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+--- Thread: Correction to PUT and GET example in QBASIC online help (/thread-9997.html)
Correction to PUT and GET example in QBASIC online help - Bill Buckels - 06-01-2007
'This source code is herewith released by me into the Public Domain.
'Bill Buckels, May 30, 2007
'You have a royalty-free right to use, modify, reproduce and distribute this
'source code and the binaries that it produces in any way you find useful
' This is a corrected and rewritten example based on the QBASIC manual
' Demonstrating GET, BSAVE, PUT and BLOAD.
' Both the GWBASIC manual and the QBASIC manual presented their
' examples in a confusing manner, with too much or too little information.
' The QBASIC manual further complicated the matter with incorrect calculations.
' Here's a less-confusing example with properly calculated values.
' A BASIC Screen Fragment is stored in a byte array
' Preceded with a header giving the image size.
' Header - 4 bytes
' X - width of image in bits - short integer (2 bytes)
' Y - height of image in rasters - short integer (2 bytes)
' Image data - Variable
' Image data is arranged in rasters. Each raster is byte-aligned.
' In the example below, CGA 4-color screen mode is used.
' Rasters are stored at 2 bits per pixel (4 pixels per byte).
' Since the Cube is 101 pixels wide, 26 bytes are required per raster.
' The last 6 bits (3 pixels) of each scanline are unused (padding)
' Since the Cube is 101 rasters high, image data is 26 bytes x 101 = 2626 bytes.
' The Header requires 4 bytes.
' The array size required to store the image is 2630 bytes.
' Since a short integer is 2 bytes in size and since an integer array is
' required to use the GET and PUT commands, an integer array size of 2630/2 =
' 1315 is required for this example.
' Allocate enough memory in an integer array to hold an image fragment
DIM fragment(1315)
fragmentSize = 2630
fragmentName$ = "MAGCUBE.PUT"
SCREEN 1
' Create an image fragment
GOSUB DRAWCUBE
' Transfer the image fragment into the integer array
GET (140, 25)-(240, 125), fragment
' BSAVE
' Point to the the fragment array and BSAVE to disk.
DEF SEG = VARSEG(fragment(1))
BSAVE fragmentName$, VARPTR(fragment(1)), fragmentSize
DEF SEG ' Restore default BASIC segment.
LOCATE 1, 1: PRINT fragmentName$ + " Saved. Press a key to load."
KeyPress$ = INPUT$(1)
CLS
' BLOAD
' The image fragment's array size is BSaved
' so the BLoad command knows how to re-load it from disk
DEF SEG = VARSEG(fragment(1))
BLOAD fragmentName$, VARPTR(fragment(1))
DEF SEG ' Restore default BASIC segment.
' Put the fragment on the screen.
' The fragment dimensions that the GET command stored are now in
' the array, so the PUT command knows the width and height to display
PUT (80, 10), fragment
LOCATE 1, 1: PRINT "Press a key to end..."
KeyPress$ = INPUT$(1)
SCREEN 2
SCREEN 0
END
DRAWCUBE:
' Draw a white box.
LINE (140, 25)-(140 + 100, 125), 3, B
' Draw the outline of a magenta cube inside the box.
DRAW "C2 BM140,50 M+50,-25 M+50,25 M-50,25"
DRAW "M-50,-25 M+0,50 M+50,25 M+50,-25 M+0,-50 BM190,75 M+0,50"
RETURN