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103 lines
1.7 KiB
NASM
103 lines
1.7 KiB
NASM
; kbd - implement FetchKC for SMS PS/2 adapter
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;
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; Implements KBD_FETCHKC for the adapter described in recipe sms/kbd. It does
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; so for both Port A and Port B (you hook whichever you prefer).
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; FetchKC on Port A
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smskbdFetchKCA:
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; Before reading a character, we must first verify that there is
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; something to read. When the adapter is finished filling its '164 up,
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; it resets the latch, which output's is connected to TL. When the '164
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; is full, TL is low.
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; Port A TL is bit 4
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in a, (0xdc)
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and 0b00010000
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jr nz, .nothing
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push bc
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in a, (0x3f)
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; Port A TH output, low
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ld a, 0b11011101
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out (0x3f), a
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nop
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nop
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in a, (0xdc)
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; bit 3:0 are our dest bits 3:0. handy...
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and 0b00001111
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ld b, a
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; Port A TH output, high
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ld a, 0b11111101
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out (0x3f), a
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nop
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nop
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in a, (0xdc)
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; bit 3:0 are our dest bits 7:4
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rlca \ rlca \ rlca \ rlca
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and 0b11110000
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or b
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ex af, af'
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; Port A/B reset
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ld a, 0xff
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out (0x3f), a
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ex af, af'
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pop bc
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ret
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.nothing:
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xor a
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ret
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; FetchKC on Port B
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smskbdFetchKCB:
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; Port B TL is bit 2
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in a, (0xdd)
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and 0b00000100
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jr nz, .nothing
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push bc
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in a, (0x3f)
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; Port B TH output, low
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ld a, 0b01110111
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out (0x3f), a
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nop
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nop
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in a, (0xdc)
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; bit 7:6 are our dest bits 1:0
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rlca \ rlca
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and 0b00000011
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ld b, a
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in a, (0xdd)
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; bit 1:0 are our dest bits 3:2
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rlca \ rlca
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and 0b00001100
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or b
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ld b, a
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; Port B TH output, high
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ld a, 0b11110111
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out (0x3f), a
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nop
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nop
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in a, (0xdc)
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; bit 7:6 are our dest bits 5:4
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rrca \ rrca
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and 0b00110000
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or b
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ld b, a
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in a, (0xdd)
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; bit 1:0 are our dest bits 7:6
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rrca \ rrca
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and 0b11000000
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or b
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ex af, af'
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; Port A/B reset
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ld a, 0xff
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out (0x3f), a
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ex af, af'
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pop bc
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ret
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.nothing:
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xor a
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ret
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