screen clearing fixes

c64/kernal/screen/clear_screen_real.s

@@ -1,70 +1,69 @@
-// Clear screen and initialise line link table
-// (Compute's Mapping the 64 p215-216)
+
+//
+// Clear screen and initialise line link table, described in:
+//
+// - [CM64] Computes Mapping the Commodore 64 - pages 215-216
+//
 
 
 clear_screen_real:
-	rts
 
-/* YYY disabled for rework
+	// First disable the cursor
+	jsr cursor_disable
 
-clear_screen_real:
+	// Clear the line link table - cheecked on original ROMs,
+	// the highest bit set means that the line is NOT a logical
+	// continuation of the previous one
 
-	// 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:
+	lda #$80
+	ldy #24	
+!:
 	sta LDTBL,y
 	dey
-	bpl clearscreen_l1
+	bpl !-
 
-	// Y now = #$FF
+	// YYY flow can probably be size-optimized, by clearing a single row each time (we will need such a subroutine)
 
-	// 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
+	// Clear screen character RAM. Unfortunately, the HIBASE value is not on the zero page,
+	// we will reuse PNT (it will be overridden in a moment nevertheless) as the pointer
+
+	iny                                // sets .Y to 0
+	sty 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+1	
+
+	ldx #$03                           // countdown for pages to update
+
+clear_screen_loop:
+
+	ldy #$00
+	lda #$01		                   // space character screen code
+!:
 	sta (PNT),y
 	iny
-	bne clearscreen_l2
-	// To draw only 1000 bytes, add 250 to address each time
-	lda PNT
+	cpy #250                           // clear 250 bytes only, do not cross 1000 bytes barrier
+	bne !-
+
+	lda PNT+0
 	clc
-	adc #<250
-	sta PNT
-	lda PNT+1
-	adc #>250
-	sta PNT+1
-	lda #$20		// get space character again
+	adc #250
+	sta PNT+0
+	bcc !+
+	inc PNT+1
+!:
 	dex
-	bpl clearscreen_l2
+	bpl clear_screen_loop
+
+	// Now clear the colour RAM - .Y is now 250, make use of this fact
 
-	// 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
+!:
+	sta $D800 - 1 + 250 * 0, y
+	sta $D800 - 1 + 250 * 1, y
+	sta $D800 - 1 + 250 * 2, y
+	sta $D800 - 1 + 250 * 3, y
+	dey
+	bne !-
 
-	// (Compute's Mapping the 64 p216)
+	// Fall to cursor home routine
 	jmp cursor_home
-
-*/