Q38068: PRINT SCREEN (Hardware Interrupt 5) Fails If Printer Is Busy

Article: Q38068
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 12-JAN-1990

If the printer is busy (still working on a print job) when a hardware
interrupt 5 (PRINT SCREEN) interrupt is issued (such as by pressing
the SHIFT+PRINT SCREEN or PRINT SCREEN key), the interrupt will not
dump the screen to the printer. The hardware interrupt 5 checks the
printer status before continuing, and if the printer is busy, it
aborts the screen dump.

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 to Microsoft BASIC PDS 7.00 for MS-DOS. It also applies to
any language that supports hardware interrupt 5.

If a program sends some information to the printer and then issues an
interrupt 5 (through CALL INTERRUPT, or an assembly-language program),
the screen dump may be skipped. If the printer has not finished
printing the information sent prior to the interrupt 5 call, the
printer will return "busy" when the print screen interrupt checks the
printer status.

This may not be a problem running on machines with slower clock speeds
(4.77, 6 MHz, etc.), but the same code running on a faster machine
(clock speed 10, 12, 16, 20 MHz, etc.) may fail because the processor
of the computer may work faster than the printer does.

To guarantee that the printer is not busy before doing a screen dump,
you can check the printer status yourself. The printer status can be
obtained by invoking interrupt 17 hex, function 2. The value returned
in the AH register by this interrupt contains the following
information corresponding to each bit:

   Bit     Status
   ---     ------

   7       Printer NOT busy
   6       Acknowledge
   5       Out of paper
   4       Printer selected
   3       I/O error
   2       Unused
   1       Unused
   0       Timed-out

Below are two sample program listings, one for assembly language, and
one for BASIC using the CALL INTERRUPT method, that return the printer
status byte. If the status byte ANDed with 128 is 128, the printer is
not busy; if it is 0, the printer is busy, and a screen dump at that
point would fail. Program logic should loop, constantly checking the
printer status, until the status (AND 128) is 128, and then perform
the interrupt 5.

For another sample BASIC program using this interrupt, query on the
following:

   time-out and printer and interrupt and 17 and function and 2

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

   The assembly listing is as follows:

; Assembled with Microsoft Macro Assembler (MASM) Version 5.x.
.MODEL  medium
.CODE
        public PStat
PStat   proc    far
        mov ah, 2
        mov     dx,0                    ;printer # 0
        int     17h
        mov al, ah
        mov     ah,0                    ;one byte return value
        ret
PStat endp
END

   The BASIC listing is as follows:

REM $INCLUDE: 'QB.BI'
REM For PDS 7.00, you must include QBX.BI
FUNCTION PStat% STATIC
  DIM Regs AS RegType
  Regs.AX = &H200
  Regs.DX = 0                           'printer # 0
  CALL INTERRUPT(&H17, Regs, Regs)
  PStat% = Regs.AX \ 256                'status = AH
END FUNCTION

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