Motorola 68881 FPU family

Motorola 68881 (MC68881) is a co-processor used with second generation of Motorola 68K microprocessors - Motorola 68020 and 68030. The 68881 could be theoretically used with earlier 68000/68010 CPUs, but only as a peripheral device. The MC68881 FPU has 8 directly-addressable 80-bit registers. The FPU supports 7 data types: 8, 16 and 32-bit integers, packed decimal strings, and single, double and extended-precision floating point numbers. When loading and storing data, the 68881 converts all these formats into extended-precision floating-point numbers, and all operations inside of the co-processor are performed with these numbers. Instructions supported by the 68881 include save/load instructions, arithmetic, trigonometric, test and branching instructions.

The Motorola 68881 co-processor works as a memory mapped device. When the main CPU loads co-processor instruction, it writes the instruction code into Co-processor Interface Registers (CIR), which are mapped in CPU address space, and then reads co-processor response from one of CIR registers. If the co-processor returns "continue operation" response the main CPU may start executing next instruction while the FPU continues executing the Floating-Point (FP) instruction. Thus, both integer and FP instructions can be executed concurrently. The response can be also used to synchronize executions between main CPU and the co-processor. If the FPU receives a request to process an instruction, and it's still busy executing previous instruction, it will tell the main CPU to wait. It's important to note that the co-processor only executes instructions that it receives from the CPU, and it doesn't directly access the memory or even calculate memory addresses. When co-processor needs to read from or write to memory, it returns a response code which instructs the main processor to calculate memory address and read or write data to that address.

Because the FPU operates as a memory-mapped device, it's not dependent on the frequency of the main CPU and may run at different frequency.