How to Assign High & Low Registers for CALL INTERRUPT, INT86 (27287)

SUMMARY

When you invoke MS-DOS and BIOS interrupts from Visual Basic for MS-DOS, you pass full-word (two-byte) register variables such as AX, instead of the half registers AH (high byte of AX) and AL (low byte of AX). This article describes how to assign or read half registers before or after calling the interrupt routines.

This information applies to CALL INT86OLD and CALL INTERRUPT in the Standard and Professional Editions of Microsoft Visual Basic for MS-DOS version 1.0; to Microsoft QuickBasic for MS-DOS versions 4.0, 4.0b, and 4.5; to Microsoft Basic Compiler for MS-DOS versions 6.0 and 6.0b; and to Microsoft Basic Professional Development System (PDS) for MS-DOS versions 7.0 and 7.1. This information also applies to CALL INT86 and CALL PTR86 in Microsoft QuickBasic for MS-DOS versions 2.0, 2.01, and 3.0.

This information is also included with the Help file provided with the Standard and Professional Editions of Microsoft Visual Basic for MS-DOS, version 1.0.

MORE INFORMATION

You can use any of the following methods to assign values to high and low half-registers and load them into the full-word (two-byte) registers:

The simplest method is to combine the hexadecimal values of the high and low registers into one hexadecimal constant:

   AX = &H0941             ' Where AH=&H09, AL=&H41
   BX = &H0002             ' Where BH=&H00, BL=&H02
   CX = &H07D0             ' Where CH=&H07, CL=&HD0

The following is a more flexible method, letting you assign variables to the high and low registers with a formula:
AX%, BX%, CX%, or DX% = (high% * 256) + low%
In this case, "high%" and "low%" contain the decimal values that you want to assign to the high and low half-registers (where high% can be a value from 0 to 127). For example:
```
   high% = 9                   ' 9 = &H09
   low% = 65                   ' 65 = &H41
   AX% = (high% * 256) + low%    ' AX = 2369 = &H0941
						
```
You will get an integer "Overflow" error if high% is 128 or larger. Use method 3 below if you need high% between 128 and 255.

If you need the high byte (AH, BH, CH, or DH) to be a value from 1 to 255, then you can POKE the value as follows:

   high% = 255
   low% = 65
   AX% = low%            ' Assigns low byte into AX%
   POKE VARPTR(AX%)+1,high%   ' Pokes the high byte into AX%

Note that the following is a quick way to convert a decimal number to hexadecimal using the Immediate window in the QB.EXE, QBX.EXE or VBDOS.EXE environment:

PRINT HEX$(number)

(To open the Immediate window select Immediate from the Window menu. Pressing F6 in the VBDOS.EXE environment lets you switch between windows. Pressing F4 toggles between viewing the editor and viewing the output screen.)

The following formulas return the contents of the half registers, which are stored in the two bytes of a full register, such as AX, returned from an interrupt routine:

AL% = AX% MOD 256 ' MOD operator returns integer remainder of division
PRINT "The AL register contains &amp;H"; HEX$(AL%)

AH% = AX% \ 256  ' Integer division by 256 removes the lower byte.
PRINT "The AH register contains &amp;H"; HEX$(AH%)