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Floating Point Coprocessors

The designer of any microprocessor would like to extend its instruction set almost infinitely but is limited by the quantity of silicon available (not to mention the problems of testability and complexity). Consequently, a real microprocessor represents a compromise between what is desirable and what is acceptable to the majority of the chip’s users. For example, the 68020 microprocessor is not optimized for calculations that require a large volume of scientific (i. e. floating point) calculations. One method to significantly enhance the performance of such a microprocessor is to add a coprocessor.

To ncrease the power of a microprocessor, it does not suffice to add a few more instructions to the instruction set, but it involves adding an auxiliary processor that works in parallel to the MPU (Micro Processing Unit). A system involving concurrently operating processors can be very complex, since there need to be dedicated communication paths between the processors, as well as software to divide the tasks among them. A practical multiprocessing system should be as simple as possible and require a minimum overhead in terms of both hardware and software.

There are various techniques of arranging a coprocessor longside a microprocessor. One technique is to provide the coprocessor with an instruction interpreter and program counter. Each instruction fetched from memory is examined by both the MPU and the coprocessor. If it is a MPU instruction, the MPU executes it; otherwise the coprocessor executes it. It can be seen that this solution is feasible, but by no means simple, as it would be difficult to keep the MPU and coprocessor in step.

Another technique is to equip the microprocessor with a special bus to communicate with the external coprocessor. Whenever the microprocessor encounters an operation that requires he intervention of the coprocessor, the special bus provides a dedicated high- speed communication between the MPU and the coprocessor. Once again, this solution is not simple. There are more methods of connecting two (or more) concurrently operating processors, which will be covered in more detail during the specific discussions of the Intel and Motorola floating point coprocessors.

Motorola Floating Point Coprocessor (FPC) 68882 The designers of the 68000-family coprocessors decided to implement coprocessors that could work with existing and future generations of icroprocessors with minimal hardware and software overhead. The actual approach taken by the Motorola engineers was to tightly couple the coprocessor to the host microprocessor and to treat the coprocessor as a memory-mapped peripheral lying inside the CPU address space.

In effect, the MPU fetches instructions from memory, and, if an instruction is a coprocessor instruction, the MPU passes it to the coprocessor by means of the MPU’s asynchronous data transfer bus. By adopting this approach, the coprocessor does not have to fetch or interpret instructions itself. Thus if the coprocessor requires data from memory, the MPU must fetch it. There are advantages and disadvantages to this design. Most notably, the coprocessor does not have to deal with, for example, bus errors, as all fetching is performed by the host MPU.

On the other hand, the FPC can not act as a bus master (making it a non-DMA device), making memory accesses by the FPC slower than if it were directly connected to the address and data bus. In order for the coprocessor to work as a memory mapped device, the designers of the 68000 series of MPU’s had to set aside certain bit patterns to epresent opcodes for the FPC. In the case of the 68000’s, the FPC is accessed through the opcode 1111(2). This number is the same as F’ in hexadecimal notation, so this bit pattern is often referred to as the F-line.

Interface The 68882 FPC employs an entirely conventional asynchronous bus interface like all 68000 class devices, and absolutely no new signals whatsoever are required to connect the unit to an MC 68020 MPU. The 68882 can be configured to run under a variety of different circumstances, including various sized data buses and clock speeds. What follows is a diagram of connections necessary to onnect the 68882 to a 68020 or 68030 MPU using a 32-bit data path. As mentioned previously, all instructions for the FPC are of the F-line format, that is, they begin with the bit pattern 1111(2).

A generic coprocessor instruction has the following format: the first four bits must be 1111. This identifies the instruction as being for the coprocessor. The next three bits identify the coprocessor type, followed by three bits representing the instruction type. The meaning of the remaining bits varies depending on the specific instruction. Coprocessor Operation When the MPU detects an F-line instruction, it writes the instruction into the coprocessors memory mapped command register in CPU space.

Having sent a command to the coprocessor, the host processor reads the reply from the coprocessor’s response register. The response could, for example, instruct the processor to fetch data from memory. Once the host processor has complied with the demands from the coprocessor, it is free to continue with instruction processing, that is, both the processor and coprocessor act concurrently. This is why system speed can be dramatically improved upon installation of a coprocessor.

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