Re: How many times must Apple make such DRASTIC architecture changes?

[Paul Schlyter <pausch@NOSPAMsaaf.se>]

In article <1118281016.658234.254430@g49g2000cwa.googlegroups.com>, 
vze35xda@verizon.net says...

>The PC has been through a lot of change over the years.  Remember the
>original machine had an 8088 (8 bit bus) limited memory and an
>interrupt system out of the Intel handbook.  After this was cloned then
>we got the AT line of computer with ISA cards with 286's and 386's.

The ISA cards were tehre already on the 8088 and 8086 PC's, although
an 8-bit version of ISA was used.  Later, on the 386 PC's, quite often
not all ISA slots were 16-bit, some were 8-bit.

>Now with the Pentium class machines with PCI and better bus connectors,
>new and different northbound/southbound controllers I can say with certanty
>these machines are NOT the original IBM PC.

Certainly not .... booting an MS-DOS 1.0 floppy on such a machine would
probably fail.... (the absence of a 5.25" floppy disk on modern PC's is
only a minor obstacle here - such a floppy reader can easily be added if
one really wants it).

>The architecture is widely different.  Also having programmed 8086's and
>80186's, now with full 32 bit registers everywhere and no segmented address
>space while backwards compatible to an extent these are very different
>processors.

The segmented addressing is not gone from modern x86 processors - it's 
merely hidden by the OS.  Since one segment now can be up to 4 GBytes
lage, it is feasible to run everything in the same segment.  And that's
what the modern PC OS'es do (Windows as well as Linux, FreeBSD and Solaris).

But whenever that "4 GByte limit" becomes too limiting, it is possible to
start using different segments again.  Currently it can be done only
from assembly language though, and kernel mode is required to set up a new
segment.  And don't expect this to happen until more than 4 GBytes of RAM
gets common.


-- 
Paul Schlyter,  Grev Turegatan 40,  SE-114 38 Stockholm,  SWEDEN
e-mail:  pausch at saaf dot se
WWW:     http://stjarnhimlen.se/