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R-Type instructions are the most common in MIPS and give us a good way of understanding how an ISA defines the process that a CPU goes through when receiving data.

All R-type instructions have an op of ‘000000’ which signifies to the processor to look at the func bits to determine which process to execute.

The three references to registers, (rs, rt, rd) directly call them by number. There are 32 registers in a MIPS system and the five bits will produce 32 numbers (0 as 00000 to 31 as 11111). The first two (rs & rt) are the operands and the last (`rd) is where to store the result.

The shift amount is used to shift the number by the amount in the shift bits, for our purposes this will always be 00000, meaning no shift. The last six bits are the function to be performed on the operands.

Let’s take a look at this example instruction:

000000 00101 10010 00110 00000 100000 op rs rt rd shamt func

Now, let’s break the instruction down:

  • op -> 000000 signifies an R-Type instruction
  • rs -> 00101 gets contents in Register 5
  • rt -> 10010 gets contents in Register 18
  • rd -> 00110 sets the result in Register 6
  • shamt -> 00000 means there is no shift
  • func -> 100000 function signature for adding

Our processor will add the contents of Register 5 (16) and Register 18 (103) and store that result (119) into Register 6. In the example, these numbers are shown as integers, but in the registers they will be stored in binary. Notice that the first register is always 0 and is a protected register, this is common in most ISAs.

Instructions

The two tables in the code editor represent the data stored in the registers before and after the ADD instruction is passed to the CPU. You will notice that the original numbers being added still remain in their location in the register. They will stay there until they are overwritten or power is turned off to the CPU. Registers are volatile memory.

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