5 ms·
The overhead for the two near-useless flags, PF and AF, would be interesting. How much did that junk slow down the clock speed?
by burfog 6y ago
The overhead for the two near-useless flags, PF and AF, would be interesting. How much did that junk slow down the clock speed?
- kens 6y agoI'm guessing the impact on clock speed was minimal, but the history of the parity flag is interesting. Automatically computing parity is very important if you're implementing a terminal, since serial communication often uses a parity bit. The Datapoint 2200 was a programmable terminal / desktop computer introduced in 1970. Earlier than the microprocessor, it had a processor built from a bunch of 7400-series TTL chips. The Datapoint company asked Intel and Texas Instruments about replacing the board of chips with a single chip. Intel eventually produced the 8008 to duplicate this functionality (including the parity flag), while Texas Instruments produced the TMX 1795 chip. Meanwhile, Datapoint redesigned their processor board around the 74181 ALU chip and decided they didn't want a microprocessor. Texas Instruments couldn't find a customer for the TMX 1795 and abandoned it, but Intel decided to sell the 8008 as a general-purpose microprocessor, essentially creating the microprocessor market. This led to the Intel 8080 and then the 8086 processors, which because of backwards compatibility kept the parity flag (and the little-endian architecture). So that's why you have the near-useless PF flag.
- kevin_thibedeau 6y agoPF is three levels of XOR gate delays. Not a big deal.