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68 lines
1.9 KiB
NASM
68 lines
1.9 KiB
NASM
; pad - read input from MD controller
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;
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; Conveniently expose an API to read the status of a MD pad. Moreover,
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; implement a mechanism to input arbitrary characters from it. It goes as
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; follow:
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;
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; * Direction pad select characters. Up/Down move by one, Left/Right move by 5\
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; * Start acts like Return
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; * A acts like Backspace
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; * B changes "character class": lowercase, uppercase, numbers, special chars.
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; The space character is the first among special chars.
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; * C confirms letter selection
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;
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; *** Consts ***
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;
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.equ PAD_CTLPORT 0x3f
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.equ PAD_D1PORT 0xdc
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; *** Variables ***
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;
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; Previous result of padStatusA
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.equ PAD_PREVSTAT_A PAD_RAMSTART
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.equ PAD_RAMEND PAD_PREVSTAT_A+1
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; *** Code ***
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padInit:
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ld a, 0xff
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ld (PAD_PREVSTAT_A), a
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ret
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; Put status for port A in register A. Bits, from MSB to LSB:
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; Start - A - C - B - Right - Left - Down - Up
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; Each bit is high when button is unpressed and low if button is pressed. For
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; example, when no button is pressed, 0xff is returned.
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; Moreover, sets Z according to whether the status changed since the last call.
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; Z is set if status is the same, unset if different.
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padStatusA:
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; This logic below is for the Genesis controller, which is modal. TH is
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; an output pin that swiches the meaning of TL and TR. When TH is high
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; (unselected), TL = Button B and TR = Button C. When TH is low
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; (selected), TL = Button A and TR = Start.
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push bc
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ld a, 0b11111101 ; TH output, unselected
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out (PAD_CTLPORT), a
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in a, (PAD_D1PORT)
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and 0x3f ; low 6 bits are good
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ld b, a ; let's store them
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; Start and A are returned when TH is selected, in bits 5 and 4. Well
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; get them, left-shift them and integrate them to B.
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ld a, 0b11011101 ; TH output, selected
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out (PAD_CTLPORT), a
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in a, (PAD_D1PORT)
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and 0b00110000
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sla a
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sla a
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or b
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pop bc
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; set Z according to whether status has change. Also, update saved
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; status.
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push hl
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ld hl, PAD_PREVSTAT_A
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cp (hl) ; Sets Z to proper value
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ld (hl), a
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pop hl
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ret
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