Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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Miscellaneous Memory Locations
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  Address
  (hex)       Size      Description
 
  0000:0478   4 bytes   Loop counters for LPT1 and LPT2. These counters
                        determine the number of times the printer
                        service (interrupt 17h) attempts to poll the
                        printer for a not-busy signal. If the signal is
                        not received within the given number of tries,
                        interrupt 17h returns AH=1, indicating a
                        printer time-out error.
 
  0000:047C   4 bytes   Loop counters for serial ports COM1 to COM4.
                        These counters determine the number of times
                        the serial communications service (interrupt
                        14h) attempts to read the modem or line status
                        registers. If the desired status is not
                        received within the given number of tries,
                        interrupt 14h returns an appropriate error code
                        in AH.
 
  0000:0500   1 byte    Indicates status of print-screen operation:
 
                        0    = Successful
                        1    = Currently in progress
                        0FFh = Failed
 
  0000:0504   1 byte    Indicates whether disk in single-disk system acts
                        as drive A or B:
 
                        0 = Acts as drive A
                        1 = Acts as drive B
 
  See also: Interrupt 05h (Print Screen),
            Interrupt 14h (Serial Communications Services),
            Interrupt 17h (Printer Service)
                                    -♦-