The opcode field being 4 bits long implies that there can only be 16 instructions, since 2⁴ = 16. However, there are more than 16 instructions in the CSC-1, 24 to be exact. But 9 of these do not require an operand. Thus, their opcodes can be longer than 4 bits. Because the computer needs to know when to interpret the remaining 12-bits as operand or part of the opcode, one of the opcodes, 1111, is reserved to mean "what follows is not an operand, but part of an extended opcode" for those instructions which need no operand.