29 Jul
2005
29 Jul
'05
4:47 p.m.
> For the PDP-11, I tend to understand that MUL is
only for signed 16
> bit numbers, so I don't think MUL can be useful with unsigned values
> which are required for a 64 bit multiple as per the above example.
> CAN SOMEONE PLEASE CORRECT ME IF I AM
WRONG???
You're right and you're wrong. :-)
If you're doing a 16*16->16 multiply, it doesn't matter whether it's
signed or not, except for detecting overflow.
If you're doing a 16*16->32 multiply, the difference between signed and
unsigned is the high half of the result. To figure details, consider,
for example, A*B where A is in the "problematic" range (32768..65535).
You want A*B; the instruction will give you (A-65536)*B = A*B - 65536*B.
Come to think of it, I think the code I posted earlier using EMUL to do
extended-precision multiplication is wrong for this reason; as I read
the VARM, EMUL does a signed multiply-and-add....
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