Q. How can this arithmetic processor be associated to the CPU? 

Two mechanisms are used for connecting arithmetic processor to CPU.  

If an arithmetic processor is treated as one of the Input / Output or peripheral units then it's termed as a peripheral processor. CPU transmitsinstructions and data to peripheral processor that performs the needed operations on data and communicates the results back to CPU. A peripheral processor has various registers to communicate with the CPU. These registers can be addressed by CPU as I/O register addresses. Peripheral and CPU processors are generally quite independent and communicate with each other by exchange of information using data transfer instructions. Data transfer instructions should be specific instructions in CPU. This kind of connection is known as loosely coupled.

Instead if the arithmetic processor has a register and instruction set that can be considered an extension of CPU registers and instruction set then it is known as a tightly coupled processor. Here CPU reserves a special subset of code for arithmetic processor. In such a system the instructions meant for arithmetic processor are fetched by CPU and decoded mutually by CPU and arithmetic processor and finally executed by arithmetic processor. So these processors can be considered a logical extension of CPU. These kind of attached arithmetic processors are called as co-processors.  

The concept of co-processor existed in 8086 machine until Intel 486 machines where co-processor was separate. But Pentium at present doesn't have a separate co-processor. In the same way peripheral processors aren't found as arithmetic processors in general. Though many chips are used for specialized I/O architecture.