### A good answer might be:

1. The operation: subtraction of two 32-bit integers
2. The location of one operand: register $16
3. The location of the other operand: register $17
4. The destination of the result: register $25

At run time: Register $25 <== (Register $16) - (Register $17)

# Machine Code

On some processors (such as the VAX) the ALU can operate directly on data contained in main memory. However, this requires extra data paths and control logic and slows down execution. On the MIPS, operands for the ALU and the destination for the result are inside the chip (usually registers).

Here is the assembly language for the add instruction:

    addu    $10,$8,$9

Here is the machine code it translates into:

    0x01095021

Here is that as a bit pattern:

    0000 0001 0000 1001 0101 0000 0010 0001

Here is the bit pattern arranged into different groups. Under each group is what the pattern means as a machine instruction. (A group of bits that is part of a larger bit pattern is often called a field). Each field of a machine instruction has a specific function.

      0    1    0    9    5    0    2    1     -- machine instruction in hex

    0000 0001 0000 1001 0101 0000 0010 0001    -- machine instruction in bits

    000000 01000 01001 01010 00000 100001      -- fields of the instructuion

    opcode oprnd oprnd dest  ----- 2ndary      -- meaning of the fields

    ALUop   $8    $9   $10           addu

Inspect the groups until you see how the instruction specifies the four things: (1) the operation, (2) operand 1, (3) operand 2, (4) result destination.

A machine instruction consists of fields that specify a machine operation and other fields that designate the data. The 12 bits that specify the `addu` operation are split into two groups. The first group is called the opcode and the second group is called the secondary opcode.

The register numbers are readable as binary integers. Some bits of the instruction are left as zero. You have to look at MIPS documentation to figure all this out.

### QUESTION 6:

Here is the machine instruction again, but the pattern that specifies the destination is blank. Fill it in so the destination is register $11.

    000000 01000  01001 _ _ _ _ _ 00000 100001

    addu    $8    $9    $11             addu
           op1   op2    dest