Why doesn't there exists a subi opcode for MIP

Question

I am very new to Assembly language. I was reading about MIPS architecture and came to know that you have addi opcode but there is no subi opcode. Can anyone please explain me why don't we have a subi opcode?

Thanks in advance.

Answer 1

When you create an instruction set, you're bound by some constraints, such as the total number of instructions you can create. The MIPS creators realized that there isn't a need for subi (because you can add a negative number with addi using 2's complement), and they simply made the decision to forego making that instruction. It may have been to conserve the number of instructions, or just simply because it isn't needed.

Answer 2

Both addi and addiu take 16-bit signed immediates as operand, so it makes no sense to add separate subi and subiu opcodes.