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I don't understand why the 74181 operates the way it does. It does provide the full set of logic functionality (AND, NAND, OR, NOR, XOR, shift),but its arithmetic functionalities are not so clear. For example, it does not contain A - B and A + B in the same mode.

Could you provide some history background about how it was designed?

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good question, but I don't think any of us chiphacker users would know any of the TI motives beyond what is in the datasheet or on wikipedia en.wikipedia.org/wiki/74181 – Jason S Jan 18 '10 at 12:45
@Jason:thanks for your comment.then where is the good place to this question? – Jichao Jan 18 '10 at 14:09
not sure... check with a TI sales rep? perhaps they could put you in touch w/ one of the old-timers who worked on the 74181 back in the 1970's. – Jason S Jan 18 '10 at 22:55

1 Answer

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A minus B minus 1 is used for subtraction, and the complement of the carry flag can be viewed as a borrow flag.

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In fact even AVRs use this - they do not have an add immediate instruction (only substract immediate) since you can convert easily one into another. If both values are coming from registers then both sub and add are nice and fast but one is still not strictly necessary. – jpc Mar 21 '11 at 3:03
Yes, see stackoverflow.com/questions/1337831/… – starblue Mar 21 '11 at 13:09

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