Thanks for the reply and information's
This link is very nice and I got lot from it
As per mid- range block diagram the program counter point the locations of the program memory and instruction register is loaded with 14 bit instruction word but in 18F it is some thing different, what do you thing about it
Thanks in advance
Many points you are asking about are discussed in detail in the PICmicro 18C MCU Family Reference Manual.
In the early days of micros, the RAM, ROM where separate chips and had to be wired up to work together so you became very familiar with the timings and methods of connection via things like tri state buffers eg 74244 and 74245 chips.
Today no one really bothers with it, as its all done internally to the micro chip and most external communication is done by some serial protocol.
To understand the various sizes of instruction sets / micro families have a look down this page