Question

What is the opcode and the rs fields encoded in the following instruction:

30620001

Answer 1

Answer :

All instructions in the MIPS R2000 Architecture are 32 bits in length. There are three different instruction formats: R-Type instructions, I-Type instructions, and J-Type instructions.

R-Type instructions, or Register instructions are used for register based ALU operations. The two operands and the destination of the result are specified by locations in the register file.

I-Type instructions, or Immediate instructions, can be either Load/Store operations, Branch operations, or Immediate ALU operations. In these instructions, one two register file locations are specified as well as a 16-bit immediate value which may be used as an operand or an address.

The rs and rt register addresses, which are present in both R- and I-type instructions, specify the two addresses which the register file is to read. In R-Type instructions the destination (write) register for the register file is specifies by rd and in I-Type instructions the destination register is specified by rt (the second read from the register file is ignored).

An R-Type instruction contains 6 fields: a 6 bit function code (funct), a 5 bit shift amount (shamt), three 5 bit register addresses (rd, rt, rs), and a 6 bit operation code (opcode) which is always zero.

An I-Type instruction contains 4 fields: a 16 bit immediate field (immed. or address), two 5 bit register addresses (rt, rs) and a 6 bit operation code (opcode).

R-Type Instruction Format:

31-26	25-21	20-16	15-11	10-6	5-0
opcode (000000)	rs	rt	rd	shamt	funct

rs = operand A location in register file; rt = operand B location in register file; rd = result destination location in register file; shamt = shift amount; funct = function code

I-Type Instruction Format (ALU immediate):

31-26	25-21	20-16	15-0
opcode	rs	rt	immed.

rs = operand A location in register file; rt = result destination location in register file; immed. = operand B location in register file

Now the binary notation of the given instruction 30620001 is :

0011 0000 0110 0010 0000 0000 0000 0001
MSB LSB

In both the type of instruction bits 31 to 26 are opcode and bits 25 to 21 is for rs , so the opcode and rs for this instruction will be :

Opcode : 001100

rs : 00011