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Q26483: CALL INTERRUPTX to Read Disk Sector Can Fail in QB; OK in .EXE

Article: Q26483
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom buglist4.00 buglist4.00b buglist4.50 | mspl13_basic
Last Modified: 20-SEP-1990

The program below uses the CALL INTERRUPTX statement to invoke a
software interrupt to read a disk sector (interrupt 13 hex, function
2). The program correctly reads the specified sector when an .EXE file
is created. However, the program does not read the correct sector from
within the QuickBASIC editor environment.

Microsoft has confirmed this to be a problem in QuickBASIC versions
4.00, 4.00b, and 4.50 for MS-DOS; in Microsoft BASIC Compiler versions
6.00 and 6.00b for MS-DOS (buglist6.00, buglist6.00b); and in
Microsoft BASIC Professional Development System (PDS) versions 7.00
and 7.10 for MS-DOS (buglist7.00, buglist7.10). We are researching
this problem and will post new information here as it becomes
available.

This problem is specific to interrupt 13 hex, function 2 -- other
interrupts invoked with CALL INTERRUPTX should work properly.

The following is a code example:

' Define the type needed for INTERRUPTX
TYPE RegTypeX
     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

' Generate a software interrupt 13 hex, function 2, loading all
' registers.
' Note: The underscore line-continuation mark is no longer supported in
'       QuickBASIC version 4.00. For illustrative purposes it appears
'       in the DECLARE statement below. If this program were to be
'       loaded into the QuickBASIC version 4.00 editor, the underscore
'       would be stripped out and the DECLARE statement would be
'       contained on one line.
DECLARE SUB INTERRUPTX (intnum AS INTEGER, inregs AS RegTypeX,_
                        outregs AS RegTypeX)
DEFINT A-Z
DIM buf(1792)
DIM inregs AS RegTypeX, outregs AS RegTypeX
CLS
inregs.ax = &H201
inregs.cx = &H101
inregs.dx = &H0
inregs.bx = VARPTR(buf(0))
inregs.es = VARSEG(buf(0))
CALL INTERRUPTX(&H13, inregs, outregs)
DEF SEG
FOR I = 0 TO (512)
  p = PEEK(VARPTR(buf(0)) + I)
  IF p > 31 AND p < 127 THEN PRINT CHR$(p);  ELSE PRINT ".";
NEXT I
PRINT HEX$(inregs.ax)

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