40 lines
1.5 KiB
ArmAsm
40 lines
1.5 KiB
ArmAsm
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; Kill the Bit -- Dean McDaniel, May 15, 1975. Public domain.
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;
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; The classic Altair 8800 front-panel reflex game. Adapted to Velxio's
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; i8080-cpu chip: instead of the Altair's address LEDs + sense switches,
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; we use the chip's memory-mapped LED port (0xC000) and button bitmap
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; (0xC003).
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;
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; How to play: a single LED walks left across the 8 LEDs. Press the
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; button at the SAME bit position as the lit LED to "kill" it. Miss,
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; and an extra bit lights up next time around.
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;
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; Logic per beat:
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; 1. Wait some cycles (DAD B + JNC beat).
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; 2. Show the current bit pattern (register D) on the LEDs.
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; 3. Read which buttons the user is pressing.
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; 4. XOR with D -- bits the user pressed at the right time get cleared.
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; 5. Rotate right -- advances the bit one position.
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ORG 0x0000
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LXI SP, 0xBFFF ; stack at top of RAM (we don't push, but safe)
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LXI H, 0x0000 ; clear delay accumulator
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MVI D, 0x80 ; D = current bit pattern (start: LED7 lit)
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LXI B, 0x0E00 ; B/C = delay step
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beat:
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DAD B ; HL += BC; loops ~18 times before carry
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JNC beat
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; One "tick" -- show D on LEDs, sample button, advance bit.
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MOV A, D
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STA 0xC000 ; LEDs <- D
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LDA 0xC003 ; A = button bitmap
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XRA D ; bits the user got right are toggled OFF
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RRC ; rotate right (bit0 -> bit7)
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MOV D, A
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JMP beat
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