# Re: C arithmetic, was Software proofs, was Are there different

## drb@ihatespam.msu.edu (Dennis Boone)

Sun, 12 Feb 2023 04:37:07 +0000

*From comp.compilers*

Related articles |

[6 earlier articles] |

Re: C arithmetic, was Software proofs, was Are there different *gah4@u.washington.edu (gah4)* (2023-02-09) |

Re: C arithmetic, was Software proofs, was Are there different *gah4@u.washington.edu (gah4)* (2023-02-09) |

Re: C arithmetic, was Software proofs, was Are there different *DrDiettrich1@netscape.net (Hans-Peter Diettrich)* (2023-02-10) |

Re: C arithmetic, was Software proofs, was Are there different *gah4@u.washington.edu (gah4)* (2023-02-10) |

Re: C arithmetic, was Software proofs, was Are there different *gah4@u.washington.edu (gah4)* (2023-02-11) |

Re: C arithmetic, was Software proofs, was Are there different *anton@mips.complang.tuwien.ac.at* (2023-02-11) |

**Re: C arithmetic, was Software proofs, was Are there different ***drb@ihatespam.msu.edu* (2023-02-12) |

| List of all articles for this month |

**From: ** | drb@ihatespam.msu.edu (Dennis Boone) |

**Newsgroups: ** | comp.compilers |

**Date: ** | Sun, 12 Feb 2023 04:37:07 +0000 |

**Organization: ** | Compilers Central |

**References: ** | 23-01-092 23-02-003 23-02-019 23-02-025 23-02-026 23-02-029 23-02-032 23-02-035 23-02-039 |

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**Keywords: ** | architecture, history, comment |

**Posted-Date: ** | 12 Feb 2023 12:18:23 EST |

> The three index registers were 1,2, and 4, and if you specified more

> than one, it OR'ed them together and subtracted the result, which was

> really strange. There were a lot of other machines with index registers

> but none I know of that subtracted or OR'ed. I have also been unable to

> tell whether the OR'ing was deliberate or just a result of minimizing

> the number of tubes that they then documented.

Not having to have the circuitry to make sure multiple registers

were never gated onto the internal bus seems like a likely reason.

But one _could_ also conceivably use multiple index registers for

multiple dimensions, with careful allocation.

De

[As I said, I've found lots of guesses but no documents. Multiple

dimensions only work if each dimension is a power of two which,

considering the 704's tiny memory, seems unlikely. My guess is the

same as yours, fewer tubes, but it's just a guess. -John]

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