[TUHS] Need a new PDP-11 or VAX?

John Cowan cowan at mercury.ccil.org
Sun Apr 28 15:57:33 AEST 2013

Larry McVoy scripsit:

> Ideally with 32 bit ints, 64 bit longs, 32 or 64 bit pointers in the 
> compiler, obviously more registers, and nothing like the vax.        

Sounds right.  I'm torn between the 8 addressing modes of the PDP-11 and
32 registers, or the 16 addressing modes of the Vax and only 16 registers.
In either case, 32-bit instructions allow all operands to have full
addressing mode on both source and destination rather than just a register
for one or the other.  The byte/word bit of the memory instructions would
become 2 bits for byte/short/int/long.  One issue is whether to have
a single 32-bit index in modes 6 and 7, or provide a full 64-bit index
with two trailing 32-bit words.  Branches can have much bigger offsets,
which is good.

The FPP needs a complete overhaul: it should look pretty much
like the main ISA, and of course use IEEE format operands.  Hmm,
perhaps get *really* modern and use 3 bits of size for byte/short/
int/long/float32/float64/decimal64/decimal128, where the last two are
IEEE 754:2008 decimal floating point values.

> I'm too dumb to get it but I never warmed up to the vax.

When I saw it had 256 opcodes I knew I was never going to master it, no
matter how orthogonal it was.  (I came from the PDP-8 to the PDP-11.)
Little did I know the Last ISA would have thousands and thousands of

Note that nobody these days would clamor for fundamental laws        John Cowan
of *the theory of kangaroos*, showing why pseudo-kangaroos are   cowan at ccil.org
physically, logically, metaphysically impossible.    http://www.ccil.org/~cowan
Kangaroos are wonderful, but not *that* wonderful.     --Dan Dennett on zombies

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