-----Mensaje original-----
De: [EMAIL PROTECTED] [SMTP:[EMAIL PROTECTED]]
Enviado el: lunes 30 de noviembre de 1998 12:53
Para: [EMAIL PROTECTED]
Asunto: Re: My MSX project + protect mode
> Make it possible to use segments. What I mean is you can set some register
> outside the cpu on a value rg and if an address ad is asked by the cpu,
> the address that is actually read out is ad+mf*rg, where mf is a
> multiplying factor, prefferibly smaller than 64 (use a power of 2 of
> course, so you don't actually have to multiply).
Why a small value like 64? A Z380 based system will probably have a
memory size in the order of megabytes, so a value like 64K will be
acceptable as well.
Ofcourse, if a value of 1 is possible, that would be ideal, but maybe
that will make hardware implementation too complex.
[...]
> (I don't know why pc-programmers always say
> segments are the worst thing ever invented...)
The decision of 16 bytes increments for segments was wrong. That
decision is the reason x86 real mode can only address 1024K, so
indirectly it is responsible for the hated 640K boundary.
Bye,
Maarten
[Isidoro Gonz�lez M�rquez] I would add that the wrong decision about
8086 PC segments, which made
them so
hated by everybody, was their
fixed length
(64K) and the absence (lack)
of other mecha-
nisms associated to modern
segmentation
(memory protection, read-only
possibility,
etc.).
[Isidoro Gonz�lez M�rquez] One of the worst aspects of Z80 is the lack of
the possibility of addressing
relative to
PC. That would make programs
capable of running
on whatever address. 6809 was
much better
in this aspect. The right
solution is al-
ready invented: virtual
memory. All programs
are written as if they were
running on the
same address and the hardware
makes the
translation "on the fly".
Gyermo.
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