As long as you are working on the software, you can burn it into (erasable, changeable) PROMs that get plugged into the piggyback-socket on top of the 80C51Px. Once you've finalized your code, you'll use OTP (one-time-programmable) or mask- (factory-)programmed chips that are, of course, much cheaper. The chip has pins in standard DIL-40 layout on the bottom, the piggyback sockets on top are also standard DIL-28 spacing.
|From chris' blog|
Interestingly, while this chip family is as old as rocks, software-compatible parts with much higher performance are still being produced, e.g. by Silabs.