Comment by gp2000
13 years ago
I'd guess this goes back to the 4004 which was designed for a desktop calculator. Easy BCD really helps those applications so they must have had that in mind as a target market. There's not much point in using BCD once reasonable amounts of RAM and ROM are available.
Except the Z80 / 80xx don't descend from the 4004, they descend from the Datapoint 2200. The 8008 didn't have BCD instructions or a half-carry flag, but it had a parity flag.
Not architecturally, but Federico Faggin and Masatoshi Shima were the key people on the 4004 and 8080 before leaving to form Zilog and build the Z-80. The Z-80 had to have DAA (decimal adjust) to be compatible with the 8080. Possibly the 8080 had DAA to compare well against the 6800. If that's the case, then we must ask where the 6800 got the idea. Could be from minicomputers or even mainframes, but from what I've read the early microcomputer designers had no pretense of making processors to compete anywhere near the high end. Instead their sights were set more along the line of embedded systems. Desktop calculators fit into that and Shima himself designed desktop calculators and helped specify the 4004 before he came to Intel. Thus my speculation that the impetus could have come from that direction.