50 lines
1.5 KiB
NASM
50 lines
1.5 KiB
NASM
; pad - read input from MD controller
|
|
;
|
|
; Conveniently expose an API to read the status of a MD pad. Moreover,
|
|
; implement a mechanism to input arbitrary characters from it. It goes as
|
|
; follow:
|
|
;
|
|
; * Direction pad select characters. Up/Down move by one, Left/Right move by 5\
|
|
; * Start acts like Return
|
|
; * A acts like Backspace
|
|
; * B changes "character class": lowercase, uppercase, numbers, special chars.
|
|
; The space character is the first among special chars.
|
|
; * C confirms letter selection
|
|
;
|
|
; *** Consts ***
|
|
;
|
|
.equ PAD_CTLPORT 0x3f
|
|
.equ PAD_D1PORT 0xdc
|
|
|
|
; *** Variables ***
|
|
;
|
|
; *** Code ***
|
|
|
|
; Put status for port A in register A. Bits, from MSB to LSB:
|
|
; Start - A - C - B - Right - Left - Down - Up
|
|
; Each bit is high when button is unpressed and low if button is pressed. For
|
|
; example, when no button is pressed, 0xff is returned.
|
|
padStatusA:
|
|
; This logic below is for the Genesis controller, which is modal. TH is
|
|
; an output pin that swiches the meaning of TL and TR. When TH is high
|
|
; (unselected), TL = Button B and TR = Button C. When TH is low
|
|
; (selected), TL = Button A and TR = Start.
|
|
push bc
|
|
ld a, 0b11111101 ; TH output, unselected
|
|
out (PAD_CTLPORT), a
|
|
in a, (PAD_D1PORT)
|
|
and 0x3f ; low 6 bits are good
|
|
ld b, a ; let's store them
|
|
; Start and A are returned when TH is selected, in bits 5 and 4. Well
|
|
; get them, left-shift them and integrate them to B.
|
|
ld a, 0b11011101 ; TH output, selected
|
|
out (PAD_CTLPORT), a
|
|
in a, (PAD_D1PORT)
|
|
and 0b00110000
|
|
sla a
|
|
sla a
|
|
or b
|
|
; we're good now!
|
|
pop bc
|
|
ret
|