On 02.07.20 22:19, John Cowan wrote:
> On Tue, Jun 30, 2020 at 12:00 PM K. Krause <klemens...@gmx.net
>> In the late 70ies I considered a 24-Bit extension of the PDP-8.
> That's a very interesting idea, but I think the time for machines whose word size is not a power of two has passed, so I'm sticking with 32 bits. However, I have switched from byte addressing back to word addressing, while keeping the "two instructions per word" format.
That was in the mid 1970ies. The first 16-Bit microprocessors were
visible at the horizont. And the large scale computer at the university
here in Stuttgart, the TR440 hat 256K Words of 48(52) Bits.
> This complicates the fetch-decode-execute cycle somewhat: there needs to be a 1-bit internal register to keep track of which halfword will be executed next, and the assembler must insert NOP padding to make sure that a JMS is always in the second halfword and a JMP target always in the first halfword. (The latter will break if attempt to modify a JMP instruction on the fly, of course.)
Thats exactly the problem: the PDP-8 is a one word - one instruction
computer an architecture which keeps things simple. THe most attractive
idea in that project was, to be compatible to the PDP-8 on assembly
level, not at binary level. But naturally if memory-structure has more
words for example in zero page and page size generally, than naturally
all address computation has to be adapted individually and simply
reassembly of the original FORTRAN-compiler is impossible.
>> I knew
>> the 24-bit ICL 1900 machines and the 24-bit Siemens 300. I also got my
>> first Intersil-Sampler with the IM 6100. Other (16-bit)-extensions of the PDP-8 architecture are the hp 2216 and the DG nova.
> I know the Nova. I'll look into the other machines when I get a chance.
THe last time I thought about a 24-bit PDP-8 was around 1989: After the
end of GDR (east germany) I was in an electronic surplus store in east
Berlin. Don't remember if that was after the reunion or nearly before.
In this shop I fond three east-german bitslice ICs: U830C. The only
8-bit bit slices I've ever seen. People say that they were intended to
make an east german PDP11.
I bought them all, and so the idea of the 24-bit PDP-8 came up again.
But at that time I had no documentation for them, the project is stopped