Re: Apple Drops Backward Compatibility for Next MacOS

[smentzer@pacbell.net (Steve Mentzer)]

In article <552l5n$cdr@ns1.nl.cis.philips.com>, matzingr@iclab.ce.philips.nl 
says...
>
>In article <54u5ud$7k6@darla.visi.com>,
>        nathan@visi.com (Nathan Mates) writes:
>> 
>>    That 640K problem is a limitation of MS-DOG. With the 286 series of
>> chips that could easily address 1MB of ram, Mickeysoft had a perfect
>> opportunity to shatter the 640K barrier,... 8<
>
>Hi,
>
>  the 640K problem is a hardware limitation. The memory map shows
>these ranges: 0-640K RAM, followed by videoRAM and ROM. If they (IBM)
>had build it the other way around: starting with ROM from 0, followed
>by videoRAM and user RAM, they'd never any problems.
>
>



Not necessarily.  Yes, the 8086 and 8088 had a "segmented" memory architecture
that limited addressing to 640k. But with the advent of the 286 and higher, 
additional "modes" were implemented allowing linear memory addressing.

The only reason DOS still runs on the newer chips is because of its 
"backwards" compatibility. In fact, when you are running dos, you are 
really running a super-fast, cached, 8086.

smentzer@pacbell.net