Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ncar!ames!oliveb!amiga!cbmvax!grr
From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga.tech
Subject: Re: SDB and tracing through the ROMs
Keywords: SDB, BCPL, breakpoint
Message-ID: <5599@cbmvax.UUCP>
Date: 31 Dec 88 23:49:19 GMT
References: <1988Dec27.223345.29513@ziebmef.uucp>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 24

In article <1988Dec27.223345.29513@ziebmef.uucp> mcp@ziebmef.UUCP (Colin Plumb) writes:
> I'm trying to reverse-engineer the disk validator, and encoutering a few
> snags.  Could someone help me out?
> 
> What I'd really like to do is put breakpoints in the ROMs and work through
> things that way, but I haven't yet found the magic flag that write-enables
> the WCS.  The only likely sounding thing, the "memory overlay bit" on
> PA0 of 8250 A, crashes the machine.  Obviously not what I want.

There is no magic flag.  What you must do to reenable writing to the
control store is issue a 68000 reset instruction, then get things started
up again.  This is, in general, non-trivial task, though I suspose for a
stand-alone game it isn't that hard.

The "memory overlay bit" or "OVR" bit simply controls whether ROM or
"chip" memory is mapped at location 0.  This is neccessary so that the
processor can find it's way to the normal "ROM" area.

It is of course fairly simple to modify the RAM/ROM tower board to
disable the ROM write protect feature...
-- 
George Robbins - now working for,	uucp: {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@uunet.uu.net
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)