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Q39859: CALL INTERRUPT 25H Doesn’t Return Error Flags in the AX Reg

Article: Q39859
Product(s): See article
Version(s): 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom SR# S881216-23 | mspl13_basic
Last Modified: 14-DEC-1989

Microsoft QuickBASIC supports the CALL INTERRUPT and CALL INTERRUPTX
statements for calling MS-DOS and ROM BIOS functions/interrupts. The
Interrupt Hex 25 is used to perform an absolute read from disk to a
memory location. If there are any errors during this operation, the
interrupt will return them in the AH register.

If the CALL INTERRUPT or CALL INTERRUPTX are used to execute Interrupt
25 Hex, the correct error code is not returned. This problem occurs
because Interrupt 25H does not POP the CPU flags from the stack after
its operation; these are flags the INT 25 Hex function originally
PUSHed onto the stack. Therefore, when INTERRUPTX returns to
QuickBASIC, it returns the "old" flags. These do not reflect the
Interrupt 25 Hex function-flag return. Most MS-DOS and ROM BIOS
interrupts do POP the CPU flags from the stack upon completion and
this problem does not exist. INTERRUPTX does not compensate for the
extra set of flags on the stack.

This information applies to Microsoft QuickBASIC Versions 4.00, 4.00b,
and 4.50, Microsoft BASIC Compiler Versions 6.00 and 6.00b for MS-DOS
and MS OS/2, and Microsoft BASIC PDS 7.00 for MS-DOS and OS/2.

The two workarounds for this problems are as follows:

1. Call an assembly-language subroutine that performs the Interrupt 25
   Hex. The correct flags will be directly accessible by the
   subroutine and can then be cleared with a POPF or ADD SP,2
   instruction. More information on this can be found on Pages 388-389
   of "Advanced MS-DOS" by Ray Duncan (published by Microsoft Press,
   1986).

OR

2. Call a different but very similar interrupt as follows:

      Interrupt 13 Hex  Function 02 Hex

   This is not an MS-DOS function, but rather a ROM BIOS interrupt,
   which (unlike Interrupt 25 Hex) does its own stack clearing. This
   is described below.

In the following programs, Prog1 demonstrates the problem. No matter
what Interrupt 25 Hex error occurs, the AX flags are always returned
the same. Prog2 follows the second workaround described above and uses
the Interrupt 13 Hex, which will return the proper error flags.

Prog1 is as follows:

'The include file below needs to be 'QBX.BI' when using
'BASIC PDS 7.00 and the Quick library is QBX.QLB.

'$INCLUDE: 'QB.BI'
DEFINT A-Z
DIM InRegs AS RegTypeX, OutRegs AS RegTypeX
DIM buffit(1 TO 8000)  AS INTEGER
CLS
InRegs.ax = &H0
InRegs.cx = &H0
InRegs.dx = &H0
InRegs.ds = VARSEG(buffit(1))
InRegs.bx = VARPTR(buffit(1))
CALL INTERRUPTX(&H25, InRegs, OutRegs)
PRINT OutRegs.ax
END

Prog2 is as follows:

'The include file below needs to be 'QBX.BI' when using
'BASIC PDS 7.00

'$INCLUDE: 'QB.BI'
DEFINT A-Z
DIM InRegs AS RegTypeX, OutRegs AS RegTypeX
DIM buffit(1 TO 8000)  AS INTEGER
CLS
InRegs.ax = &H201
InRegs.cx = &H1
InRegs.dx = &H0
InRegs.es = VARSEG(buffit(1))
InRegs.bx = VARPTR(buffit(1))
CALL INTERRUPTX(&H13, InRegs, OutRegs)
PRINT OutRegs.ax
END

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