Re: Why still using Apple II?

[Roger Johnstone <rojaws@orcon.net.nz>]

In <MPG.1d1211ba7f493dee9896a2@news.individual.net> Paul Schlyter  wrote:
> In article <_dWdnUq3Cs1RQTrfRVn-2w@comcast.com>, alexf@mirrow.com says...
> 
>>>>4GB address space
>>>
>>> That came later.  The first versions of the 68K CPU had only a 24-
>>> bit
>>
>>I'm talking about LOGICAL memory space, not the number of physical 
>>address pins.
> 
> Did the 68K have some paging mechanism which allowed access to 4 
> GBytes address space through its 24 address pins?  If not, that 
> LOGICAL memory space becomes quite useless, since no physical RAM can 
> ever be attached to most of that memory space.

The logical address space was very important for forwards compatibility. 
It meant that when newer CPUs came out with the full 32-bit address 
space, the old programs writen for the original 68000 could access more 
than 16MB.

Of course Apple still managed to stuff it up! Some of the earliest 68020/
030 Macs had ROM code which used the unused top 8 bits in the addresses. 
Well, they were unused until System 7 came out with 32-bit addressing 
support. Macs without '32-bit clean' ROMs could only support 24-bit 
addressing and so were limited to just 8MB of RAM.

-- 
Roger Johnstone, Invercargill, New Zealand
http://vintageware.orcon.net.nz/
________________________________________________________________________
  No Silicon Heaven? Preposterous! Where would all the calculators go?

              Kryten, from the Red Dwarf episode "The Last Day"