Variations in instruction encoding
Variable: Can support any number of operands. Number of operands is a part of the opcode. Each operand has address specifier. Example: VAX (1 - 53 bytes)
Fixed: Fixed format always has the same number of operands. How they are decoded may differ with opcode. Example: SPARC, MIPS, PowerPC. (For SPARC - 4 bytes)
Hybrid: Multiple formats of fixed length. Format specified as a part of the opcode. EX: IBM 370, Intel 80X86 series. (Intel 7 different formats of 1 - 5 bytes)