Why doesn't there exists a subi opcode for MIPS?

assembly mips cpu-architecture instructions instruction-set
user379888 picture user379888 · Aug 2, 2011 · Viewed 26.5k times · Source

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. Why don't we have a subi opcode?

Answer

Chris Gregg picture Chris Gregg · Aug 2, 2011

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.

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