The arduino microcontroller is based on Atmel’s AVR chips. The AVR is a modified Harvard Architecture 8-bit RISC. It is a modified Harvard Architecture because it is equipped with separate, dedicated memories and buses for program and data information.
This microcontroller also follows instructions set based on the RISC concept. RISC means Reduced Instruction Set Computing. It is an strategy to have simplified instructions or operations that work in fewer CPU cycles increasing the chip’s performance.
Although Arduino uses its own modified version of a library-driven C++, because it is based on an AVR mocrocontroller, we can program it in C or assembly, reducing the size that the program will take inside the microcontroller’s flash memory. Assembly language would probably be one of the most effective, but it requires to know more internals details about the hardware and it is not very portable. C language would still allow direct control of the hardware and it is portable.