Data Overwrite Problem

[Walt Decker]

The following code illustrates the problem. There are no comments; however, I've tried to use variable names that are fairly descriptive. This code creates a random terrain cross-section, then displays the data in screen mode 13. To view the problem, increase the value of NumImages.

DEFINT A - Z
 
DIM Posn AS DWORD
DIM ScnAdr AS DWORD
DIM ScnPtr AS BYTE PTR
DIM GndAdr AS DWORD
DIM GndPtr AS BYTE PTR
DIM HUGE Gnd(MAX, 1) AS BYTE
DIM Colour AS BYTE
DIM Kolor AS BYTE
DIM BytesPerImage AS WORD
DIM Counter AS WORD
DIM GndCounter AS BYTE
DIM Rad AS SHARED SINGLE
DIM Deg AS SHARED SINGLE
 
Pi! = 4 * ATN(1)
 
Deg = 180 / Pi!
Rad = Pi! / 180
 
GndWide = 320
GndHigh = 36
 
Colour = 8
NumImages = 4
BytesPerImage = GndWide * GndHigh
TotalBytes& = BytesPerImage * NumImages + NumImages
PRINT TotalBytes&
REDIM Gnd(TotalBytes&)
 
RANDOMIZE TIMER
 
GndCounter = 0
ScnAdr = &HA000 * 65536
 
Ly = RND(0, 9)
 
DO
	Counter = 0
	Lx = 0
	X = 7
  Posn = GndCounter * BytesPerImage
	GndAdr = VARPTR32(Gnd(Posn))
 
	DO
		Elvn = RND(0, 9)
  	Y = RND(0, Elvn)
  	Azi# = Az(Lx, Ly, X, Y) * Rad
  	Dist = SQR((Lx - X) ^ 2 + (Ly - Y) ^ 2)
  	FOR I = 0 TO DIST
  		XX = Lx + I * SIN(Azi#)
    	YY = Ly - I * COS(Azi#)
    	FOR J = YY TO GndHigh - 1
    		Posn = J * 320 + XX
      	GndPtr = GndAdr + Posn
      	@GndPtr = Colour
    	NEXT J
  	NEXT I
 
  	Lx = X
  	Ly = Y
  	X = Lx + 8
    Counter = X
    IF X > 319 THEN X = 319
	LOOP UNTIL Counter > 319
  INCR GndCounter
  INCR Colour
LOOP UNTIL GndCounter > NumImages - 1
 
! mov AX, &H13
! int &H10
 
Y = 0
GndCounter = 0
 
DO
	Posn = GndCounter * BytesPerImage
	GndAdr = VARPTR32(Gnd(Posn))
	GndPtr = GndAdr
 
	FOR I = 0 TO GndHigh - 1
		Posn = I * 320
		ScnPtr = ScnAdr + Posn
  	FOR J = 0 TO GndWide - 1
  		Kolor = @GndPtr
  		@ScnPtr = Kolor
   		INCR ScnPtr
   		INCR GndPtr
  	NEXT J
	NEXT I
  INCR GndCounter
  SLEEP 3
LOOP UNTIL GndCounter > NumImages - 1
END
 
FUNCTION Az# (BYVAL X1#, BYVAL Y1#, BYVAL X2#, BYVAL Y2#)
  X = X2# - X1#
  Y = Y2# - Y1#
SELECT CASE Y
  CASE < 0
    Az# = -ATN(X / Y) * Deg!
    IF X < 0 THEN Az# = 360 - ATN(X / Y) * Deg!
 
  CASE > 0
    Az# = 180 - ATN(X / Y) * Deg!
 
  CASE 0
    IF X > 0 THEN Az# = 90
    IF X < 0 THEN Az# = 270
END SELECT
END FUNCTION

[Michael Mattias]

As Lance says, some code would be useful.

However, if you are only getting the VARPTR32 at 11520-byte increments, I would expect problems at the highest multiple of 11520 which is still under 64K, say, right around 5 * 11520, which is, by golly, 57600, which is, by golly, the exact number at which you are having problems!

The sixth block of 11520 bytes does not fit in the same segment and a new pointer must be obtained.

One of the internal procedures available in PB/DOS is ArrayInfo&, which will give you the number of elements per page (segment).




-------------
Michael Mattias
Racine WI USA
[email protected]

[Lance Edmonds]

Please post some compilable code that demonstrates your problem.

-------------
Lance
PowerBASIC Support
( mailto:[email protected][email protected]</A> )

[Walt Decker]

Where the pointer goes past a data segment boundry should have no effect since I use VARPTR32 to get the 32 bit address of each 11520 segment. In otherwords, I continually update the pointer target address.

[Michael Mattias]

If the array is HUGE, your pointers change at each segment boundary and you may not just increment your pointer anymore.

Since PB allocates arrays with an offset of 0 in the current segment (undocumented but well known behavior), you should neve have a problem using ponters as long the array stays within 64K - that is, the range of a non-HUGE array.

However, when you DIM HUGE, you will cross a segment boundary and when you do, your pointer becomes invalid (unless you take specific steps to adjust it).

Why this is happening at 57K rather than at 64K is a mystery to me, but if the offset portion of your pointer - a DWORD - is going over 64K, I'll bet that's why it's overwriting. (MS-DOS pointer DWORD = Segment*64K + Offset)

MCM

[Walt Decker]

I'v just begun exploring PB and I've been having trouble with data overwrite. Just as an experiment I've used pointers to fill an array of type BYTE with data. Once I get past 57600 bytes additional data seems to overwrite the first 11520 bytes the seems to randomly corrupt parts of each following 11520 byte segment. I've used DIM HUGE ... AS BYTE with no effect. Are there any suggestions as to why this occurs?