At run time: Register $25 is set to (Register $16) - (Register $17)
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:
Here is the machine code it translates into:
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 instruction 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) destination of the result.
A machine instruction consists of fields
that specify a machine operation
and other fields that designate the data.
The 12 bits that specify the
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.
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 ALUop $8 $9 $11 addu op1 op2 dest