On Tue, 28 Jul 2026 01:18:11 GMT, scott@slp53.sl.home (Scott Lurndal)
wrote:

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

James Kuyper <jameskuyper@alumni.caltech.edu> writes:

...

I asked Google Gemini "Which implementations of C implement a null
pointer whose representation is not 0?", and it identified 5 ...

I'll add a sixth; while we looked at doing a C compiler,
it didn't get any traction from customers

- Burroughs Medium Systems (hardware nulptr = 0xEEEEEE)
The architecture featured an instruction to search linked
lists (SLT) and the hardware recognized the base offset
0xEEEEEE as the NIL pointer. It was a BCD architecture
addressed to the digit (nibble).

BCD should have been a much bigger problem than nullptr. Everything
since C89 has required 'pure binary (magnitude)' for integer types,
and effectively a binary fraction for FP. How could you have gotten
bit operators (& | ^ ~ << >>) to work right?

--- PyGate Linux v1.5.19
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

On 2026-09-15 21:16, dave_thompson_2@comcast.net wrote:

On Tue, 28 Jul 2026 01:18:11 GMT, scott@slp53.sl.home (Scott Lurndal)
wrote:

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

James Kuyper <jameskuyper@alumni.caltech.edu> writes:

...

I asked Google Gemini "Which implementations of C implement a null
pointer whose representation is not 0?", and it identified 5 ...

I'll add a sixth; while we looked at doing a C compiler,
it didn't get any traction from customers

- Burroughs Medium Systems (hardware nulptr = 0xEEEEEE)
The architecture featured an instruction to search linked
lists (SLT) and the hardware recognized the base offset
0xEEEEEE as the NIL pointer. It was a BCD architecture
addressed to the digit (nibble).

BCD should have been a much bigger problem than nullptr. Everything
since C89 has required 'pure binary (magnitude)' for integer types,
and effectively a binary fraction for FP. How could you have gotten
bit operators (& | ^ ~ << >>) to work right?

Can you implement binary % and / for unsigned integers? If so, you can
emulate bitwise operators by using those operators with right operands
that are powers of 2.. It won't be fast, but it can be done.

--- PyGate Linux v1.5.19
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

dave_thompson_2@comcast.net writes:

On Tue, 28 Jul 2026 01:18:11 GMT, scott@slp53.sl.home (Scott Lurndal)
wrote:

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

James Kuyper <jameskuyper@alumni.caltech.edu> writes:

...

I asked Google Gemini "Which implementations of C implement a null
pointer whose representation is not 0?", and it identified 5 ...

I'll add a sixth; while we looked at doing a C compiler,
it didn't get any traction from customers

- Burroughs Medium Systems (hardware nulptr = 0xEEEEEE)
The architecture featured an instruction to search linked
lists (SLT) and the hardware recognized the base offset
0xEEEEEE as the NIL pointer. It was a BCD architecture
addressed to the digit (nibble).

BCD should have been a much bigger problem than nullptr. Everything
since C89 has required 'pure binary (magnitude)' for integer types,
and effectively a binary fraction for FP. How could you have gotten
bit operators (& | ^ ~ << >>) to work right?

It was circa 1984/1985 that we looked into C support. The hardware
had some support (mainly conversion instructions) for binary, but
there were no bit shift operators (other than digit shifting, which
worked in units of four bits).

There were bit set and bit clear as well as and/or/not instructions
as well as B2D and D2B instructions.

We were updating the architecture at that point in time and
adding additional instructions was on the table, but there was
no customer demand for C.

--- PyGate Linux v1.5.19
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

dave_thompson_2@comcast.net writes:

On Tue, 28 Jul 2026 01:18:11 GMT, scott@slp53.sl.home (Scott Lurndal)
wrote:

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

James Kuyper <jameskuyper@alumni.caltech.edu> writes:

...

I asked Google Gemini "Which implementations of C implement a null
pointer whose representation is not 0?", and it identified 5 ...

I'll add a sixth; while we looked at doing a C compiler,
it didn't get any traction from customers

- Burroughs Medium Systems (hardware nulptr = 0xEEEEEE)
The architecture featured an instruction to search linked
lists (SLT) and the hardware recognized the base offset
0xEEEEEE as the NIL pointer. It was a BCD architecture
addressed to the digit (nibble).

BCD should have been a much bigger problem than nullptr. Everything
since C89 has required 'pure binary (magnitude)' for integer types,
and effectively a binary fraction for FP. [...]

That's right about integer types, but the C standard admits radix 10
(or most other bases, within reason) for floating point. So the
floating point types could have both a decimal exponent and a
decimal fraction in their representations.

Of course converting such representations to an integer type would
be a PITA, but then no sensible programmer wants their code to do
such things anyway. ;)

--- PyGate Linux v1.5.19
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

On Tue, 15 Sep 2026 21:16:41 -0400, dave_thompson_2 wrote:

How could you have gotten bit operators (& | ^ ~ << >>) to work
right?

They can all be defined mathematically in terms of operations on
integers, regardless of any underlying base representation.

--- PyGate Linux v1.5.19
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)