A friend asked me about packed decimal sign nibbles, specifically x'0F'. I said
"Unsigned" was what I'd always been told. He said he was finding varied
answers, and then pointed me at SA22-7832-03 (PofOp, but an old version from
2004) where, on page 8-2 (PDF page 1150) is the following table. It displays
correctly in monospace font, so hopefully you can read it:
Recognized As
Code
(Binary) Digit Sign
-------- ------- -------
0000 0 Invalid
0001 1 Invalid
0010 2 Invalid
0011 3 Invalid
0100 4 Invalid
0101 5 Invalid
0110 6 Invalid
0111 7 Invalid
1000 8 Invalid
1001 9 Invalid
1010 Invalid Plus
1011 Invalid Minus
1100 Invalid Plus (preferred)
1101 Invalid Minus (preferred)
1110 Invalid Plus
1111 Invalid Plus (zone)
X'0A'? X'0B'?? X'0E'??? I'd only ever heard of x'0C', x'0D', and sometimes
x'0F'. Is it just me? I certainly don't claim to be Mr. Packed Decimal, but I
have encountered it off and on over the last 45 years, so I was very surprised.
I do see the discussion of "zones" in that section of the book, but that
doesn't clarify for me because zones have always been a mystery to me--never
needed to grok them. And it still doesn't explain the other values.
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