NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/sys/atari/8bit/5931

Text File | 1993-01-22 | 3.3 KB | 78 lines

Newsgroups: comp.sys.atari.8bit
Path: sparky!uunet!spool.mu.edu!yale.edu!ira.uka.de!news.belwue.de!ifi!straif
From: [e-mail] (Peter Straif)
Subject: Re: File Headers.
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: Informatik, Uni Stuttgart, Germany
References: <[e-mail]> <[e-mail]>
Date: Fri, 22 Jan 1993 11:53:53 GMT
Lines: 66

In <[e-mail]> [e-mail] (Scott M. Ballew) writes:

>In article <[e-mail]> [e-mail] (Marek Tomczyk) writes:
>>> What determines if it runs or returns to DOS?
>>
>>In order to automatically run a program you should specify a
>>starting address. This starting address is stored into the
>>memory locations $02E0-$02E1. An initiation address should be
>>written into $02E2-$02E3 (it could be vice versa, I don't have my
>>notes with me).
>>
>> ...
>>
>>> If you chain (by appending) files together, Which one gets control?
>>
>>Very simple, that one whose address is stored in the $02E0 vector.
>>You could specify an init address somewhere in the middle of the
>>chained file. Then DOS jumps to this routine. In order to continue
>>loading you have to exit your init routine with an RTS instruction.

>You missed a very important subtlety in this system.  First, there
>is a difference between the RUN address and the INIT address (and I,
>too, cannot remember which is at $2E0-2E1 vs $2E2-2E3).  When DOS
>loads the INIT address, it does a jsr to that address _immediately_
>(without continuing the load).  When that code does and rts to DOS,
>DOS picks up with the load where it left off.  This means that you can
>have multiple INIT addresses strewn through your load file and they
>are treated like subroutines to the load process (be sure the code
>they run has already been loaded, though :-).  In the case of the RUN
>address, only the last value stored here matters.  This is the one
>that DOS jsr's to _after_ the load is complete.

>Now, can someone with a memory map handy verify which address is
>which?  I'm not sure where my memory map is.

>Scott


    Ok, $02E0 - $02E1 is the run address,
        $02E2 - $02E3 is the init address.

I'm have an addition to: There is one problem, that occurs, when you
use any SIO-handler (like P:) in an Init-Part:
    All IOCB-Blocks are copied into the zero page, when an operation
    is done on any channel.Normally, this doesn't matter, because
    on every SIO/CIO call this is done. But when you do it in the init part
    of a COM-File and then return to DOS ( with RTS ) , sometimes
    something strange can happen. For instance, I've programmed
    one time an Init-Part that sended a text to the printer ( p:),
    and after the RTS, Dos continued to load, but the floppy didn't
    READ from the file, but WROTE to it, because I've opened before
    an IOCB to the Printer with open #x,8,0,"P:". The only solution
    I saw, was to save the zero page IOCB before outputting to the
    printer, and afterwards to restore it.
    The same thing can happen, if you use handler-routines in the VBI
    ( for instance a T:-handler to get the current time from an
    real time clock).


Peter

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