Re: Dead Amiga? --- Live Apple

[dempson@actrix.gen.nz (David Empson)]

Supertimer <supertimer@aol.com> wrote:

> Obviously, if WDC did release an improved 65C processor, it would be
> a next generation extension.  The Pentium II is an extension of the
> 8088 in a very real sense, but it has overcome the original limitations.
> The 80286 had the same "queer segmentation scheme" as the '816,
> but the Pentium II does not, right?

The 8086/8088 segmentation scheme is quite different to how the 65816
operates.  The 80286 adds "protected mode", which basically extends the
address space to 16MB (with protection rules and limit registers) but
keeps the same addressing scheme.

In all cases, the program is only able to access a maximum of 256K
without reloading a segment register (64K code, 64K data, 64K stack, 64K
"extra").

The segments must be aligned on paragraph boundaries (16 bytes), and may
overlap.

The 386 and later processors relax the 64K limit for segments when in
protected mode, but they still support the 286's limited version of
protected mode, and the old 8086 "real mode", either directly (e.g. at
boot time, or when running DOS), or via a captive virtual address space
("virtual 8086 mode") - this is typically used to run DOS programs under
OS/2 or Win95, for example.  When running in its native protected mode,
the segmentation scheme can usually be ignored, except to provide memory
protection and virtual memory support.

I'm not aware of any fundamental differences in the addressing schemes
in later processors, but I haven't looked at the Pentium family in
detail.


The 65816 is a much cleaner system than the 8086/80286.  The memory is
logically organised into 64K banks, but this is only really significant
for code (which cannot leave a 64K bank without doing a far
jump/call/return).  Data instructions may access the entire 16MB address
space directly, and there are few restrictions on crossing over bank
boundaries.  There are registers for getting faster access to limited
areas, e.g. the old 6502 "absolute" mode uses a bank register, and the
old 6502 "zero page" mode, now called "direct page" mode, is limited to
the first 64K bank, and uses a pointer register.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand