tested cursor homing, started work on PNT recalculation

STATUS.md

@@ -236,7 +236,8 @@ Not all of them - only these we want to have implemented.
 | `$E51B`   | init screen keyboard, no VIC | DONE     |                                                    |
 | `$E544`   | clear screen                 | DONE     |                                                    |
 | `$E50C`   | set cursor position          | PARTIAL  |                                                    |
-| `$E566`   | home cursor                  | NOT DONE |                                                    |
+| `$E566`   | home cursor                  | DONE     |                                                    |
+| `$E56C`   | set PNTR value               |          |                                                    |
 | `$E5A0`   | setup VIC II & I/O           | DONE     |                                                    |
 | `$E6B6`   | advance cursor               | NOT DONE |                                                    |
 | `$E701`   | previous line                | NOT DONE |                                                    |

c64/kernal/,stubs/e566.home_cursor.s

@@ -1,4 +0,0 @@
-
-// XXX provide documentation and implementation
-
-    STUB_IMPLEMENTATION()

c64/kernal/screen/e544.clear_screen.jmp.s

@@ -1,6 +1,9 @@
 
-// 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:

c64/kernal/screen/e566.cursor_home.s

@@ -0,0 +1,15 @@
+
+//
+// Home the cursor, described in:
+//
+// - [CM64] Computes Mapping the Commodore 64 - page 216
+//
+
+
+cursor_home:
+
+	lda #$00
+	sta PNTR                           // current column
+	sta TBLX                           // current row
+
+	// FALLTROUGH to the next routine

c64/kernal/screen/e56c.interop

@@ -0,0 +1,4 @@
+
+This is a well-known address, probably used by lots of software. See for example here:
+
+- https://www.lemon64.com/forum/viewtopic.php?t=69526&sid=836d087e9759db667ba3bb1732eea266

c64/kernal/screen/e56c.screen_calculate_pnt.s

@@ -1,7 +1,16 @@
 
 //
-// Home the cursor (Compute's Mapping the 64 p216)
+// Set pointer (PNT) to current screen line , described in:
 //
+// - [CM64] Computes Mapping the Commodore 64 - page 216
+//
+
+
+screen_calculate_PNT:
+
+	// YYY implement this!
+	rts
+
 
 // Variables: (http://sta.c64.org/cbm64mem.html)
 // - $C7     - RVS    - reverse mode switch
@@ -25,16 +34,59 @@
 // - $02A5   - TLNIDX - number of line currently being scrolled during scrolling the screen
 
 
-cursor_home:
-	jmp cursor_home
-
-
 /* YYY disabled for rework
-cursor_home:
 
-	lda #$00
+screen_calculate_line_pointer:
-	sta PNTR // x position
+	jsr screen_normalize_xy
-	sta TBLX // y position
+	
+	//  Reset pointer to start of screen
+	lda HIBASE
+	sta PNT+1
+	lda #0
+	sta PNT+0
 
-	jmp screen_calculate_line_pointer
+	// Add 40 for every line, or 80 if the lines are linked
+	ldx TBLX
+
+!:
+	// Stop if we have counted enough lines
+	beq !+
+
+	// Add 40 or 80 based on whether the line is linked
+	// or not.
+	ldy #40
+	// -1 offset is because we count down from N to 1, not
+	// N-1 to 0.
+	lda LDTBL-1,x
+	bpl sclp_l1
+	ldy #80
+sclp_l1:
+	// Add computed line length to pointer value
+	tya
+	clc
+	adc PNT+0
+	sta PNT+0
+	lda PNT+1
+	adc #0
+	sta PNT+1
+	
+	// Loop back to next line
+	dex
+	jmp !-
+!:
+
+	// FALLTHROUGH
+
+update_colour_line_pointer:
+	// Now setup pointer to colour RAM
+	lda PNT+0
+	sta USER+0
+	lda PNT+1
+	sec
+	sbc HIBASE
+	clc
+	adc #>$d800
+	sta USER+1
+
+	rts
 */

c64/kernal/screen/screen_calculate_line_pointer.s

@@ -1,57 +0,0 @@
-
-/* YYY disabled for rework
-
-screen_calculate_line_pointer:
-	jsr screen_normalize_xy
-	
-	//  Reset pointer to start of screen
-	lda HIBASE
-	sta PNT+1
-	lda #0
-	sta PNT+0
-
-	// Add 40 for every line, or 80 if the lines are linked
-	ldx TBLX
-
-!:
-	// Stop if we have counted enough lines
-	beq !+
-
-	// Add 40 or 80 based on whether the line is linked
-	// or not.
-	ldy #40
-	// -1 offset is because we count down from N to 1, not
-	// N-1 to 0.
-	lda LDTBL-1,x
-	bpl sclp_l1
-	ldy #80
-sclp_l1:
-	// Add computed line length to pointer value
-	tya
-	clc
-	adc PNT+0
-	sta PNT+0
-	lda PNT+1
-	adc #0
-	sta PNT+1
-	
-	// Loop back to next line
-	dex
-	jmp !-
-!:
-
-	// FALLTHROUGH
-
-update_colour_line_pointer:
-	// Now setup pointer to colour RAM
-	lda PNT+0
-	sta USER+0
-	lda PNT+1
-	sec
-	sbc HIBASE
-	clc
-	adc #>$d800
-	sta USER+1
-
-	rts
-*/