Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!bbn!apple!dlyons
From: dlyons@Apple.COM (David Lyons)
Newsgroups: comp.sys.apple
Subject: Re: Slot Assignments
Keywords: Slots, Booting
Message-ID: <33278@apple.Apple.COM>
Date: 19 Jul 89 07:17:50 GMT
References: <447ed450.f759@gtephx.UUCP>
Distribution: na
Organization: Apple Computer Inc, Cupertino, CA
Lines: 36

In article <447ed450.f759@gtephx.UUCP> campbellb@gtephx.UUCP (Brian Campbell) writes:
> [...]  Having flexible slot assignments does
>make programming the card ROM more difficult.  It also seems that code must
>be relocatable, and that means that JSRs in the ROM to subroutines in the ROM
>are not possible.  Is this true?  [...]

True--you can't just JSR $Cnxx from the $Cnxx space without assuming you know
what slot you're in.  For simple ROMs, the answer is to simply not make JSRs
to your own ROM code.  For more complicated ROM, you can take advantage of the
$C800-$CFFE space, which is *shared* (more or less carefully) between different
slots.  Only one slot "owns" this space at a time, any twiddling $CFFF tells
all cards they no longer own it.  As soon as $Cnxx is twiddled, slot n owns the
space until $CFFF is twiddled.

So you can put your subroutines in the $C800-$CFFE space and JSR them with no
problems.  'course, if some cards forget to twiddle $CFFF before assuming they
own that space, there can be problems.  (There *are* cards like that out there,
and I *can't imagine* what company would have produced them.... :-)

> [...] since it never makes sense to boot off of a printer,
>you put that down in a low slot position, like #1.  Since, it makes sense to
>boot off of a disk, you put that in a high slot position.

Except that the system is smart enough to realize when a card is not a bootable
device--it looks at $Cn01, $Cn03, and $Cn05 for the values $20, $00, $03 to
identify it as bootable.  (I think the ROMs *before* the enhanced IIe also
looked at $Cn07, but I'd have to look it up.)

 --Dave Lyons, Apple Computer, Inc.          |   DAL Systems
   AppleLink--Apple Edition: DAVE.LYONS      |   P.O. Box 875
   AppleLink--Personal Edition: Dave Lyons   |   Cupertino, CA 95015-0875
   GEnie: D.LYONS2 or DAVE.LYONS         CompuServe: 72177,3233
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   My opinions are my own, not Apple's.