x86: http://en.wikipedia.org/wiki/Intel_BCD_opcode
68k: http://meseec.ce.rit.edu/eecc250-winter99/250-1-20-2000.pdf
6502: http://www.6502.org/users/dieter/bcd/bcd_0.htm
6800: http://fundamentals-digital-electronics.blogspot.com/2010/09...
Several others, such as Power6 and Sparc64: http://speleotrove.com/decimal/ (also shows this is is both still useful today)