Re: Macintosh LC...need info on this =) TIA

[pausch@saaf.se (Paul Schlyter)]

In article <3a2b2149@naylor.cs.rmit.edu.au>,
Matt Jenkins  <mdj@bilby.cs.rmit.edu.au> wrote:
 
> Roy and/or Janet Miller <millers@inetnebr.com> wrote:
> > frankc wrote:
> 
>>> I think to keep in the spirit of early Apple machines maybe we should
>>> service the needs of users of older Macs.  I do not own any newer Macs,
>>> but I do own some of the older 8/16 bits versions (Is the 68000 8-bit?)
> 
>> The 68000 is generally rated as a 16/32 bit chip (kinda like the 8088
>> was an 8/16 bit chip)
> 
> I think you mean 32/24 bit chip. The 68000 has real 32 bit instructions,
 
Yep -- 32-bit ADD/SUBTRACT but only 16-bit MULTIPLY/DIVIDE....
 
> real 32 bit registers, etc. It is indeed a 32 bit processor. The only
> 'problem' is that the address bus of the thing can only use 24 bits to
> address memory, and hence you're limited to 16mb of ram on a 68k.
 
The 68000 was a 16/32 bit chip, because the size of the external data
bus was 16 bits, not 24 bits.  The 8088 is considered an 8/16 bit chip,
even thouhg its address bus was 20 bits.
 
And then we had the 68008, with an 8-bit data bus, which was used in
e.g. the Sinclair QL.  The 68008 was a 8/16 bit chip.
 
 
> This is where the Mac concept of '32-bit clean' apps comes from. Many
> old applications (including MacOS 6.x) used to hide data in the high
> order byte of the 68k's address registers for speed, since only 24
> bits were actually used for addressing. This is why 68030 Macs (eg.
> Mac IIci, IIsi, LCIII) have the 32bit addressing switch in the control
> panel, so you can disable 32bit addressing and run older applications
> or older System software. Starting with the 68040 machines (Quadra,
> Centris, etc) the CPU's could no longer do this, and hence you couldn't
> run any OS older than 7.0, and dozens of old classic Mac apps also.
 
As usual, "dirty programming" strikes back sooner or later....
 
-- 
----------------------------------------------------------------
Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40,  S-114 38 Stockholm,  SWEDEN
e-mail:  pausch at saaf dot se   or    paul.schlyter at ausys dot se
WWW:     http://hotel04.ausys.se/pausch    http://welcome.to/pausch