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Re: Octal

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  • Herb Johnson
    ... Note: This is an arcane discussion today, but hex versus octal was an ACTIVE discussion in the mid-late 1970 s. I was there, and part of it, and there
    Message 1 of 7 , Jun 27, 2008
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      Herb Johnson wrote:
      >> Look at the 8080 instructions in binary. There are eight
      >> references to registers or memory - three bits. instructions
      >> which move or compare
      >> values use two sets of those three bits, leaving
      >> two bit for instruction types. An OCTAL representation
      >> of 8080 op codes is the most consistent format.

      Dan said:

      > I did, but only the register opcodes follow this convention,
      > that's only about 20% of all the opcodes in there. The others
      > each have various bit fields which don't fit into the octal
      > format.

      Note: This is an arcane discussion today, but "hex versus octal" was
      an ACTIVE discussion in the mid-late 1970's. I was there, and part of
      it, and there were reasons for it.

      It is true that only 8080 register-explicit instructions follow the
      EXACT pattern I described. But if you inspect the other instructions
      carefully, they follow a consistent scheme of binary patterns, like
      the register scheme. I mentioned the "register" convention to suggest
      that pattern. I did not want to post a lecture on 8080 op codes.

      I might have said that in the 8080, there are consistent instruction
      bit fields, shown from left to right, as bit 6&7, 3-5, and 0-2. (7 is
      the most signifigant bit.) For example, the "jump" instructions all
      have bits 6&7 = "11", and almost all have bits 2-0 ="010". Call
      instructions, bits 6&7 = "11", bits 2-0 = "100". The exceptions in
      both cases are the unconditional jump and call. There are similar
      patterns in other instruction groups.

      But, it's a perceptual issue as well, and different people "see"
      patterns or not in different ways. This should not become a personal
      discussion.

      My point was that these patterns are clearer in OCTAL representations
      of op codes, than in HEXidecimal. Intel groups these instructions in
      BINARY charts in their data books, but organized by instruction type.
      I'm using an Intel 1978 8085 data book right now. These patterns
      reflect the underlying digital binary logic which decodes the
      instructions; there are reasons for such patterns.

      The fact it WAS a debate, can be found in articles from microcomputer
      magazines of the period. If some today find this discussion arcane, or
      boring, or trivial; it was an issue at a time when people DID program
      in binary and think in binary. Front panels weren't for show, they
      were used. They were used in an era when many people were writing
      their own assemblers, and when the only digital storage was paper tape
      or audio cassette - or pencil and paper.

      Herb Johnson
      retrotechnology.com
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