Updated thoughts on what the upper 3 bits should be for (visible obstructions indicated in high-bit)

Updated main geometry calculation macro for the drawing routine (should be mostly ok now)
Allocated variables for main loop drawing routine
Conjured up use of memory holes for tracking drawing (clever?) with address calculation notes

Platform/Apple/virtual/src/tile/tile.s

@@ -19,7 +19,7 @@ MAX_MAP_ID=254	; This means that the total map area can be as big as 5588x5842 t
 
 REL_X=$50	; Will always be in the range 0-43
 REL_Y=$51	; Will always be in the range 0-45
-; Map quadrant data pointers
+; Map quadrant data pointers (Maybe move these to screen holes in 2078-207f?  There might be no advantage to using ZP for these)
 NW_MAP_LOC=$52
 NE_MAP_LOC=$54
 SW_MAP_LOC=$56
@@ -36,6 +36,14 @@ EAST	=1
 SOUTH	=2
 WEST 	=3
 
+;-- Variables used in drawing
+DRAW_X_START	= $5E	; Starting column being drawn (between 0 and VIEWPORT_WIDTH)
+DRAW_Y_START	= $5F	; Starting row being drawn (between 0 and VIEWPORT_WIDTH)
+SECTION_X_START = $60	; X Offset relative to current section being drawn 
+SECTION_Y_START = $61	; Y Offset relative to current section being drawn 
+DRAW_WIDTH		= $62	; Number of columns to draw for current section
+DRAW_HEIGTH		= $63	; Number of rows to draw for current section
+
 ; >> INIT (reset map drawing vars)
 INIT
 		LDA #NOT_LOADED
@@ -131,9 +139,9 @@ FINISH_LOAD
 ;	(Returns Tile # in Y, Flags in A)
 ;	Each tile in memory can be 0-64, the flags are the upper 3 bits
 ;	0 0 0
-;	| | `- Boundary (Can not cross)
+;	| | `- Script assigned, triggers script lookup
-;	| `--- Requires special (rope, raft, etc)
+;	| `--- Boundary (Can not walk on it)
-;	`----- Script assigned, triggers script lookup 
+;	`----- Visible obstruction (Can not see behind it)
 ;----------------------------------------------------------------------
 ; >> SET X,Y COORDINATES FOR VIEWPORT CENTER
 SET_XY
@@ -272,27 +280,44 @@ CROSS_WEST
 		SBC #(deltaX+VIEWPORT_HORIZ_PAD)
 		TAX
 		BPL .10
-		;This section isn't at the edge, note that
+		LDA #0
-		EOR #$FF
+.10 	STA SECTION_X_START
-		ADC #$00 ; Carry is still set, so this is really +1
-		BPL .11	; Should be always true
-.10		LDA #0
-.11 	STA X1
 		TXA
 		CLC
-		ADC VIEWPORT_WIDTH
+		ADC #VIEWPORT_WIDTH
-		CMP SECTION_WIDTH
+		CMP #SECTION_WIDTH
-		BLT .12
+		BLT .11
-		LDA #SECTION_WIDTH-1
+		LDA #SECTION_WIDTH
-.12		STA X2		
+.11		
-
+		SEC
-		}
+		SBC SECTION_X_START
-
+		STA DRAW_WIDTH
 	; Determine Y1 and Y2 bounds for what is being drawn
+		LDA REL_Y
+		SEC
+		SBC #(deltaY+VIEWPORT_VERT_PAD)
+		TAX
+		BPL .20
+		LDA #0
+.20		STA SECTION_Y_START
+		TXA
+		CLC
+		ADC #VIEWPORT_HEIGHT
+		CMP #SECTION_HEIGHT
+		BLT .21
+		LDA #SECTION_HEIGHT
+.21
+		SEC
+		SBC SECTION_Y_START
+		STA DRAW_HEIGHT
+		jsr mainDraw
 }
 
 DRAW
 ; For each quadrant, display relevant parts of screen
+		LDA #00
+		STA DRAW_X_START
+		STA DRAW_Y_START
 .checkNorthQuads
 		LDA REL_Y
 		CMP SECTION_HEIGHT+VIEWPORT_VERT_PAD
@@ -302,22 +327,56 @@ DRAW
 		CMP SECTION_WIDTH+VIEWPORT_HORIZ_PAD
 		BGE .checkNEQuad
 		!drawMapSection NW_MAP_LOC, 0, 0
-		LDA NW_MAP_LOC+1
-		BNE .drawNW
-
-.drawNW
 ;	Check for NE quadrant area
 .checkNEQuad
-
+		LDA REL_X
+		CMP #VIEWPORT_HORIZ_PAD+1
+		BLT .finishTopQuads
+		LDA DRAW_WIDTH
+		STA DRAW_X_START
+		!drawMapSection NE_MAP_LOC, SECTION_WIDTH, 0
+.finishTopQuads		
+;Update Y start for bottom quadrants
+		LDA DRAW_HEIGHT
+		STA DRAW_Y_START
+;-----
 .checkSouthQuads
+		LDA #00
+		STA DRAW_X_START
 ;	Check for SW quadrant area
 		LDA REL_X
 		CMP SECTION_WIDTH+VIEWPORT_HORIZ_PAD
 		BGE .checkSEQuad
-;	Check for SE quadrand area
+		!drawMapSection SW_MAP_LOC, 0, SECTION_HEIGHT
 .checkSEQuad
+;	Check for SE quadrand area
+		LDA REL_X
+		CMP #VIEWPORT_HORIZ_PAD+1
+		BGE .drawSEQuad
+		RTS
+.drawSEQuad		
+		LDA DRAW_WIDTH
+		STA DRAW_X_START
+		!drawMapSection SE_MAP_LOC, SECTION_WIDTH, SECTION_HEIGHT
 	RTS
+
 .mainDraw
+;----- Tracking visible tile data -----
+;There are a total of 512 screen holes in a hires page located in xx78-xx7F and xxF8-xxFF
+;We only need 81 screen holes to track the 9x9 visible area.  So to do this a little translation is needed
+;      78  79  7a  7b  7c  7d  7e  7f  f8 
+;2000   
+;2100  
+;2200  
+; .
+; .
+;2800
+;
+; The calculation goes like this:  Page + $78 + (Row * $100) + (Col & 7) + ((Col & 8) << 4)
+; When the display is drawn, the screen hole is compared to see if there is a different tile to draw 
+; and if there isn't then the tile is skipped.  Otherwise the tile is drawn, etc.
+;--------------------------------------
+
 ; Identify start of map data (upper left)
 ; Display row of tiles
 ; Get tile