Intel,Atmel 8051 Family

In 1981, Intel Corporation introduced an 8-bit microcontroller called the 8051. This microcontroller had 128 bytes of RAM, 4K bytes of on-chip ROM, two timers, one serial port, and four ports (each 8-bits wide) all on a single chip. At the time it was also referred to as a “system on a chip.” The 8051 is an 8-bit processor, meaning that the CPU can work on only 8 bits of data at a time. Data larger than 8 bits has to be broken into 8-bit pieces to be processed by the CPU. The 8051 has a total of four I/O ports, each 8 bits wide. 
The Atmel Corp. has a wide selection of 8051 chips, as shown in Tables 1-6 and 1-7. For example, the AT89C51 is a popular and inexpensive chip used in many small projects. It has 4K bytes of flash ROM. Notice the AT89C51-12PC, where “C” before the 51 stands for CMOS, which has a low power consumption, “12″ indicates 12 MHz, “P” is for plastic DIP package, “C” is for commercial.