26 Oct
2010
26 Oct
'10
11:14 a.m.
From: Johnny Billquist <bqt at softjar.se>
On 10/26/10 04:20, Roger Holmes <roger.holmes at
microspot.co.uk> wrote:
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 for a 5 tape deck mag tape machine with card reader, card punch, line
printer, one drum and 2000 (decimal) words of core.
Sorry for rambling and thanks for all who commented on my 41 years of programming
experience.
> 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. There was talk of the VAX design being the
inspiration for the Motorola 68k. Isn't it more likely that the PDP11 influenced the
design of both the VAX and the 68k?
That the PDP-11 influenced the VAX there can be no doubt about.
The the PDP-11 influenced the 68K seems very probable when looking at
the architectures, but that is guessing on our part.
William Donzelli<wdonzelli at gmail.com> write:
>> There was talk of the VAX design being
the inspiration for the Motorola 68k. Isn't it more likely that the PDP11 influenced
the design of both the VAX and the 68k?
> Big microcoded CISC was the dominant
thinking in computer architecture
> at the time, so it is really hard to say that X influenced Y.
Well, the fact that DEC explicitly stated that the VAX would be like the
PDP-11, but extended to 32 bits seems like an obvious statement that the
PDP-11 influenced the VAX. Not to mention the fact that they used the
same peripherial buses, and at VMS V1, most of the applications were
just the RSX programs moved straight over. Oh, and don't forget the
compatibility mode in the VAX, which made it execute PDP-11 code. :-)
As for the 68K, we are making more of a guess, but it seems like a
fairly educated guess that the 68K was inspired by the PDP-11 (although I'd say the 68K is way inferior to the PDP-11...)
If you say so, I never used a PDP-11.
Johnny
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