PIC18 Harvard Architecture
PIC microcontroller contains an 8-bit ALU (Arithmetic Logic Unit) and an 8-bit Working Register (Accumulator). There are different GPRs (General Purpose Registers) and SFRs (Special Function Registers) in a PIC microcontroller. The overall system performs 8-bit arithmetic and logic functions. These functions usually need one or two operands. One of the operands is stored in WREG (Accumulator) and the other one is stored in GPR/SFR. The two data is processed by ALU and stored in WREG or other registers.
The above process occurs in a single machine cycle. In PIC microcontroller, a single machine cycle consists of 4 oscillation periods. Thus an instruction needs 4 clock periods to be executed. This makes it faster than other 8051 microcontrollers.
Early processors and controllers could fetch or execute a single instruction in a unit of time. The PIC microcontrollers are able to fetch and execute the instructions in the same unit of time thus increasing their instruction throughput. This technique is known as instruction pipelining where the processing of instructions is split into a number of independent steps.