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Code/snippets/6845_drvr.asm

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+;8645 Routines
+; http://bitsavers.trailing-edge.com/components/motorola/_dataSheets/6845.pdf
+CS_CRTC_ADDR	.EQU 80h
+CS_CRTC_DATA	.EQU 81h
+
+VAL_R0			.EQU 99		;Nr of Horizontal Characters Total.
+VAL_R1			.EQU 80		;Nr of Horizontal Characters Displayed.
+VAL_R2			.EQU 83		;Horizontal Sync Position.
+VAL_R3			.EQU 6		;Sync width.
+VAL_R4			.EQU 31		;Vertical Total.
+VAL_R5			.EQU 13		;Vertical Total Adjustment.
+VAL_R6			.EQU 25		;Nr of Vertical Characters Displayed.
+VAL_R7			.EQU 29		;Vertical Sync Position (might need manual fine tuning).
+VAL_R8			.EQU 0		;Interlace Mode.
+VAL_R9			.EQU 15		;Max Scanline Address.
+VAL_R10			.EQU 205	;Cursor Start Scan Line.
+VAL_R11			.EQU 207	;Cursor Stop Scan Line.
+VAL_R12			.EQU 0		;Start Address (High). Real start address is 0x0000.
+VAL_R13			.EQU 0		;Start Address (Low). Real start address is 0x0000.
+VAL_R14			.EQU 0		;Cursor Start Address (High). Cursor will be at position (0, 0).
+VAL_R15			.EQU 0		;Cursor Start Address (Low). Cursor will be at position (0, 0).
+
+; A contains DATA
+; B contains REGISTER ID
+CRTC_WRITE_REGISTER:
+	PUSH AF
+	LD A,B
+	AND 1Fh
+	OUT (CS_CRTC_ADDR), A
+	POP AF
+	OUT (CS_CRTC_DATA), A
+	RET
+	
+; A returns DATA
+; B contains REGISTER ID
+CRTC_READ_REGISTER:
+	LD A,B
+	AND 1Fh
+	OUT (CS_CRTC_ADDR), A
+	IN A, (CS_CRTC_DATA)
+	RET
+	
+CRTC_INIT_80COL:
+	LD DE,[VAL_R0]
+	LD B, 0
+	
+CRTC_INIT_80COL_LOOP:
+	LD A, (DE)
+	CALL CRTC_READ_REGISTER
+	INC DE
+	INC DE
+	INC B
+	LD A, B
+	CP 16
+	JR NC, CRTC_INIT_80COL_LOOP
+