6 Jul
2024
6 Jul
'24
5:17 a.m.
According to Larry McVoy <lm(a)mcvoy.com>:
If you're looking for simplicity and elegance, I can still remember most
of the PDP-8's instruction codes so I could disassemble at sight. But
it's not a very good model for a modern computer.
I realized a while ago that the only really important things in an
architecture are the addressing model, e.g., flat byte addressed
little-endian, and the data formats, e.g., two's complement integers,
ASCII or UTF-8 text, IEEE floating point.
Back in the day getting a program to act the same on different
computers, was really hard, with the switch from IBM 7090 (36 bit word
addressed binary floating point) to IBM 360 (32 or 64 bit byte
addressed hex floating point) the most famous example. These days we
write code and compile it for x64 or ARM or RISC-V and for the most
part, it just works because the data formats and addressing are all
the same.
It did, but it
was word addressed which makes it an historical
curiosity like its spiritual predecessors PDP-4/5/7/8/9.
I also have a mental model of a PDP-11 but these days it's more a simplified 386
leaving out the dumb or useless stuff.
I took a look at x86 in 386/486 days and found it to be enough of a mess that
I stopped looking. In no way did it compare the simplicity and elegance
of the PDP-11.