Happy Birthday VAX 11/780

Johnny Billquist bqt at softjar.se
Tue Oct 26 19:36:51 CDT 2010

On 2010-10-27 00:16, Roger Holmes<roger.holmes at microspot.co.uk> wrote:
>> >  From: Johnny Billquist<bqt at softjar.se>
>> >  On 10/26/10 04:20, Roger Holmes<roger.holmes at microspot.co.uk>  wrote:
>> >
>>>>> >>>>  From: Mr Ian Primus<ian_primus at yahoo.com>
>>>>> >>>>  And glaringly so. To say that the 11/780 is the first 32 bit machine is just silly. Prime had a 32 bit machine in 1972. And I know that there were others - but the Prime is the machine that I know the best:)
>>> >>  I think I'm right saying the Manchester 'Baby' had a 32 bit word in 1948, actually 32 of them on one Williams tube. However as it was a serial machine the data path to memory was actually one bit wide so it depends how you define bit size, but I was taught it was the largest addressable unit of memory and by that definition it had a 32 bit word.
>> >
>> >  What does "largest addressable unit of memory" means? I totally fail to
>> >  understand that. Sounds like "the largest memory chip that can be
>> >  utilized", but that can hardly be the meaning.
> Compare with another thing I was taught. A byte is the smallest addressable unit of memory. By this definition I have worked on machines with bytes sizes of 3, 8, 18, 24, 36 and (I think) 60. In most of the latter ones, a byte was also a word. I guess you exclude memory to  memory block move instructions, then consider the instructions which can load and save data and find the one which acts on the largest number of bits. I think by this definition a 68000, a Z8000, and the Manchester Baby all had 32 bit words. The VAX may have had 32 bit of 64 bit words even if it had just a 32 bit data path. My ICT 1301 has 48 bits words and 48 bit bytes even though its mill was only 4 bits wide. The data path from core memory to the 'A' register was 50 bits in parallel (it had two parity bits), but the data path between registers was only 4 bits, or in one case two sets of 4 bits. It was a serial/parallel architecture which allowed the end user price to be kept just below 250,000 pounds 
>   or a 5 tape deck mag tape machine with card reader, card punch, line printer, one drum and 2000 (decimal) words of core.

Hmm. Gotcha. But in that case, a VAX would be a 128 bit machine, since 
the largest quantities it can operate on is 128 bits.
And a PDP-11 would, I guess, be a 32-bit machine, and a Z80 a 16-bit 
Hmm, I do not think I agree with that definition.

> Sorry for rambling and thanks for all who commented on my 41 years of programming experience.

I think it's more convenient to look at what the "natural size" of the 
registers are.
I say "natural", because for instance, on the Z80, you can combine 
registers, but I would define that as not the natual size. So, HL is a 
16-bit register, and you can do pretty much anything with the, but it's 
actually the registers H and L combined together, and you can address 
each register separately as well. And they are 8 bits.

And the PDP-11 have 16-bit registers, but some operations work on a 
combination of two registers combined. Once again, not "natural". And 
the same goes for the VAX (obviously).

On the PDP-10 the accumulaltors are 36 bits, but you have instructions 
that can deal with bytes of any size between 1 and 36 bits. And they 
automatically manage the manipulations on parts of accumulators. But 
those smaller byte sizes aren't natural either, so it is 36 bits on that 
machine. :-)

But it might have made more sense back in older days..?


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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