43 lines
1.7 KiB
NASM
43 lines
1.7 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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; The data port is PORTB. The CLK (clock) signal is on PORTD0
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; and the RS (register select) on PORTD1
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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 Z (16-bit, r31 and r30): Coarse position. Points to one of the chunks
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; (character columns). 52 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.)
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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 r24, PINB ; 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: Copy PORTB to the next framebuffer byte
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st Z+, r24
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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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; Sets the cursor to 0,0 and clears fine position
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cursor_pos_home:
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; Set Z to framebuffer start
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ldi POS_COARSE_H, high(FRAMEBUFFER<<1)
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ldi POS_COARSE_L, low(FRAMEBUFFER<<1)
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clr POS_FINE
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ret
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