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From: hojo@cbnewsl.att.com (HC Johnson)
Newsgroups: comp.sys.atari.st
Subject: Re: Mac 128K ROMs ---> questions
Summary: roms switch lines info
Message-ID: <1991Jan10.183716.18268@cbnewsl.att.com>
Date: 10 Jan 91 18:37:16 GMT
References: <1991Jan4.012611.23146@contact.uucp> <1908@diemen.utas.edu.au> <2611@krafla.rhi.hi.is>
Organization: AT&T Bell Laboratories
Lines: 50

In article <2611@krafla.rhi.hi.is>, adamd@rhi.hi.is (Adam David) writes:
> In <1991Jan7.162136.3773@cbnewsl.att.com> hojo@cbnewsl.att.com (HC Johnson) writes:
> 
> 
> Hang on a bit, Mac 128k ROMs are 64k each or am I missing something. Spectre
> only addresses 128k on two chips, by a rather strange method (IMHO).
> The myth of a security lock possibly originates with the Mac board layout, which
> requires some of the data lines to be swapped on the ROMs. This would cause an
> EPROM made from a ROM dump to fail. Anyone with their firmware on EPROM will be
> in for a shock some years later when the data falls off it (if they don't have
> the master ROMs handy), correct me if I'm wrong.
> 

The 64K roms used the Intel 2764 type Eprom or equivalent Rom.
In the Intel design, there pin 1 was used to control programming an Eprom;
Gnd for programming, +5 for usage.  The Intel ROM (2564) required (allowed)
pin 1 to be grounded.  The result was that an Eprom couldn't be used unless
you knew to lift pin 1.

The 128K roms used by Intel are 27128 (or maybe 27256) Eprom or equivalent
rom.  These use pin 1 for an address line so the roms and eproms have the 
exact same pin outs.

Yes, Mac switched around 5 lines (2 in one; 3 in the other) making up
the full 16 bit word being accessed.  The Spectre cartridge rearranges
them to make them read correctly.  (This really makes using the S cartridge
for other roms exciting and the bits have to be rearranged in software).

A Rom programmer will not care how the bits were rearranged when the original
was made, it only makes a 1:1 clone.  However, there are many programmers
that will NOT allow a ROM to be read (to prevent cloning!).  Thats when the 
Spectre cartridge is useful in reading the Rom into memory; then producing
a data file such a programmer will accept.  This is the convenient way
to patch TOS ROMS.

------

As has been pointed out earlier, one does not say they are making a copy
of thus and such ROMS (or software).  What is done and used in your own
house is really your business.  The nasties come about when these copies
are freely available to "friends" or are being sold.

Most of my work into reading ROMS dealt with copying both TOS 1.2 and 1.4
into 27512 EPROMS so that I could check out product compatibility.  I own
the sources and run my copies instead.

Howard Johnson
ATT BELL LABS
att!lzsc!hcj
hcj@lzsc.att.com