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                 February 03, 2006
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Lucas Scharenbroich, author of GTE "generic tile engine"
for the IIgs, had updated his software.  After seeing his
post to CSA2 newsgroup, I sent him a few emails asking about
graphics and the possibility of making a SuperMario Brothers
clone for the IIe...  

He sent some code which has some neat tricks in it
I'm in the process of reading over the code and 
gaining more understanding of its' operation.

it seems as if people are beginning to be more interested
in apple II programming, and I'd like to hear from anyone
who has ideas on how to put this example into operation..

download zipped dsk image of merlin8 + source

here is Lucas's code (with my edits, which aren't necessarily
marked):


 ORG $6000


*THIS CODE draws one horizontal row of tiles



*******************************************


SHIFT DFB #00 ;SHIFT FOR DISPLAY FIELD


VIEWWIDTH DFB #10 ;WIDTH OF DISPLAY FIELD


 LDA #TILE_DATA
 STA LEVELPOS+1




 LDY #VIEWWIDTH-1 ; push the tile numbers
;on the stack in reverse order

;THIS ROUTINE PUSHES TILE#
;THEN ADDRESS OF CODE TO DRAW THE CURRENT SHIFT

;THE SHIFT ROUTINE SHOULD PLA TO GET TILE#


LOOP
 LDA (LEVELPOS),Y ;GET TILE #
 PHA  ;PUSH ON STACK


 LDX SHIFT ; get the shifting for
;this particular frame

;
;THIS MODIFIES RTS AT END
;

 LDA SHIFTADDR,X ;LOAD  ADDRESS OF CODE THAT DRAWS PARTICULAR SHIFT
 PHA
 LDA SHIFTADDR+1,X ;
 PHA ; AND push the address on the stack

 DEY  ;DO ENTIRE WIDTH OF PLAYFIELD
 BPL LOOP


 LDY #0 ; start at the left-most tile

 RTS ; dispatch to the drawing code,
 ; SHIFT0, SHIFT1, ETC

GO_BACK:
; return here when done
 LDA LEVELPOS ; advance to the next row of tile data
 CLC
 ADC WIDTHOFLEVEL
 STA TILE_DATA




*Here is a table of 16-bit addresses
*which point to subroutines to
*draw the tile shifted across different
*bytes in the HGR screen

SHIFTADDR

 DFB #SHIFT0

 DFB #SHIFT1

 DFB #SHIFT2

 DFB #SHIFT3

 DFB #SHIFT4

 DFB #SHIFT5

 DFB #SHIFT6



*The shift routine assumes that
*Y = horizontal displacement and
*X =offset into a table of tile data.
*Since we can only access 256 bytes
*of data, if we are using 8x8 tiles
*--> 2 * 8 bytes, then we can only
*have 16 unique tiles.  Getting around
*this limitation is an problem
*left for the reader. ;)

SHIFT0 PLA ; PULL the tile # OFF THE STACK
 TAX

 LDA TILEDATA,X ;              R=0 C=0
 STA ($80),Y ;PUT ON SCREEN

 LDA TILEDATA+2,X ;            R=1,C=0
 STA ($82),Y

 LDA TILEDATA+4,X ;           R=2,C=0
 STA ($84),Y

 LDA TILEDATA+6,X ;           R=3,C=0
 STA ($86),Y
 LDA TILEDATA+8,X ;           R=4,C=0
 STA ($88),Y
 LDA TILEDATA+10,X ;          R=5,C=0
 STA ($8A),Y
 LDA TILEDATA+12,X ;          R=6,C=0
 STA ($8C),Y
 LDA TILEDATA+14,X ;          R=7,C=0
 STA ($8E),Y


 INY ; advance one byte

; NOW DO 2ND BYTE OF SHAPE!
; (WHICH IS = COLUMN1)


 LDA TILEDATA+1,X ;            R=0 C=1
 STA ($80),Y   ;PUT ON SCREEN

 LDA TILEDATA+3,X ;            R=1,C=1
 STA ($82),Y

 LDA TILEDATA+5,X ;           R=2,C=1
 STA ($84),Y

 LDA TILEDATA+7,X ;           R=3,C=1
 STA ($86),Y

 LDA TILEDATA+9,X ;           R=4,C=1
 STA ($88),Y

 LDA TILEDATA+11,X ;          R=5,C=1
 STA ($8A),Y

 LDA TILEDATA+13,X ;          R=6,C=1
 STA ($8C),Y

 LDA TILEDATA+15,X ;          R=7,C=1
 STA ($8E),Y

 INY ; advance to next SPOT ON SCREEN
 CPY #WIDTH ; have we done the whole row
 BCC shift0

 JMP DRAWTILES ; go back to the drawing routine


SHIFT1: PLA ;GET SHAPE # TO DRAW
 TAX

 LDA ($80),Y ; load the data from the screen
 AND SHIFT1MASK ; mask out the portion that this tile occupies
 ORA TILEDATASHIFTED,X ; copy a pre-shifted piece of tile data in
 STA ($80),Y ; store it back on screen



 INY ; advance to next tile
 CPY #WIDTH ; have we done the whole row
 BCC SHIFT0
 JMP GO_BACK ; go back to the drawing routine



********************************************

ROW1_ADDR EQU $3002 ;ADDRESSES OF 8 CONSECUTIVE SCREEN LINES
ROW2_ADDR EQU $3402
ROW3_ADDR EQU $3802
ROW4_ADDR EQU $3C02
ROW5_ADDR EQU $2082
ROW6_ADDR EQU $2482
ROW7_ADDR EQU $2882
ROW8_ADDR EQU $2C82

ROW1PTRL EQU $80
ROW1PTRH EQU $81

ROW2PTRL EQU $82
ROW2PTRH EQU $83

ROW3PTRL EQU $84
ROW3PTRH EQU $85

ROW4PTRL EQU $86
ROW4PTRH EQU $87

ROW5PTRL EQU $88
ROW5PTRH EQU $89

ROW6PTRL EQU $8A
ROW6PTRH EQU $8B

ROW7PTRL EQU $8C
ROW7PTRH EQU $8D

ROW8PTRL EQU $8E
ROW8PTRH EQU $8F

DRAW_ROW:

; first, fill in the zero page locationS with the
;pointer to the left edge of each row

 LDA #ROW1_ADDR
 STA ROW1PTRH



 LDA #ROW2_ADDR
 STA ROW2PTRH



 LDA #ROW3_ADDR
 STA ROW3PTRH



 LDA #ROW4_ADDR
 STA ROW4PTRH



 LDA #ROW5_ADDR
 STA ROW5PTRH



 LDA #ROW6_ADDR
 STA ROW6PTRH



 LDA #ROW7_ADDR
 STA ROW7PTRH



 LDA #ROW8_ADDR
 STA ROW8PTRH






 PUT BLOAD

TILEDATA DS 2

TILE_DATA
 DS 256*16 ; LEVELS = 4K