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.

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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:

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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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