Tetris evil joypad code

joypad.asm

; This is the code in Tetris which infinitely resets if you return 0 from the joypad register

Evil:                                   ; CODE XREF: sub_20C+E9↓j
ROM:02C4                 call    pollJOYP        ; stores inverted joypad state into 0xFF80 (upper nibble = dpad, low nibble = buttons)
ROM:02C7                 call    sub_2F8
ROM:02CA                 call    sub_7FF0
ROM:02CD                 ld      a, [$FF80]      ; read the joypad state that pollJOYP previously cached
ROM:02CF                 and     $F              ; only keep the buttons, dump the dpad
ROM:02D1                 cp      $F              ; see if all buttons are pressed
ROM:02D3                 jp      z, reset        ; if yes reset lmfao

ROM:29A6 ; =============== S U B R O U T I N E =======================================
pollJOYP:                               ; CODE XREF: sub_20C:Evil↑p
ROM:29A6                 ld      a, $20 ; ' '    ; enable dpad
ROM:29A8                 ld      [rJOYP], a
ROM:29AA                 ld      a, [rJOYP]      ; poll it
ROM:29AC                 ld      a, [rJOYP]      ; poll it harder
ROM:29AE                 ld      a, [rJOYP]      ; poll it harder harder
ROM:29B0                 ld      a, [rJOYP]      ; poll it harder harder harder
ROM:29B2                 cpl                     ; invert so that pressed buttons = 1
ROM:29B3                 and     $F              ; ignore misc bits
ROM:29B5                 swap    a               ; store the dpad in the upper nibble of a, then cache it into b
ROM:29B7                 ld      b, a
ROM:29B8                 ld      a, $10          ; enable buttons
ROM:29BA                 ld      [rJOYP], a
ROM:29BC                 ld      a, [rJOYP]      ; poll them even harder
ROM:29BE                 ld      a, [rJOYP]      ; haaaaaaaaaaaaaaaaaaaaaaaaaaaaaaarder
ROM:29C0                 ld      a, [rJOYP]
ROM:29C2                 ld      a, [rJOYP]
ROM:29C4                 ld      a, [rJOYP]
ROM:29C6                 ld      a, [rJOYP]
ROM:29C8                 ld      a, [rJOYP]
ROM:29CA                 ld      a, [rJOYP]
ROM:29CC                 ld      a, [rJOYP]
ROM:29CE                 ld      a, [rJOYP]
ROM:29D0                 cpl                     ; invert so that pressed buttons = 1
ROM:29D1                 and     $F              ; ignore misc bits
ROM:29D3                 or      b               ; a's upper nibble now corresponds to the dpad, and the lower nibble corresponds to the buttons
ROM:29D4                 ld      c, a            ; cache pad into c
ROM:29D5                 ld      a, [$FF80]
ROM:29D7                 xor     c
ROM:29D8                 and     c
ROM:29D9                 ld      [$FF81], a
ROM:29DB                 ld      a, c            ; restore pad into a
ROM:29DC                 ld      [$FF80], a      ; store pad into 0xFF80
ROM:29DE                 ld      a, $30 ; '0'    ; disable both pad config bits
ROM:29E0                 ld      [rJOYP], a
ROM:29E2                 ret
ROM:29E2 ; End of function pollJOYP