Co-Processor | Computer Architecture

Introduction :
If in microprocessor chip, new circuitry can be added with special purpose to perform special tasks or to perform operations on numbers in order to offload the work of the core CPU. The CPU can then work faster.

We may use a conveyor belt to do some extra work while motor is running. So , the motor is more effectively utilized. Similarly, supplementary processor, i.e.,  a co-processor handles the mathematical part of the work when we run complex applications.

What is a coprocessor ?

• A co-processor is many times referred as a Math Processor. As the coprocessor performs routine mathematical tasks, the core processor is freed up from this computation and its time is saved. By taking specialized processing tasks from core CPU, coprocessor reduces the strain on the main microprocessor, so that it can run at a greater speed.
• A coprocessor can perform  special tasks like complex mathematical calculations or graphical display processing. They perform such jobs faster than core CPU. As a result, overall computer speed of the system increases.
• To an ARM processor, we can attach the coprocessors. A coprocessor when added, we need to   expand instruction set of Core CPU or add configurable registers,  to increase the processing power. The coprocessor interface permits a couple of coprocessor to be connected to the ARM CPU.

How does coprocessor work ?
Coprocessors assists the system in running more efficiently by offloading specific tasks of the CPU. They can be 

1. Independent type - 
   It work in an asynchronous manner with the CPU . Whenever it performs any task/ some calculations, that is not synchronized with the CPU. Co-processor can take decisions independently here. CPU can then do its own work without worrying about co-processor's work. Instead of synchronously waiting, CPU that issued the request is freed up to perform another task.
2. Direct control type -  
   Direct control type such as floating-point units - controlled by coprocessor's instructions that are CPU instruction set's part. The CPU that initiated the request waits or  checks until co-processor completes operation. The coprocessor is not independent here & is governed or controlled by the main CPU. CPU & coprocessor works in synchronization here.

The processor is designed and implemented in manner so that it can send, both data and instructions to several coprocessors. These coprocessors are designed to perform in coordination with the core and are pipelined in the identical manner.

By adding a new set of specialized instructions, coprocessor can expand instruction set . For example, to handle vector floating-point (VFP) operations, a series of specialized instructions can be added to the normal ARM instruction set. When the instruction is decoded and it is encountered as coprocessor instruction, that instruction is passed on to the appropriate coprocessor. However, if the coprocessor isn't present or doesn't find instruction in its instruction set, an undefined instruction exception is thrown by ARM.

Properties of Coprocessor  :

1. Without primary microprocessor, coprocessor cannot function.
2. Main processor has to identify and segregate computationally intensive instructions in a program.
3. The instructions which have an intensive amount of calculations are performed by coprocessor.
4. The main processor handles all other activities.

Functionalities of a Coprocessor :

• Coprocessors are incapable of retrieving instructions from memory, managing memory, executing instructions(flow control types) directly, performing I/O operations, etc.
• The coprocessor relies on the host (main) processor to retrieve coprocessor instructions and take care of all other operations not related to the coprocessor.
• A coprocessor is not the main processor of the system.
• Coprocessor can perform : Floating point arithmetic operations such as addition, subtraction, multiplication, Calculating the square root of the given number, estimating the logarithmic value  of the given number etc. (for float point values) , or signal, string, graphical processing or encryption/decryption etc.
• Coprocessors enable a computer customization , so the clients  do not need to pay if they do not require the additional performance.

Example - Intel 8087 Coprocessor :
The Intel 8087 was first  math co-processor suited for operations of 16 bit. It was built to be paired with the Intel 8086 Microprocessor. Its main purpose was to decrease time required by  the applications to give output (that require high floating point computations).
With introduction of co-processor along with the main processor, it was noticed that performance of the applications improved from 20% to 500%. 
8087 coprocessor Performance - About 50 Kilo Floating Point Operations per second(KFLOPS).

Intel 8087 co-processor supports :

• Integer
• BCD
• Single Precision floating point number
• Double Precision  floating-point number
• Extended precision ( 80 bit)  floating-point number

The diagram shows the connectivity of a co-processor with 8086 Microprocessor. 
Most of the co-processors has - Arithmetic module, Temporary registers and logic module.