Q27287: How to Assign High & Low Registers for CALL INTERRUPT, INT86

Article: Q27287
Product(s): See article
Version(s): 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 31-OCT-1988

The QuickBASIC interrupt routines (CALL INT86 and PTR86 in QuickBASIC
2.x and 3.00; and INT86old and INTERRUPT in QuickBASIC 4.00/4.00b and
the BASIC Compiler 6.00/6.00b) are passed full-word register variables
such as AX instead of half registers AH (high byte of AX) and AL (low
byte of AX). The CALL statement documentation in the reference manual
for QuickBASIC Versions 2.x, 3.00, and 4.00 does not clearly state how
to assign or read half registers before or after calling the interrupt
routines.

The following are two methods to assign values to high and low
registers and to load them into the full word (two-byte) registers:

1. 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

2. 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 which you
   want to assign to the respective half registers. For example:

   high% = 9                   ' 9 = &H09
   low% = 65                   ' 65 = &H41
   AX = (high% * 256) + low%    ' AX = 2369 = &H0941

Note that the following is a quick way to convert a decimal number to
hexadecimal using the immediate mode window of the QuickBASIC Version
4.00 editor:

PRINT HEX$(number)

(Pressing F6 in the QuickBASIC Version 4.00 editor lets you activate
the immediate mode window. Pressing F4 toggles between viewing the
editor and viewing the output window.)

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 &H"; HEX$(AL%)

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

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