Re: Apple Drops Backward Compatibility for Next MacOS

[matzingr@iclab.ce.philips.nl (Edgar Matzinger)]

In article <5536k4$qr6@client2.news.psi.net>,
	smentzer@pacbell.net (Steve Mentzer) writes:
> In article <552l5n$cdr@ns1.nl.cis.philips.com>, matzingr@iclab.ce.philips.nl 
> says...
>>
>>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.

I was NOT talking about the microprocessor they used, but about the
way they constructed the motherboard. still MB's today have this
memory-layout:

     | 0 -- RAM -- 640 KB | Video | ROM | 1 MB -- RAM -- n MB |

The memory management units used in today MB's can be configured
to compensate for this (anomaly?).

  And how DOS tries to extend it's memory beyond 640 KB (or 1 MB)
using "emm386", etc. This wouldn't be necessary when the memory-layout
started with ROM and followed by VideoRAM and RAM.

> 
> 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.

Quite true.

cu l8r,
-- 
Edgar.

    |\  /| :                                : Addr: Philips Sound & Vision
 /  | \/ | :     Edgar R. Matzinger         :       t.a.v. E.R. Matzinger
/   |    | :                                :       building: SFJ-6.58
\  /|  /\| :                                :       P.O. Box 80002
 \/   /  \ :     Philips Sound & Vision     :       5600 JB Eindhoven
  \  /\  / : Advanced Systems & Appl. Labs. :       The Netherlands
   \/ |\/  : Integrated Circuits Laboratory :Email:
      |    :                                : matzingr@iclab.ce.philips.nl
Disclaimer: Any comments, opinions made are mine, etc ...