8086 (was Re: more talking to the press.)

Hey Eric, now it get's fun :) Excuse me? The PDP-11 is a classic example for an external MMU, completely invisible to a user task. Nice if you just want to run old Software that expects a 28K addres space. But without expesive OS calls, and the MMUs equivalent of bank switching, it just allows to access said 28K (well, 32K since the IO may have been not mapped in). (And BTW, IBM had it before :) Try to see the segment address as a handle given to a user task as result of a mempry request to access the requested memory. As a user task, I don't care where the memory is located, I don't need the information - nor does any other part of the operation system, except for memory management. Now, still beeing a strict 16 Bit CPU, this little trick allows a 16 Bit user process to access up to 65K of 65K segments. How many memory realy was available and how it was organized did not concern to a user programm. While still beeing on a real mode CPU, the segments allowed it to programm with all the advantages of a mature OS. The only thing missing where page faults and boundry checks. The 286 then added these features. In fact, memory management on classic Macs was similar, a user programm got an handle to the requested memory, which it used to gain information where the memory is located - this, while still beeing a real mode OS, MacOS was able to reorganize/relocate memory. Usualy register relative addressing was used then and the register did hold the (thru the handle) aquirred block address. Basicly that's nothing else than using software to do exactly what the 8086 does in hardware. The value of the segment has no meaning - why do people always waste their time in calculateing 'real' addresses? I don't care for that on a /370, nor on a PDP 11, so why should I do so on my PC? Gruss H. -- VCF Europa 5.0 am 01./02. Mai 2004 in Muenchen http://www.vcfe.org/