more work towards Mega65 screen support

src/kernal/screen_m65/mega65_clrscr.s

@@ -1,9 +0,0 @@
-// #LAYOUT# M65 KERNAL_1 #TAKE
-// #LAYOUT# *   *        #IGNORE
-
-
-M65_CLRSCR:
-
-	// XXX impplement this
-
-	rts

src/kernal/screen_m65/mega65_clrscr_clrwin.s

@@ -0,0 +1,83 @@
+// #LAYOUT# M65 KERNAL_1 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+
+M65_CLRSCR:
+
+	// Set the text window to whole screen area
+
+	lda #$00
+	sta M65_TXTWIN_X0
+	sta M65_TXTWIN_Y0
+
+	ldx M65_SCRMODE
+	
+	lda m65_scrtab_txtwidth, X
+	sta M65_TXTWIN_X1
+	lda m65_scrtab_txtheight, X
+	sta M65_TXTWIN_Y1
+
+	// FALLTROUGH
+
+M65_CLRWIN:
+
+	// To clear the window, two zeropage long pointers will be used:
+	// - M65_LPNT_SCR  for screen memory
+	// - M65_LPNT_KERN for colour memory (starts from $FF80000)
+
+	// First initialize both pointers
+
+	ldx #$03
+!:
+	lda M65_SCRTXTBASE,x
+	sta M65_LPNT_SCR,x
+    lda #$00
+    sta M65_LPNT_KERN,x
+    dex
+    bpl !-
+
+    lda #$0F
+    sta M65_LPNT_KERN+2
+    lda #$F8
+    sta M65_LPNT_KERN+1
+
+    // Go trough all the rows
+
+    phz
+    ldy #$00
+
+    // FALLTROUGH
+
+m65_clrwin_loop:
+
+	// Check if .Y cordinate is not below the window
+
+	cpy M65_TXTWIN_Y0
+	bcc m65_clrwin_loop_next                     // branch if we should skip this row
+
+	// Clear the row
+
+	ldz M65_TXTWIN_X0
+!:
+	lda #$20
+	sta_lp (M65_LPNT_SCR),z
+	lda #$00
+	sta_lp (M65_LPNT_KERN),z
+	inz
+	cpz M65_TXTWIN_X1
+	bne !-
+
+	// FALLTROUGH
+
+m65_clrwin_loop_next:
+
+	iny
+
+    // XXX
+
+    plz
+
+
+	// XXX implement this
+
+	rts

src/kernal/screen_m65/mega65_screen_tables.s

@@ -2,10 +2,20 @@
 // #LAYOUT# *   *        #IGNORE
 
 
-// Table of parameters for VIC control register B
-
-m65_scrtab_vic_ctrlb:
+m65_scrtab_vic_ctrlb:        // parameters for VIC control register B
 
 	.byte %00100000 // 40x25 + extended attributes
 	.byte %10100000 // 80x25 + extended attributes
 	.byte %10100100 // 80x50 + extended attributes
+
+m65_scrtab_txtwidth:         // width of the text screen
+
+	.byte 40
+	.byte 80
+	.byte 80
+
+m65_scrtab_txtheight:        // height of the text screen
+
+	.byte 25
+	.byte 25
+	.byte 50