improved VIC II initialization

c64/kernal/screen/e544.clear_screen.s

@@ -0,0 +1,62 @@
+// Clear screen and initialise line link table
+// (Compute's Mapping the 64 p215-216)
+
+clear_screen:
+
+	// XXX it is probably a good idea to hide cursor first
+
+	// Clear line link table 
+	// (Compute's Mapping the 64 p215)
+
+	lda #$00
+	ldy #24
+clearscreen_l1:
+	sta LDTBL,y
+	dey
+	bpl clearscreen_l1
+
+	// Y now = #$FF
+
+	// Clear screen RAM.
+	// We should do this at HIBASE, which annoyingly
+	// is no ZP, so we need to make a vector
+	// (Compute's Mapping the 64 p216)
+	// Get pointer to the screen into PNT
+	// as it is the first appropriate place for it found when
+	// searching through the ZP allocations listed in
+	// Compute's Mapping the 64
+	sta PNT+0
+	lda HIBASE
+	sta PNT+1
+	ldx #$03		// countdown for pages to update
+	iny 			// Y now = #$00
+	lda #$20		// space character
+clearscreen_l2:
+	sta (PNT),y
+	iny
+	bne clearscreen_l2
+	// To draw only 1000 bytes, add 250 to address each time
+	lda PNT
+	clc
+	adc #<250
+	sta PNT
+	lda PNT+1
+	adc #>250
+	sta PNT+1
+	lda #$20		// get space character again
+	dex
+	bpl clearscreen_l2
+
+	// Clear colour RAM
+	// (Compute's Mapping the 64 p216)
+	lda COLOR
+clearscreen_l3:	
+	sta $d800,y
+	sta $d900,y
+	sta $da00,y
+	sta $db00-24,y    	// so we only erase 1000 bytes
+	iny
+	bne clearscreen_l3
+
+	// (Compute's Mapping the 64 p216)
+	jmp cursor_home

c64/kernal/screen/screen.s

@@ -0,0 +1,19 @@
+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 295
+// - [CM64] Compute's Mapping the Commodore 64 - page 215
+//
+// CPU registers that has to be preserved (see [RG64]): .A
+//
+
+SCREEN:
+
+	// There are only 2 sane ways to implement this routine,
+	// I hope this one is different than what Commodore picked :)
+
+	ldy #25 // 25 columns
+	ldx #40 // 40 rows
+
+	rts