Forth: add MIN_ROW variable to allow screen split

You can change MIN_ROW to something between 0 (top of the screen) and 28
(near the bottom). The interpreter will keep the forth lines in that
range, leaving the upper part of the screen to the user (for anything
application related?).

forth.s

@@ -70,8 +70,9 @@ COL = ROW - 1
 VRAM = $F800
 BP   = $F000 - 2	; top of LOCALS stack (grows down). Right below the character memory
 
-MAX_ROW = 30
-MAX_COL = 40
+MIN_ROW = 0		; we can set that any value in [0,28] to split the screen
+MAX_ROW = 30	; Row will go from MIN_ROW to MAX_ROW-1
+MAX_COL = 40	; Col will go from 0 to MAX_COL-1
 CURSOR  = '_'
 SPACE   = ' '
 ; End Cerberus Screen management
@@ -118,13 +119,19 @@ RES_vec:
 	CLI
 
 	; start at line 0, col 0
+.if (MIN_ROW>0)
+	lda #MIN_ROW
+	sta ROW
+.else
 	stz ROW
+.endif
+
 	stz COL
 
-	; store addr of line 0
-	lda #<VRAM
+	; store addr of first line
+	lda #<(VRAM+MIN_ROW*MAX_COL)
 	sta LINE
-	lda #>VRAM
+	lda #>(VRAM+MIN_ROW*MAX_COL)
 	sta LINE+1
 
 .ifndef EMULATOR
@@ -496,13 +503,18 @@ defword "BREAK",,
 
 ; Clear Screen (and bring cursor back HOME)
 defword "CLSCR",,
+.if (MIN_ROW>0)
+	lda #MIN_ROW
+	sta ROW
+.else
 	stz ROW
+.endif
 	stz COL
 
-	; store addr of line 0
-	lda #<VRAM
+	; store addr of first line
+	lda #<(VRAM+MIN_ROW*MAX_COL)
 	sta LINE
-	lda #>VRAM
+	lda #>(VRAM+MIN_ROW*MAX_COL)
 	sta LINE+1
 
 	jsr clear_screen
@@ -2409,7 +2421,7 @@ putc:
 	; we are at the beginning of a line:
 	; if first line: can't do anything
 	lda ROW
-	cmp #0
+	cmp #MIN_ROW
 	beq @erase
 	; otherwise: move up 1 line:
 	dea
@@ -2483,68 +2495,14 @@ clear_screen:
 
 
 scroll_up:
-  ; Scroll screen UP (and clears last line), long... ¿fast?
+  ; Scroll screen UP (and clears last line), (unrolled)
   phy
   ldy #00
 @next:
-  lda $F828,y
-  sta $F800,y
-  lda $F850,y 
-  sta $F828,y
-  lda $F878,y 
-  sta $F850,y
-  lda $F8A0,y 
-  sta $F878,y
-  lda $F8C8,y 
-  sta $F8A0,y
-  lda $F8F0,y 
-  sta $F8C8,y
-  lda $F918,y 
-  sta $F8F0,y
-  lda $F940,y 
-  sta $F918,y
-  lda $F968,y 
-  sta $F940,y
-  lda $F990,y 
-  sta $F968,y
-  lda $F9B8,y 
-  sta $F990,y
-  lda $F9E0,y 
-  sta $F9B8,y
-  lda $FA08,y 
-  sta $F9E0,y
-  lda $FA30,y 
-  sta $FA08,y
-  lda $FA58,y 
-  sta $FA30,y
-  lda $FA80,y 
-  sta $FA58,y
-  lda $FAA8,y 
-  sta $FA80,y
-  lda $FAD0,y 
-  sta $FAA8,y
-  lda $FAF8,y 
-  sta $FAD0,y
-  lda $FB20,y 
-  sta $FAF8,y
-  lda $FB48,y 
-  sta $FB20,y
-  lda $FB70,y 
-  sta $FB48,y
-  lda $FB98,y 
-  sta $FB70,y
-  lda $FBC0,y 
-  sta $FB98,y
-  lda $FBE8,y 
-  sta $FBC0,y
-  lda $FC10,y 
-  sta $FBE8,y
-  lda $FC38,y 
-  sta $FC10,y
-  lda $FC60,y 
-  sta $FC38,y
-  lda $FC88,y 
-  sta $FC60,y
+.repeat (MAX_ROW-MIN_ROW-1), I
+  lda VRAM + MAX_COL * (MIN_ROW + I+1),y
+  sta VRAM + MAX_COL * (MIN_ROW + I  ),y
+.endrepeat
   lda #SPACE
   sta $FC88,y
   iny