Q37475: User-Defined Character Fonts for Hercules Graphics Mode

Article: Q37475
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 5-JAN-1990

The method described in this article will allow you to display a
user-defined character set of up to 256 characters in Hercules
graphics mode. Note that this method works only in Hercules graphics
mode (SCREEN 3) and does not work under MS OS/2.

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.

It is possible to modify two assembly-language programs available in
"Programmer's Guide to PC & PS/2 Video Systems," by Richard Wilton
(Microsoft Press), to display user-defined character fonts in the
graphics mode.

The first program, PixelAddrHGC, is Listing 4-3. This program computes
the starting address of the first pixel for the data to be displayed.
The main assembly program, DisplayCharHGC, is shown in Listing 9-4.

As written, these programs are meant to interface with Microsoft C. To
change them to work with BASIC, do the following:

1. Change all near references to far (use medium memory model).

2. Remove the underscore in the public name.

3. Create storage space for VARmask, VARtoggle, and VAR9bits in
   DGROUP.

4. Reverse the input order of the arguments. Instead of using
   interrupt vector 43 hex to point to the first 128 characters, and
   1F hex to point to the font table containing the rest of the
   characters, use an unused interrupt vector to point to a single
   table containing 256 characters.

5. Set DS == ES.

The BASIC program below calls a modified form of DisplayCharHGC to
print a user-defined font to the screen. This program works correctly
in the QuickBASIC environment and in stand-alone executable form.

The following is a code example:

' $INCLUDE: 'q:qb.bi'
REM for BASIC PDS 7.00 include QBX.BI instead
DECLARE SUB DisplayCharHGC (BYVAL A%, BYVAL x%, BYVAL y%,_
             BYVAL f%, BYVAL b%)
DEFINT A-Z
DIM RegX AS RegtypeX

' set table large enough to hold the character set
DIM table(2048) ' 256 * 8

' table is common to put it in the DGROUP
COMMON table()

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

CLS
SCREEN 3
DEF SEG = VARSEG(table(0))
FOR i = 1 TO 24
   READ A%   'Place the created characters into the new
   POKE VARPTR(table(0)) + i, A%    'graphics table
NEXT i
DEF SEG

 'get the initial value at interrupt 50 hex
   RegX.Ax = &H3550
   RegX.Ds = -1
   RegX.Es = -1
   CALL interruptX(&H21, RegX, RegX)
   PRINT "Vector 50H was "; RegX.Es, RegX.Bx
   oldoff& = RegX.Es
   oldPtr& = RegX.Bx

 ' reset interrupt 50 hex to point to table of characters
   RegX.Ax = &H2550
   RegX.Ds = VARSEG(table(0))
   RegX.Dx = VARPTR(table(0))
   CALL interruptX(&H21, RegX, RegX)

 ' verify the reset took
   RegX.Ax = &H3550
   RegX.Ds = -1
   RegX.Es = -1
   CALL interruptX(&H21, RegX, RegX)
   PRINT "New vector "; RegX.Es, RegX.Bx
   A% = 10

' print characters
FOR i = 0 TO 2
        A% = A% + 1
        CALL DisplayCharHGC(i, 20, 20 * A%, 0, 7)
NEXT

'restore original vector
   RegX.Ds = oldoff&
   RegX.Dx = oldPtr&
   RegX.Ax = &H2550
   CALL interruptX(&H21, RegX, RegX)

'verify restoration
   RegX.Ax = &H3550
   RegX.Ds = -1
   RegX.Es = -1
   CALL interruptX(&H21, RegX, RegX)
   PRINT "Vector 50H reset to "; RegX.Es, RegX.Bx

' pause
   SLEEP

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