[Simh] ALTAIR thinko?

[Dell Setzer]

As I remember, MITS mostly used octal notation for the ALTAIR 8800. The 
front panel switches and lights are divided into 3-bit rather than 4-bit 
groups (there's a photo on Wikipedia that shows this pretty clearly). And 
most of the 8800's docs use octal notation. So, I'm guessing that the 
author of the Simh emulator was probably just following that convention. 
The 8080-based Heathkit H8 also used octal notation. Probably folks back 
then were used to octal from having worked on minicomputers (like DG or 
DEC).

On the other hand, the 6800-based MITS ALTAIR 680b (one of which I 
actually own and am looking at as I write this) has its switches and 
lights in 4-bit groups, and the docs use hex notation. Go figure.

...dell

On Mon, 2 Feb 2015, Gregg Levine wrote:

> Hello!
Would the Octal reference for $FF be used because one of the authors
of emulator would be more familiar with those terms from working with
the one for the PDP-11?

I've seen the crowd courtesy the VCF and have a box here who's waiting
to be restored.

Oddly enough I came within a big something-else to own one (an Altair
or IMSAI or the Heathkit version) a long time ago.
-----
Gregg C Levine gregg.drwho8 at gmail.com
"This signature fought the Time Wars, time and again."


On Mon, Feb 2, 2015 at 10:40 AM, Clem Cole <clemc at ccc.com> wrote:
>
> On Mon, Feb 2, 2015 at 10:16 AM, Bailey, Scott <scott.bailey at hp.com> wrote:
>>
>> Non-existent memory.  On the 8080, reads to non-existent memory
>>       return 0377, and writes are ignored.
>
>
> That was definitely true and how the HW worked [although the comment should
> have been 0xFF or actually $FF in Intel syntax - since the 8080s were
> defined in hex not octal like PDP-11s).  There was no non-existent memory
> trap for the bus, so the HW read the data back as 0xFF because of TTL
> floating to high.
>
> I do miss the Altair and Imsai machines in a strange sort of way ;-)
>
> Clem
>
>
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