Glenn writes:

> Interesting mnemonic for the sign nibbles.  Hadn't heard it described that
>  way before.

I didn't make it up. I have no idea now where I heard it, but it wasn't 
original with me.

 
>  I feel certain that the choice of x'C' and x'D' as sign characters has its
>  roots in the way signed zoned numbers are stored.  And the method we use 
for
>  storing signed zoned numbers has its roots in Hollerith card codes (punch
>  cards).  Numbers punched on Hollerith cards were signed by overpunching the
>  last digit with either a '+' (zone 12) or a '-' (zone 11) punch giving us
>  the possibly familiar ( {, A, B, ..., I and }, J, K, ..., S ) sign
>  characters.  The EBCDIC values for those character ranges start with x'C'
>  and x'D' respectively. Creating hardware to pack and unpack numbers while
>  retaining the sign was made relatively easy by simply using the last
>  high-order nibble (whatever it was) as the sign.

In my nightly communing with William of Occam, this explanation seems much 
less likely than mine. Where does the "F" come from then? And who really cares 
about EBCDIC? Or even ASCII? The governing idea is hex encoding.

By using only the ABCDEF characters, the least significant nibble (the sign 
nibble) is composed of an "out-of-band" symbol and could easily be 
differentiated from the data nibbles, which are encoded 0-9 BCD. If I were to choose, I 
too would have chosen the "F" to represent no sign information. The only 
question then is which characters to use to represent the "+" and "-" events.

In looking at the remaining characters, I would obviously have either chosen 
the A/B or the C/D pair. Heck, there isn't anything else.

Wirt Atmar

 

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