The 8031 does not have any inter ROM, while the 8051 has 4k of internal factory programmed ROM.

You need to provide your code to the manufacturer so that he could burn it into the chip at the time of manufacturing, so you wont get any 8051s in retail. if you get them treat them as 8031s. Do not attempt to use its internal ROM because you wont know whats in it and you cannot change it!

The 8751 is the EPROM version of the 8051, while the 89C51 is the Flash ROM version. Specwise all the chips are similar.

girish