more screeen work, provided unofficial line clear routine

STATUS.md

@@ -245,7 +245,7 @@ Not all of them - only these we want to have implemented.
 | `$E8DA`   | color code table             | DONE     |                                                    |
 | `$E8EA`   | scroll line                  | NOT DONE |                                                    |
 | `$E96C`   | insert line on top           | NOT DONE |                                                    |
-| `$E9FF`   | clear line                   | NOT DONE |                                                    |
+| `$E9FF`   | clear line                   | DONE     |                                                    |
 | `$EA31`   | default IRQ                  | PARTIAL  |                                                    |
 | `$EA7E`   | ack CIA1 + below             | DONE     |                                                    |
 | `$EA81`   | ret from IRQ/NMI             | DONE     |                                                    |

c64/kernal/,stubs/e9ff.clear_line.s

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

c64/kernal/screen/chrout_screen_clr.s

@@ -3,9 +3,7 @@
 // CLR key handling within CHROUT
 //
 
-chrout_screen_CLR: /* YYY disabled for rework
+chrout_screen_CLR:
 
 	jsr clear_screen
 	jmp chrout_screen_done
-
-*/

c64/kernal/screen/chrout_screen_home.s

@@ -4,12 +4,7 @@
 //
 
 
-chrout_screen_HOME:
-	rts
-
-/* YYY disabled for rework
 chrout_screen_HOME:
 
 	jsr cursor_home
 	jmp chrout_screen_done
-*/

c64/kernal/screen/chrout_screen_return.s

@@ -4,13 +4,11 @@
 //
 
 
-chrout_screen_RETURN: /* YYY disabled for rework
+chrout_screen_RETURN:
 
 	// RETURN clears quote and insert modes, it also clears reverse flag
 	lda #$00
 	sta QTSW
 	sta INSRT
 	sta RVS
-	jmp chrout_screen_advance_to_next_line
+	jmp screen_advance_to_next_line
-
-*/

c64/kernal/screen/clear_screen_real.s

@@ -19,7 +19,7 @@ clear_screen_real:
 	dey
 	bpl !-
 
-	// YYY flow can probably be size-optimized, by clearing a single row each time (we will need such a subroutine)
+	// YYY flow can probably be size-optimized by calling screen_clear_line in a loop
 
 	// 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

c64/kernal/screen/e56c.screen_calculate_pnt_user.s

@@ -9,7 +9,11 @@
 
 screen_calculate_PNT_USER:
 
-	// YYY jsr screen_normalize_xy - do we need this? probably not
+	ldx TBLX
+
+	// FALLTROUGH
+
+screen_calculate_PNT_USER_from_X: // entry point for screen_clear_line
 
 	// Reset pointer to start of screen
 
@@ -20,7 +24,7 @@ screen_calculate_PNT_USER:
 
 	// Add 40 to PNT for every line
 
-	ldx TBLX
+	cpx #$00	
 	beq screen_calculate_USER
 !:
 	lda #40

c64/kernal/screen/e9ff.screen_clear_line.s

@@ -0,0 +1,34 @@
+
+//
+// Well-known routine, described in:
+//
+// - [CM64] Computes Mapping the Commodore 64 - page 219
+// - http://sta.c64.org/cbm64scrfunc.html
+//
+// input: .X - screen line to clear
+//
+// Checked that original routine preserves X (convenient for loops),
+// and leaves space screen code in .A, so duplicate this behaviour
+//
+
+
+screen_clear_line:
+
+	// First, calculate PNT and USER
+	phx_trash_a
+	jsr screen_calculate_PNT_USER_from_X
+	plx_trash_a
+
+	// Now clear the line
+	ldy #39
+!:
+	lda COLOR
+	sta (USER), y
+	lda #$20                           // space character screen code
+	sta (PNT), y
+
+	dey
+	bpl !-
+
+	// Done
+	rts

c64/kernal/screen/screen_advance_to_next_line.s

@@ -1,8 +1,8 @@
 
 
 screen_advance_to_next_line:
-.break
+
-	rts
+	jmp screen_advance_to_next_line
 
 /* YYY disabled for rework
 

c64/kernal/screen/screen_grow_logical_line.s

@@ -6,8 +6,9 @@ screen_grow_logical_line_done:
 
 
 screen_grow_logical_line:
-.break
+
-	rts
+	jmp screen_grow_logical_line // YYY
+
 
 /* YYY disabled for rework