8051 architecture

History  

In 1981, Intel Corporation introduced an 8-bit micro-controller called the 8051.
This micro-controller 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.
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. 
Although the 8051 can have a maximum of 64K bytes of on-chip ROM, many manufacturers have put only 4K bytes on the chip.
The 8051 became widely popular after Intel allowed other manufacturers to make and market any flavors of the 8051 they please with the condition that they remain code-compatible with the 8051.
This has led to many versions of the 8051 with different speeds and amounts of on-chip ROM marketed by more than half a dozen manufacturers

 Inside Micro-Controller

In the following Figure you can see that all the supporting peripherals are at the single chip i.e CPU, ROM, RAM, Serial Port, TIMERS etc. 

Look at the following Diagram which explains in more details the architecture of Micro-Controller. some pins of the MC are shown in the diagram which are responsible for the functioning of the MC.