Article Q43534
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How to Use CALL INTERRUPT with Visual Basic for MS-DOS - Q43534
The CALL INTERRUPT statement is a complicated statement that allows
programmers to have access to low-level MS-DOS and ROM BIOS
information and control from Visual BASIC for MS-DOS. Effective use
of the complex CALL INTERRUPT interface requires an understanding of
the Visual Basic for MS-DOS programming environment, the VBDOS
language, and lower-level MS-DOS and ROM BIOS functions. This article
explains many of these necessary features, including the following:
1. Libraries and Quick libraries
2. User-defined TYPEs
3. INCLUDE files
4. CALL INTERRUPT input and output
5. Example of CALL INTERRUPT
6. Differences between CALL INTERRUPT and CALL INTERRUPTX
7. References for documentation on INTERRUPTs
LIBRARIES AND QUICK LIBRARIES
-----------------------------
The object code for the INTERRUPT routines is located in the VBDOS.LIB
and VBDOS.QLB files, which are supplied with Visual Basic for MS-DOS.
The difference between LINK libraries (LIB files) and Quick libraries
(QLB files) is that Quick libraries serve as executable code modules
for use within the VBDOS environment, and LINK libraries are used
at link time to produce executable programs.
To load a Quick library for use with the VBDOS environment, you
must start VBDOS with the /L option (that is, VBDOS /L VBDOS.QLB). This
will allow you to make CALLs to the routines in that Quick library.
When you open the Run menu and choose Make EXE File, your program
automatically will be linked with the library (LIB file) of the same
name as your Quick library (in this case, VBDOS.LIB).
User-Defined Types
------------------
To use the CALL INTERRUPT statement, you must first create a user-
defined TYPE to contain the registers for the INTERRUPT. The TYPEs
defined in the INCLUDE file (VBDOS.BI) that comes with VBDOS are as
follows:
TYPE RegType
ax AS INTEGER
bx AS INTEGER
cx AS INTEGER
dx AS INTEGER
bp AS INTEGER
si AS INTEGER
di AS INTEGER
flags AS INTEGER
END TYPE
TYPE RegTypeX ' See Note below.
ax AS INTEGER
bx AS INTEGER
cx AS INTEGER
dx AS INTEGER
bp AS INTEGER
si AS INTEGER
di AS INTEGER
flags AS INTEGER
ds AS INTEGER
es AS INTEGER
END TYPE
Note: RegTypeX is used with the CALL INTERRUPTX statement, which
allows you to specify the DS and ES registers. For more information
on the CALL INTERRUPTX statement, please refer to the section
"Differences Between CALL INTERRUPT and CALL INTERRUPTX" on Page 4 of
this application note.
INCLUDE Files
-------------
To simplify the TYPE definition for INTERRUPTs, the INCLUDE file
VBDOS.BI is shipped with VBDOS. VBDOS.BI has the TYPE definitions
and SUB declarations needed for INTERRUPTs. To use this file, place
the metacommand $INCLUDE at the beginning of your code. The syntax
of this statement is as follows:
REM $INCLUDE: 'VBDOS.BI'
Please note the following:
1. The $INCLUDE metacommand is placed in a REM (comment) statement.
2. A colon (:) follows $INCLUDE.
3. The filename VBDOS.BI is enclosed in single quotation marks
('QB.BI').
CALL INTERRUPT Input and Output
-------------------------------
Besides the INTERRUPT number, there are two other parameters for the
CALL INTERRUPT statement: the input registers and the output
registers. Before you use these registers, you must dimension two
variables AS the RegType defined earlier, as follows:
DIM inregs AS RegType, outregs AS RegType
For most INTERRUPTs, you need to pass some information (function
number, function parameters) in one or more of the registers. This
assignment is done into individual elements of the user-defined TYPE
inregs, such as the following:
inregs.AX = &H1A00
Note that the above assignment uses hexadecimal values -- denoted by
the "&H"-- instead of decimal values. Most references for INTERRUPTs
use hexadecimal numbers rather than decimal numbers because the high
(1A) and low (00) bytes are easier to distinguish in hexadecimal
notation.
For some INTERRUPTs, it is necessary to set the high-order or low-
order byte of a register. These bytes are referred to in technical
literature with H and L replacing X. For example, the high and low
bytes of the BX register are BH and BL, respectively. To assign the
registers when given high and low bytes, concatenate the hexadecimal
values. For example, if you need to assign CH the value 2Bh and
assign CL the value 3Dh, you would assign CX as follows:
inregs.CX = &H2B3D ' High byte = &H2B Low byte = &H3D
If you are given only 1 byte (high or low), the other byte should be
assigned 00 (two zeros). For example, you would set AH to the value
01h as follows:
inregs.AX = &H0100 ' High byte = &H01 Low byte = &H00
Note: The above statement is NOT equivalent to inregs.AX=&H01.
You must specify the low-order byte or the value will be stored
as &H0001.
Once you have set the values of the input registers, you are ready to
make the CALL. The CALL INTERRUPT syntax is as follows:
CALL INTERRUPT(IntNum%, inregs, outregs)
If an INTERRUPT returns any values, those values will be passed back
in the outregs variable. As with inregs, values will often be passed
in the high or low bytes of a register. The routine BreakWord() in the
"Example CALL INTERRUPT" section below breaks a register into the
2-byte values.
With many INTERRUPTs, you need to check only a single bit (or a few
bits) of any register. This is done using the bitwise operators AND,
OR, XOR, and NOT. For example, the following statement will check to
see if the third bit of AX is set:
IF (outregs.AX AND &H4) THEN ' 4 in hex = 100 in binary.
PRINT "3rd Bit is on"
END IF
The following example program (see below) gives an example of the
the CALL INTERRUPT statement and provides two utility SUB programs
for processing output.
◄See Example►
Differences Between CALL INTERRUPT and CALL INTERRUPTX
------------------------------------------------------
The CALL INTERRUPT and CALL INTERRUPTX statements are very similar.
Either statement allows you to make calls to MS-DOS and ROM BIOS
INTERRUPTs.
The only difference is that with INTERRUPTX, you can specify the DS
and ES registers. (The documentation for INTERRUPTs -- see the
following reference section -- will state whether those registers are
necessary. For most INTERRUPTs, DS and ES are not needed.)
References for Documentation on INTERRUPTs
------------------------------------------
The following books are excellent resources for the different
INTERRUPTs available from MS-DOS and the ROM BIOS. Be aware that
the code in these books is written in assembly language; however,
the necessary input and output is given by register.
1. Advanced MS-DOS Programming, Second Edition by Ray Duncan,
published by Microsoft Press (1988).
2. The New Peter Norton Programmer's Guide to the IBM PC & PS/2 by
Peter Norton, published by Microsoft Press (1988).