[fpc-devel] Episode 4. Addressing and it's limits Part Two

[Tomas Hajny]

On 9 Feb 12, at 17:47, Mark Morgan Lloyd wrote:
> Tomas Hajny wrote:
> > On Thu, February 9, 2012 15:08, Mark Morgan Lloyd wrote:
> >> steve smithers wrote:
> >  .
> >  .
> >> "Also, the standard character set on the 360/370/Z-System is EBCDIC,
> >> while the Pentium uses ASCII."
> >>
> >> If the community can't get its head around the idea that character
> >> encoding is much more an operating system than a hardware issue, that
> >> the Intel/AMD range of processors could happily run an EBCDIC-based
> >> operating system, and that IBM gleefully supports ASCII-based Linux and
> >> ASCII-based Internet services then it's going to be damn difficult to
> >> get this (sub)project off the ground.
> > 
> > Just a comment on this: While I understand your statement and the Linux
> > port obviously confirms that an ASCII based operating system is possible
> > on S370 too, I wouldn't consider the character set being so completely
> > independent from the underlying hardware (all IBM PC compatible graphic
> > adapters can show ASCII characters directly but not EBCDIC, and also Intel
> > CPU instruction set includes support for BCD arithmetics based on the
> > ASCII character set if I understand it correctly).
> 
> I agree: when taking terminals and- in the general case- other I/O 
> devices into account. But both the examples I gave- one from Wikipaedia 
> and the other from Wikibooks- specifically associated ASCII with the CPU 
> type ("Intel" in one case, "Pentium" in the other) and I really don't 
> think that's healthy.
> 
> I was just checking the BCD arithmetic situation a few minutes ago, and 
> I believe that there's a "half carry" flag for carry out of the first 
> four bits: and that will work equally well for both ASCII and EBCDIC. I 
> might have come across other cases, but I can't remember whether they 
> apply to x86.

The Intel x86 (and thus also Pentium) opcodes like "AAD" ("ASCII 
adjust before division"), etc., suggest some dependency, but I admit 
that I didn't try to analyze what results it would have with EBCDIC.

Tomas