8051 Microcontroller

is a Harvard architecture, CISC instruction set, single chip microcontroller (µC) series which was developed by Intel in 1980 for use in embedded systems. Intel's original versions were popular in the 1980s and early 1990s and enhanced binary compatible derivatives remain popular today.
The Intel 8051 microcontroller is one of the most popular general purpose microcontrollers in use today. The success of the Intel 8051 spawned a number of clones which are collectively referred to as the MCS-51 family of microcontrollers, which includes chips from vendors such as Atmel, Philips, Infineon, and Texas Instruments.

The Intel 8051 is an 8-bit microcontroller which means that most available operations are limited to 8 bits. There are 3 basic "sizes" of the 8051: Short, Standard, and Extended. The Short and Standard chips are often available in DIP (dual in-line package) form, but the Extended 8051 models often have a different form factor, and are not "drop-in compatible". All these things are called 8051 because they can all be programmed using 8051 assembly language, and they all share certain features (although the different models all have their own special features).
Some of the features that have made the 8051 popular are:

• 4 KB on chip program memory.
• 128 bytes on chip data memory(RAM)

[ 32 bank reg + 16 bit addressable reg + 80 general purpose reg ]

• 4 reg banks.
• 128 user defined software flags.
• 8-bit data bus
• 16-bit address bus
• 16 bit timers (usually 2, but may have more, or less).
• 3 internal and 2 external interrupts.
• Bit as well as byte addressable RAM area of 16 bytes.
• Four 8-bit ports, (short models have two 8-bit ports).
• 16-bit program counter and data pointer.
• 1 Microsecond instruction cycle with 12 MHz Crystal.

8051 models may also have a number of special, model-specific features, such as UART, ADC, Op_Amps, etc... it is a very powerful micro controller.