creating architecture or ISA (Instruction Set Architecture).

The architecture must be GPR architecture (General Purpose Register-based). Your instruction size (total instruction width) should be in the vicinity of 16 bits (You may vary the width anywhere from 16 to 24 bits, if you choose to have variable encoding.) It is a traditional stored-program computer, i.e. both instructions and data are accessed from memory. Your computer cannot be OISC/URISC (One Instruction Set Computer/Ultimate RISC) or ZISC (Zero Instruction Set Computer) etc. The word-size for data is 1 byte Memory is byte-addressable (i.e., every byte in the memory has a unique address