[Info-vax] Continued development of PDP-10 architecture [was Re: Hard links on VMS ODS5 disks]

[Arne Vajhøj]

On 7/31/2023 2:33 PM, gah4 wrote:
> On Sunday, July 30, 2023 at 11:28:32 PM UTC-7, terry-... at glaver.org wrote:
>> The Dolphin (KM-10 or KXF10 at various times) had hopes of being both a
>> KL-10 and a VAX: "Dolphin is an advanced computer system based on a new
>> generation CPU incorporating Macro Cell Array technology. It is the logical
>> successor to the KL-10 based PDP-10/20 system, and may well become the
>> high-end VAX system." That seems like an odd idea - the additional 4 bits in
>> registers, data paths and main memory would seem to be an unnecessary
>> expense in VAX mode.
>   
> Having all instructions the same length, like most RISC systems, makes
> it very easy to design pipelined parallel processors.
> 
> VAX seems to be designed very well to be hard to pipeline.
> 
> IBM S/360 and successors have three instruction lengths, all known
> from the first byte.  Slightly harder, but not all that hard.
> 
> VAX, with addressing mode bytes, is well designed for serial processing,
> going one byte at a time and decoding it.  Each addressing mode byte
> indicates how many operand bytes it needs, and so where to find the
> next addressing mode byte.
> 
> It would have been a very easy change to VAX to put all the address
> mode bytes immediately after the opcode.  It is still slightly more
> complicated due to indexed mode, but not all that hard.
> You get all the address mode bytes in parallel, then process any
> index mode bytes, and you know where the next instruction is.
> 
> But no, for VAX you process each address mode byte, which tells
> you were the next one is, which then tells where the next one is ...
> Nice for serial microprograms, not for pipelining.
> 
> Well, VAX might also be designed to make it easier for assembly
> programming, and especially debugging from core dumps.
> I don't believe that moving the bytes around would have changed
> that much, though.
> 
> Seems like the extra bits in the word are not the worst problem.

Maybe the extra silicium in a hypothetical modern VAX CPU
should have been used on cores instead of pipelining.

A VAX 7000 model 800 CPU with 64 cores????

Arne