The issue here was that the OP claimed that at89c51 was the complete name of the chip, which it isn't.

Most people would also know what a 747-400 is, but it is still not the complete name, but just a model. And it still doesn't contain information about selected options.

I drive a V70 XC, which is probably sufficient. But it wouldn't be complete without mentioning that it is a Volvo. And in this case, I would probably need to add the model year too, just as at least newer processor generations makes it important to give the full name including stepping when discussing problems where errata may be involved.

In the case of the AT89C51, it seems like all the extra characters at the end of a full name "Atmel AT89C51-16PC", only controls maximum clock speed, package type and temperature range. But some other processors may have extra I/O for a 44-pin PLCC chip compared to the 40pin DIP version. Some Atmel AVR chips have 8 ADC inputs for the PLCC version and 4 ADC inputs for DIP version.