Article ID: 35664
Article Last Modified on 11/21/2006
APPLIES TO
- Microsoft QuickBasic 4.0, when used with:
- Microsoft MS-DOS 6.22 Standard Edition
- Microsoft QuickBASIC 4.0b, when used with:
- Microsoft MS-DOS 6.22 Standard Edition
- Microsoft QuickBasic 4.5 for MS-DOS, when used with:
- Microsoft MS-DOS 6.22 Standard Edition
This article was previously published under Q35664
SUMMARY
The program below demonstrates how to create your own non-ASCII characters for use in CGA screens 1 and 2. This way you can make characters such as 1/3, or foreign letters and symbols not found in the ASCII or extended-ASCII character set.
This information applies to Microsoft QuickBasic Versions 4.00 4.00b and 4.50, to Microsoft Basic Compiler Versions 6.00 and 6.00b for MS-DOS, and to Microsoft Basic PDS Version 7.00 for MS-DOS.
MORE INFORMATION
In graphics mode, the bit patterns for ASCII character codes 128 through 255 are obtained from a table located in RAM. The address of this table is located in the Interrupt Vector Table at location 0000:007C (the vector for interrupt 1F). This location can be modified to install modified character sets. Note, that on a standard IBM PC or compatible, ASCII characters 0 through 127 are contained in the ROM BIOS and cannot be replaced. (The IBM PCjr, which is not supported by QuickBasic, handles these characters differently.)
To create your own character set, you would create a table of characters and replace the current address in the Interrupt Vector Table with the address of your table. Keep in mind that the segment and offset are stored "backwards" in the table. The following example
F000: FF54
would be stored as follows:
54 FF 00 F0
The information stored in the table should be an 8 x 8 bitmap for each character desired. For example, the following character would be represented by the following eight decimal values:
Character Decimal Value
--------- -------------
11110000 240
10010000 144
10010000 144
11111110 254
10010010 146
10010010 146
10011110 158
00000000 0
The decimal value is the binary value for each line converted to decimal.
Code Example:
The three DATA statements hold the binary coding for the characters to be represented. They are a small triangle, a sigma, and a 1/3. In binary, the data looks like the following:
Triangle Sigma 1/3
-------- ----- ---
00000010 11111110 10000100
00000110 00100000 10001000
00001110 00010000 10011110
00011110 00001000 10100010
00111110 00010000 01000110
01111110 00100000 10000010
11111110 11111110 00001110
00000000 00000000 00000000
DATA 2,6,14,30,62,126,254,0
DATA 254,64,32,16,32,64,254,0
DATA 132,136,158,162,70,130,14,0
DIM table(100)
COMMON SHARED table()
location = VARPTR(table(0))
segment = VARSEG(table(0))
DEF SEG = 0: SCREEN 1
A = PEEK(124) 'Save this information and restore it when done.
B = PEEK(125)
C = PEEK(126)
D = PEEK(127)
POKE 124, location MOD 256 'This is where the address of the graphics
POKE 125, location \ 256 'screen characters are stored. The program
POKE 126, segment MOD 256 'changes this so now it thinks the graphics
POKE 127, segment \ 256 'characters are stored in the table() array.
DEF SEG
FOR I = 1 TO 24
READ A% 'Place the created characters into the new
POKE location + I, A% 'graphics table
NEXT I
FOR I = 128 TO 130 'Print the new characters out.
PRINT CHR$(I) + " ";
NEXT
DEF SEG = 0 'Restore to the original graphics character set.
POKE 124, A
POKE 125, B
POKE 126, C
POKE 127, D
DEF SEG
Additional query words: QuickBas BasicCom
Keywords: KB35664
