Introduction to 8051 Microcontroller

The 8051 is an 8 bit microcontroller originally developed by Intel in
1980. It is the world's most popular microcontroller core, made by
many independent manufacturers (truly multi-sourced). There were 126
million 8051s (and variants) shipped in 1993!!

A typical 8051 contains:

• CPU with boolean processor


• 5 or 6 interrupts:
  2 are external
  
  2 priority levels


• 2 or 3 16-bit timer/counters


• programmable full-duplex serial port
  (baud rate provided by one of the timers)


• 32 I/O lines (four 8-bit ports)


• RAM


• ROM/EPROM in some models
  

One strong point of the 8051 is the way it handles interrupts.
Vectoring to fixed 8-byte areas is convenient and efficient. Most
interrupt routines are very short (or at least they should be), and
generally can fit into the 8-byte area. Of course if your interrupt
routine is longer, you can still jump to the appropriate routine from
within the 8 byte interrupt region.

The 8051 instruction set is optimized for the one-bit operations so
often desired in real-world, real-time control applications. The
boolean processor provides direct support for bit manipulation. This
leads to more efficient programs that need to deal with binary input
and output conditions inherent in digital-control problems. Bit
addressing can be used for test pin monitoring or program control
flags.

See full information from 8051 FAQ
- http://www.faqs.org/faqs/microcontroller-faq/8051/