Shareware Grab Bag/Shareware Grab Bag.iso/015/printcom.arc/PRINTCOM.INF

Text File | 1986-11-02 | 11.3 KB | 272 lines

Date: Sun, 25 Aug 85 23:02:37 edt
From: [e-mail] (James A. Mullens)
To: [e-mail]
Subject: PRINT.COM 3.1 Internals


I offer the following description of the internals of PRINT.COM for
DOS 3.1 as a guide for those wishing to implement background tasks
under DOS 3.1.  This is the non-networked version of PRINT.COM.

This information is a good starting point for serious hackers,
determined to implement their own background task or collapse over the
keyboard trying.  I won't bore you with all of the reasons you should
not write such a background task -- just let me say that if you do, be
prepared to assume full responsibility for your actions and the actions
of your program!

I am giving this to the public domain mostly because I think some people
might have some fun trying to use this stuff, and because I hope some
public domain programs will turn up.  This little project required
the interpretation of 47 pages of uncommented assembly language, which
relocates itself, uses 3 different CSs, and has innumerable subroutines.
I doubt that I am the first to do this, but I appear to be the first to
offer my findings to the net.

The description is only partially complete.  I tried to concentrate on
the parts of PRINT.COM related to the problem of calling DOS functions.
I cannot say that all you need to know is here because I have not
tried to write a background program that calls DOS.  Even if I had an
(apparently) working background program, I would hesitate to say that
it didn't have a bug which would rear its ugly head in the future.

I look forward to exchanging notes with anyone else who takes an
interest!  If you do something in the near future, please send a
message to me directly (in addition to the network) so that I won't
be attempting to do something you have already done.


               DESCRIPTION OF DOS 3.1 PRINT.COM INTERNALS
                     James A Mullens / jcm@ornl-msr
                            August 1985

     First Stage Setup

    Compute stack address and entry point for second stage of setup.
    Adjust some values pointed to by a table.
    Relocate most of program, moving down 310 bytes.
    JMP FAR to second stage setup, changing CS:IP.

     Second Stage Setup

    Check DOS version.
    Free Allocated Memory.
    Save pointers to old INTs 28,2F,13,15,17,14,05, and 1C if
      spooler is not installed already.
    If spooler already installed, but OK to reinstall, do so.
      If not OK to reinstall do something (?) which may result
      in task terminate with result code FF.
    Do INT 37 (DOS internal). Reason for doing?
    Scan command line, possibly adding filename passed to spool
      queue.  Operations done here may result in closing of
      file handles 0 - 5 and terminate with result code 0.
    Many operations involving byte memory comparisons and
      subroutine calls...
      Get current disk (DOS func 19). (Label L15FF)
      Get current directory (DOS func 47). (Label L1627)
      Get extended error (DOS func 59). (Label L1730)
      Skip remainder of setup if PRINT is already installed.
        Simply submit new file for spooling.
    Ask for name of device to spool to.

     Third Stage Setup - installation of memory resident portion.

    Find DOS driver for output device.
      This involves DOS func 52, perhaps to find a pointer to the
      device driver list.  Reject any block-structured device
      as an output device.
    INT redirection.  Save and reset vectors for INTs 28,13,15,
      17,14, and 05.
    DOS function 34 to get location of the INDOS flag in ES:BX.
      The INDOS flag is not used in the same way as DOS v2.0
      used it.
    Save old INT 1C handler address.
    Get print spooler installed state.  If not installed, install
      new INT 1C handler.  If spooler already installed,
      test a flag to determine if should not install new
      INT 1C handler.
    Install new INT 1C handler if necessary.
    Print 'resident part of print installed'.
    Far jump to address stored at CS:[0E73H].  I assume this exits.

     Timer (INT 1C) actions

    (Hex numbers refer to memory locations).

    Do an INT 2A and exit if carry set.  This is an undocumented
      INT, but interesting.  It seems one is obliged to do
      INT 2A before exiting interrupt if this call was successful.
      This may be a simple guard against re-entering this 1C
      handler.
    Increment 2D7 and 2D8.  Used in the print action routine.
    If 2D9 is not 0, decrement it and
      exit after calling INT 2A.  (2D9 is a tick countdown 'timer'.
      Print actions are done only when it counts down to 0).
    If 2CC is not 0, exit without doing INT 2A, else read 8259
      interrupt status register.  Exit after doing INT 2A if
      any interrupts except the timer are active.
      2CC is also increment/decremented in the INT 28 handler and
      the ROM BIOS INT 13 handler (note bene).
    Check INDOS flag byte and the byte _before_ it.  If any bits
      are set, exit after doing INT 2A.
    Increment 2CC (flag that print actions are being done).
    Zero 2D7 and 2D8.
    Enable interrupts.
    Test byte flag 2D5.  If any bits set, do not send EOI to
      8259.  This flag appears to be used to communicate with the
      INT 2F handler.  Possibly the INT 2F handler can invoke
      this INT 1C handler, in which case hardware interrupt stuff
      should not be done.
    Do any print actions needed (including write, read, open a
      new spool file, print error messages, etc.).
    Disable interrupts.
    Move 2DA to 2D9.  (Reset countdown timer).
    Decrement 2CC.  (Flag that print actions are over).
    Do INT 2A.
    If 2D5 has no bits set call old INT 1C handler, otherwise
      exit via IRET.

     INT 2F (new handler inserted by PRINT)

    This handler is documented in the DOS manual.

     INT 28 Actions (new handler inserted by PRINT)

    INT 28 is undocumented, 'used internally by DOS'.  This INT
    will also cause print actions to do done under certain
    conditions.  It is believed that INT 28 calls are made by the
    DOS 'get from keyboard' routines whenever it is safe to do
    file i/o.  (See the net's archive file documenting DOS
    functions and interrupts.)  Notice that no attempt is made
    to examine the 'INDOS' flag.

    Call INT 2A.  If carry set, exit immediately.
    If 2CC is not 0, exit after calling INT 2A.
    Increment 2CC and 2D6.
    Disable INTs.
    Do Print actions (identical to INT 1C handler actions).
    Zero 2D6, move 2DA to 2D9 (byte), decrement 2CC.
    Do INT 2A and continue with old INT 28 handler.

     Print Actions called by INT 1C and INT 28 handlers

    This is a very elaborate piece of code.  A queue of files
    to print is maintained.   Files are opened and closed.
    Errors are handled.  Etc, etc.  I assume most people will
    not be interested in how the file spooling is done.

    The extended file management group of DOS functions are
    used for file i/o.

    In the print action routine, DOS functions 50 and 51
    are used whenever file i/o is done.  Some condition is indicated
    by a flag of 0 or 1.  The print action routine and INT 2F
    handler explicitly switch to state 0 before terminating.
    This is done by moving a word to BX and calling DOS function
    50.  Whenever file i/o is to take place, the state is switched
    to 1.  This is done by calling DOS
    function 51, saving a result in BX to the word used above,
    moving a new word into BX, and calling DOS function 50.

    I have 2 clues as to what DOS functions 50 and 51 do.  One
    source says DTA stuff for DOS 2.x, another source says
    PSP stuff for DOS 3.x.  (The net's archive file documenting
    DOS functions says with assurance that 50 sets a PSP address
    and 51 gets the current PSP address.  This seems to be so.)
    If these are PSP actions, then state 1 has the PSP set to its
    original value (on PRINT.COM entry), and this is the state
    set when doing file i/o.  I do not know why it is changed
    when print actions are over.

    DOS function 5D is called just prior to exiting the print
    actions routine.

    See the discussion of the INT 15 handler below for a special
    case of control-break handling when doing DOS function calls.

    Special character handling:
    Backspace erases the previous character.
    Non-printing control characters are filtered.
    Tabs are expanded and all of the spaces sent
    immediately to the output device -- the code seems to have the
    philosophy that all output resulting from the current input
    character must be done in the current call of the INT 1C
    handler.

    Output is done by calling device driver 'strategy' and
    'interrupt' routines.  (A device 'interrupt' routine has
    nothing to do with machine interrupts -- it only performs
    the actions indicated by the strategy call.)
    The device state is checked prior to write commands to see if
    the device is busy or has an error.  PRINT will attempt to
    get a 'ready' state 20,000 times before giving up.

     New ROM BIOS interrupt handlers.

    Disk handler (INT 13)
        Increment 2CC on entry, decrement 2CC on exit.
        This flag is checked and used by INT 1C handler and
        INT 28 handler also.
    Cassette and other device handler (INT 15)
        The calls expected by the new handler do not conform
        to IBM PC ROM BIOS cassette tape handler.
        I notice that the AT supports a variety of device
        calls through this vector, but I haven't seen a
        documented call for the function this new INT 15
        handler intercepts (AH = 20).

        The new handler would pass all 'old cassette' calls
        to ROM BIOS.  Any call with AH = 20 would be
        intercepted.  If AL > 1, go to the old handler.
        (On the old cassette handler, AH > 1 meant do a
        read/write).  If AL = 1, zero flag CBB and return.
        (On the old cassette handler, AH = 1 meant turn the
        motor off). If AL > 1, increment CBB and move the
        passed ES:BX to CBC, and return.
        When the Print Action routine does file i/o, it goes
        to great pains to disable control-break recognition
        whenever flag CBB is set.  When CBB is set, the file
        i/o routine increments the byte pointed to by CBC,
        then does the i/o, and then decrements the byte.
    Printer handler (INT 17)
        Reject all calls for printer unit which is the output
        device.  (Use flag 2CC to determine if call is from
        PRINT code.
    Serial Port handler (INT 14)
        Similar to printer handler.
    Print Screen handler (INT 05)
        Reject call unless not spooling to printer 0.

     SUMMARY

    Run the background task off timer (1C) interrupts.

    You may be required to implement an INT 28 handler if
    it is necessary to perform file operations while the
    foreground is waiting on keyboard input.

    You may be required to issue INT 2A on entry, and exit
    immediately if returns with carry set.  If carry is not set
    you must also call INT 2A before exiting your interrupt
    routine.

    Intercept ROM BIOS calls to reject any calls on the device
    being used by the background job.

    Do not attempt to execute while the foreground
    job has called the ROM BIOS disk (INT 13) functions.

    Do not attempt to execute if INDOS byte or the byte before it
    have any bits set.

    Do not attempt to execute if any other hardware interrupts
    are being processed.

    It is possible to do normal DOS file handling calls, but
    it may be necessary to perform additional actions similar to
    (undocmented) DOS functions 50 and 51.  A call to DOS function
    5D may also be necessary after all file stuff is finished.


Enjoy!!