38 lines
1.5 KiB
NASM
38 lines
1.5 KiB
NASM
; *******************************************
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; * PAT80 COMPOSITE PAL VIDEO ADAPTER *
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; * Communication module *
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; *******************************************
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; This module manages the communication between Pat80 and
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; the video adapter.
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; INTERNAL POINTER:
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; Internally, the last screen position is represented by 24 bits in two registers:
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; POS_COARSE: Register Y (16-bit, r31 and r30): Coarse position. Points to one of the chunks
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; (character columns). 46 chunks per row, 304 rows. Used for character position, as
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; 1 chunk = 1 byte = 1 character.
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; POS_FINE: Register r24: Fine position. Represents the bit inside the chunk selected by POS_COARSE.
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; Ignored in character mode (the character is always aligned to column). Used in graphic mode.
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; Initializes and waits for a byte on PORTB
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comm_init:
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call cursor_pos_home ; Set cursor to 0,0
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comm_wait_byte:
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in HIGH_ACCUM, DATA_PORT_IN ; read PORTB
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; Check continuously CLK until a LOW is found
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sbic PORTD, CLK_PIN
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jmp comm_wait_byte
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; CLK triggered: Draw char
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call draw_char
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jmp comm_wait_byte
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; ; CLK triggered: Copy PORTB to the next framebuffer byte
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; st Y+, A
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; ; if reached the last framebuffer byte, exit cycle
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; cpi r31, 0b00111110
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; brne comm_wait_byte ; if not 0, repeat h_picture_loop
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; cpi r30, 0b11000000
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; brne comm_wait_byte ; if not 0, repeat h_picture_loop
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; jmp comm_init ; filled all memory: reset framebuffer position
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