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

[Mark Morgan Lloyd]

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.

I've worked for a pretty unpleasant mainframe outfit, and am very much 
used to the way that that type of corporation attempts to rewrite 
history to their advantage and forces their position on their employees 
and customers. "The company that invented virtual memory and the virtual 
machine" to which I added "and the intermittent fault"... didn't last 
very long, but they had some interesting kit.

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]