Giving octal a try

General discussions related to the Altair 8800 Clone

Re: Giving octal a try

Postby Tech » May 7th, 2022, 2:36 pm

I'm not sure that I understand the question correctly, but for me, I use the 'od' program. It's a UNIX utility that produces octal, decimal, hex and ASCII dump.

After I assemble a program, I feed the resulting .com file to 'od'. Here is an example for Kill the Bit game dump in macOS:
Code: Select all
% od -b -An
          041 000 000 026 200 001 016 000
          032 032 032 032 011 322 010 000
          333 377 252 017 127 303 010 000
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Joined: January 14th, 2022, 12:24 am

Re: Giving octal a try

Postby Wayne Parham » May 9th, 2022, 11:44 am

Ken really hit the nail on the head earlier when he said, "this is useless for an 8 bit CPU, since the 8080 knows nothing about 16 bits."

That's really the problem. It's also probably why octal began to lose popularity in the 1970s as eight-bit microprocessors hit the scene.

Octal is really a minicomputer thing. It came from a time when processors were often designed with word lengths that were multiples of six-bits, and octal fit nicely into that paradigm. This is before the ASCII standard - when each system had its own character set definition - and six bits was commonly used to express a character. Most everything back then used six-bit character sets.

Even after ASCII was embraced and 16-bit systems started becoming popular, they had both 16-bit address and 16-bit word length. So there wasn't a need to express a 16-bit argument as two eight-bit bytes. It just didn't come up.

When I look back at the programming manuals of systems like this - old Data General manuals, for example - I see 16-bit arguments for addresses and the only place where I see eight-bit arguments is when a device is accessed that only provides eight-bit data. And in this case, the top-eight bits are "greyed out" because the interface code "knows" the top eight bits are unused.

The only reason that the Altair was designed to use octal is its inspiration was the Data General Nova 2. It was ill-fit, to be honest. It probably would have made more sense to use hexadecimal for the Altair. But since all the minicomputers of the day "talked octal," Ed Roberts must have thought the Altair should too.

As I've often said, I always thought this was weird, but its one of the things I love about the Altair. It's weird-cool and I love it.
Wayne Parham
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Joined: March 18th, 2022, 3:01 pm


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