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Re: What does this do on a 68K-based Mac?

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From	John Ames <commodorejohn@gmail.com>
Newsgroups	alt.folklore.computers
Subject	Re: What does this do on a 68K-based Mac?
Date	Wed, 23 Sep 2026 08:27:54 -0700
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On Wed, 23 Sep 2026 00:28:33 -0000 (UTC)
Lawrence D’Oliveiro <ldo@nz.invalid> wrote:

> Dusts off my actual paper copy of “Inside Mac Vol III” (cough, cough);
> on page III-17:
> 
>      The MC68000 can directly access 16 megabytes (Mb) of address
>      space. In the Macintosh, this is divided into four equal
>      sections. The first four Mb are for RAM, the second four Mb are
>      for ROM, the third are for the SCC, and the last four are for the
>      IWM and the VIA. Since each of the devices within the blocks has
>      far fewer than four Mb of individually addressable locations or
>      registers, the addresses within each block “wrap around” and are
>      repeated several times within the block.
> 
> So basically the top 8MiB were used as I/O address space. This address
> must lie within some area that was not allocated to any device, and I
> guess that applied to the later 68000-based machines as well: Mac
> Plus and 512KE, Mac Portable, PowerBook 100, Mac Classic?

I'd have to try and dig up service manuals/etc. to be sure, but AFAIK
from the Plus on up there are third-party accelerators that can add
more than 4 MB of RAM - which suggests that the simple 4x 4MB mapping
was gone, or there'd be address conflicts with the ROM and I/O devices.

Re: the address in question, AFAIK the 68000 simply doesn't concern it-
self with the MSBs making it out to the bus; 0x50FFC001 and 0x00FFC001
are different values in one of the address registers, but to anything
outside the CPU itself they'll come out as the same address.

Earlier versions of MacOS and the Toolbox ROM exploited this by packing
non-address data into the MSBs of pointers, which caused issues down
the line when they wanted to expand RAM on '020-'040 Macs - and some
applications unwisely depended on that, which is why the Control Panel
applet for virtual memory also includes a switch for 32-bit addressing.

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Thread

What does this do on a 68K-based Mac? sean@conman.org - 2026-09-22 23:28 +0000
  Re: What does this do on a 68K-based Mac? Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-23 00:28 +0000
    Re: What does this do on a 68K-based Mac? John Ames <commodorejohn@gmail.com> - 2026-09-23 08:27 -0700

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