C needs BCD -- why BCD? Accountants!

[Geoff Kuenning]

> I have always been surprised that business people didn't just use 64
> bit integers, and keep the amounts in CENTS.  No BCD required, can
> take advantage of nice fast machine instructions, etc.

Primarily because there are *still* relatively few machines that have good
support for 64-bit integers.  The average 32-bitter has 32x32 multiply that
produces 64 bits, and a corresponding 64/32 divide, but other 64-bit math
has to be implemented with "add with carry" instructions.  If I recall
correctly, the 360 didn't even have such an instruction, so you really had
to hack it.  The 2-3 instruction sequences needed to perform 64-bit
arithmetic are not really more efficient than single BCD instructions.
Furthermore, many business programs (e.g., general ledger or accounts
receivable update) have a general model of "read in two fields, add them,
write it out".  You can do a BCD add *much* faster than you can do two
conversions and a 64-bit binary add.  Remember that decimal conversion
inherently involves multiplication and division.
-- 

	Geoff Kuenning
	First Systems Corporation
	...!ihnp4!trwrb!desint!geoff