Data Overwrite Code

[Walt Decker]

The following code illustrates the data overwrite problem when using pointers to store data in a byte array. Beginning at about the 57600 byte, additional data seems to overwrite the first part of the array. 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

[Fred Buffington]

Hey Walt, nice to see you took my suggestion to come here.
Glad to have you. Buff

[Walt Decker]

Thanks, Fred. So far I've not received a good answer to why my data is overwritten. Hopefully someone out there will respond. I'd like to be able to use PB to do real-time geologic modeling. I can't do that if data gets corrupted.

[Michael Mattias]

I tried running your program, but...

1. The change to mode 13 video and back seems difficult for my computer

2. I can't tell that the image is incorrect! What is it supposed to look like?

I did notice that you get a new pointer for each image, but the sample you supplied did not have enough images to go over the 57600-byte then across the 64K boundary I mentioned when you asked this same question in another thread. (I don't think. It would have been easier had you used the "code" UBB tag and preserved the indents).

Basically, if your array is over 64K in size (that is, HUGE), you cannot use pointer access on any element beyond 64K, if that pointer was obtained in the prior 64K segment. You *may* get lucky, or, if your array elements are powers of two in size (one is a power of two) you may get lucky again; but when incrementing unsigned data(i.e., DWORDS or Pointers), PB does not return the expected overflow error: it will just return MOD 65556 of the incremented value (binary truncation). This will result in a return to previous values if you are just incrementing the low word of the pointer value.

I would look there for problems.




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

[Walt Decker]

Michael,

The data is supposed to look like a terrain cross-section, i.e., the silloette of a mountain range. I'm an engineering geologist and I thought I'd look into PB as a possible viable alternative to HTbasic/Rocky Mountian Basic/PDS 7x for modeling geologic data.

What you are telling me then, is that once the pointer passes the 65536 mark, it can't find the next 64K segment and, therefore, I'll have to continually use VARPTR32 to find where the data is. That seems like it will severly slow the display of data, especially if it is continually being fed in via telecummications lines for real-time modeling.

Since your computer doe not display MODE 13 well, I've modified the display to use MODE 12. Perhaps that will enable you to see the problem more clearly. Place the following code above:

! mov AX, &H13

SCREEN 12
Y = 0
GndCounter = 0
DO
Posn = GndCounter * BytesPerImage
GndAdr = VARPTR32(Gnd(posn))
GndPtr = GndAdr
FOR I = 0 TO GndHigh - 1
FOR J = 0 TO GndWide - 1
Kolor = @GndPtr
PSET(J, I + Y), Kolor
INCR GndPtr
NEXT J
NEXT I
Y = Y + GndHigh + 2
INCR GndCounter
LOOP UNTIL GndCounter > NumImages - 1

END

[Lance Edmonds]

Walt, if you actually do perform a VARPTR32() call for each 'byte', does the "corruption" problem disappear? (I've not had time to examine your code in depth, sorry!)

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

[Walt Decker]

Lance, yes, the problem disappears when using VARPTR32() for each byte. By placing a boundry query in the code I've also ascertained that Michael is correct that the problem occurs at (or near) the 64K data boundry. This suggests that I should be able to reset the value of GndAdr to the point where the data is beyond the 64K boundry then increment the GndPtr pointer with the GndAdr + Posn value. That works except that other problems appear. The bottom half of the last image is cut off and a shadow of the top half of the last image appears in the first image.

The query I used is:

DIM GndPosn AS DWORD
DIM Boundry AS DWORD
Boundry = 65536
......
......
GndPosn = 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
IF GndPosn + J * 320 + XX > Boundry THEN
Posn = GndCounter * BytesPerImage
Posn = Posn + J * 320 + XX
GndAdr = VARPTR32(Gnd(Posn))
GndPosn = Posn
Boundry = Boundry + 65535
END IF
Posn = J * 320 + XX
GndPtr = GndAdr + Posn
@GndPtr = Colour
NEXT J
NEXT I