How to know MIPS-instruction format R, I or J

StackOverflow https://stackoverflow.com/questions/20336508

  •  07-08-2022
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Question

How to know if the opcode of the MIPS instruction is Register, Imidiate or Jump ? Given this table from the book, but is there any way to define the format of the opcode ?

http://i.imgur.com/uNK6eby.png

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Solution

Take a look at the opcode bits - the most significant 6 bits. In C, if n is the 32-bit command dword, the expression for the opcode is (n >> 26) & 0x3f.

If opcode is 0, then it's a register command.

If it's 2 (j), 3 (jal), or 26 (trap) - it's a jump.

Else immediate.

A cheat sheet of MIPS encoding, including a list of all opcodes, is available here, and probably elsewhere.

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