Q58103: How to Convert Unsigned Integer from Another Language to BASIC

Article: Q58103
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900111-205 B_BasicCom S_C H_MASM S_QuickC | mspl13_basic
Last Modified: 10-JAN-1991

This article describes how to convert an unsigned integer value
(returned from another language) to a BASIC LONG integer, keeping the
sign correct. Bit manipulation is necessary in this conversion because
BASIC does not support an unsigned integer data type.

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 MS OS/2, and to Microsoft BASIC Professional Development
System (PDS) versions 7.00 and 7.10 for MS-DOS and MS OS/2.

BASIC does not support an unsigned integer data type. However, there
are times when a BASIC program must accept an unsigned integer type
returned after calling a function in languages such as C or assembly
language.

If your other-language routine returns an unsigned integer to a BASIC
INTEGER data type, and if the value of the integer exceeds 32,767 (or
7FFF hex), it will PRINT in BASIC as a negative number, even though
the CALLed routine meant to return a positive number in the range of
32,768 to 65,535 (passed as an unsigned integer). In such a case, your
BASIC program must convert the number to a positive number stored in a
BASIC long integer.

BASIC stores both INTEGER and LONG data types as signed
two's-complement numbers. In signed two's-complement format, the
highest bit in the number is a a sign bit. When a BASIC program is
passed an unsigned integer from another language, and the number is in
the range of 32,768 to 65,535, the highest bit will be set, causing
BASIC to treat the number as negative.

For more information, see Pages 16-17 of "Microsoft QuickBASIC 4.0:
BASIC Language Reference," in the "Data Types" Chapter 2; or see an
assembly language book that discusses signed two's-complement data
types.

You can convert an unsigned integer to a positive BASIC LONG integer
as follows:

1. Check if the integer is positive or zero. If BASIC already
   recognizes the number as positive or zero, then either use it as
   is, or assign it directly to a long integer and skip the next three
   steps.

2. Otherwise, if the number (x%) is negative, then set the high bit to
   zero, as follows:

      x% = x% AND &H7FFF&

3. Assign the number to a long integer, as follows:

      y& = x%

4. Then set the 15th bit (counting from bit zero) back to a one, as
   follows:

      y& = (y& OR &H8000&)

   The long integer (y&) now contains the correct positive integer
   that the other-language routine meant to pass back to BASIC.

A simple way to do this is to use the following function:

   FUNCTION Unsigned&(param%)
      Unsigned& = &HFFFF& AND param%
   END FUNCTION

Example 1
---------

The following program converts the unsigned integer stored in x% to a
positive LONG integer stored in y&:

x% = -1     ' -1 in two's complement is 65535 as unsigned integer
IF x% < 0 THEN
   ' Set the 15th bit to zero (counting from bit 0):
   x% = (x% AND &H7FFF&)
   ' Assign it to a LONG integer:
   y& = x%
   ' Set the 15th bit back to a one:
   y& = (y& OR &H8000&)
ELSE
   y& = x%
END IF
PRINT y&   ' Prints 65535

Example 2
---------

The following is a BASIC code example that CALLs the INTERRUPT routine
provided in QB.QLB or QB.LIB. This program accesses the PC system
clock to get the number of clock ticks since midnight. It then
calculates the number of seconds (in hundredths) since midnight.

To run the program in the QuickBASIC editor, you must load the QB.QLB
library, as follows:

   QB TIMEINT.BAS /L QB.QLB

For BASIC PDS 7.00 and 7.10 do the following instead:

   QBX TIMEINT.BAS /L QBX.QLB

To compile and LINK the program, use the following:

   BC TIMINT.BAS;
   LINK TIMINT,,,QB.LIB;

For BASIC PDS 7.00 and 7.10 LINK as follows instead:

   LINK TIMEINT,,,QBX.LIB;

Code Example
------------

DEFLNG A-Z
' $INCLUDE: 'qb.bi'
CONST tps = 18.2064699073# ' tps is ticks per second
DIM inregs AS RegType, outregs AS RegType
DIM flag AS INTEGER, Previous AS LONG
CLS
WHILE 1
     inregs.ax = 0
     CALL INTERRUPT(&H1A, inregs, outregs)
     ' CX should never be larger than &H0017, so direct
     ' assignment is possible.
     ticks& = outregs.cx * &H10000
' This IF statement is the part that accomplished the conversion.
     ' First, only convert if BASIC thinks it's a negative number.
     IF outregs.dx < 0 THEN
        ' Set the 15th bit to zero (counting from 0)
        a2& = (outregs.dx AND &H7FFF&)
        ' Assign it to a LONG integer.
        ticks& = (ticks& + a2&)
        ' Set the 15th bit back to a one.
        ticks& = (ticks& OR &H8000&)
     ELSE
        ' Otherwise, just assign it to a long or use it 'as is'.
        ticks& = ticks& + outregs.dx
     END IF
     IF Previous > ticks& THEN BEEP ' It's midnight!
        Previous = ticks&

     Previous = ticks&
     seconds# = ticks& / tps 'ticks per second

     LOCATE 10, 10
     PRINT "Clock ticks since midnight: ";
     PRINT ticks&
     LOCATE 11, 10
     PRINT "Seconds since midnight: ";
     ' Print out the seconds to the hundredths place.
     PRINT USING "######.##"; seconds#
WEND
END

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