According to the instruction sets used, computers are normally classified into RISC and CISC.RISC stands for 'Reduced Instruction Set Computing' .The design philosophy of RISC architecture is such that only one instruction is performed on each machine cycle thus taking very less time and speeding up when compared to their CISC counterparts.
Here the use of registers is optimised as most of the memory access operations are limited to store and load operations.
Fewer and simple addressing modes, and simple instruction formats leads to greater efficiency, optimisation of compilers, re-organisation of code for better throughput in terms of space and time complexities. All these features make it the choice of architecture in majority of the Embedded systems.
CISC again have their own advantages and they are preferred whenever the performance and compiler simplification are the issues to be taken care of.