, credited to Damian Yerrick, github: pinobatch
• Technical document on the flags

SECTION "Input Variables", WRAM0

• Contains variables that help keep track of the current input.
• Located in WRAM, so it can be read and write.
• The input is stored as bit flags in the variables.
• first 4 bits: down (7), up (6), Left (5), right (4)
• last 4 bits: start (3), select (2), B (1), A (0)
• Variables:
  • wCurrentInputKeys, the input currently initialised, stores all current inputs regardless if it is previously pressed or not
  • wNewlyInputKeys, inputs that was not pressed previously, but pressed this time

; How to use the variables
   ld a, [wCurrentInputKeys/wNewlyInputKeys] ; get current/new inputs
   bit n, a ; check bit n of register a, this will set the z flag in the register f. 
            ; if it is pressed, the z flag will be 0
            ; bit 0, a means checked the 0 bit of a. If the 0 bit of a is 1, which means button A is press, z flag will be 0

SECTION "Input Handler", ROM0

• Read the latest input, store and update the input variables created.
• Located in ROM0
• Function:
  • UpdateInput

Things to note

• We did not use the joypad interrupt to update and handle our input.
• The joypad interrupt is not really used for collecting input, as the interrupt is not called when there is both d-pad and a/b/start/select input at the same time.
• Thus we use our own function to collect input instead.

UpdateInput

• Gets the current input, perform the instruction to handle debouncing and storing the correct input in the variables.
• To be called in the main update loop.
• Reason for debouncing: Sometimes the button states will keep switching when it is initially read, so we need to load the button state a few times to check and make sure it's stable. We want the final button state as it's the most stable.
• To be able to get d-pad input, we write the value $20 to the register at the address $FF00 (reg for reading button info). To get the button inputs (A, B, start, select), we write the value $10 to the register instead.
• To read the input information, we get the value in address $FF00. The last 4 bits of this register are the flags for the inputs. If you wrote value $20 to it, the last 4 bits are flags for the d-pad, if you wrote $10 to it, the flags are for the button.
• Please note: We need to wait for a few cycles in between writing the value to the register, and reading the input information. This is so we can wait for the system to 'stabilise' in a sense for the Gameboy to provide the correct input.

; Input handler logic
UpdateInput:
      ; Write $10 to $FF00, wait a few cycles, read bits 3-0 to get the button inputs
      ; Write $20 to $FF00, wait a few cycles, read bits 3-0 to get the d-pad inputs
      ; Write $30 to P1 to release the key matrix
      ; Combine the data read into 1 byte
      ; Do some logic to check which buttons were newly pressed (so was not pressed in previous frames, the button isn't held down)
      ; Store the data in the corresponding variables
   ret