Motorola 68881

The Motorola 68881 and Motorola 68882 were floating-point coprocessor (FPU) devices that were used in some computer systems in conjunction with the 68020 or 68030 microprocessors. The addition of one of these devices added substantial cost to the computer, but added a floating point unit that could rapidly perform floating point mathematical calculations. At the time, this feature was useful mostly for scientific and mathematical software.

The 68020 and 68030 CPUs were designed with the separate 68881 chip in mind. Their instruction sets reserved the "F-line" instructions — that is, all opcodes beginning with the hexadecimal digit "F" could either be forwarded to an external coprocessor or be used as "traps" which would throw an exception, handing control to the computer's operating system. If an FPU is not present in the system, the OS would then either call an FPU emulator to execute the instruction's equivalent using 68020 integer-based software code, return an error to the program, terminate the program, or crash and require a reboot.

The 68881 had eight 80-bit data registers (a 64-bit mantissa plus a sign bit, and a 15-bit signed exponent). It allowed seven different modes of numeric representation, including single-precision, double-precision, and extended-precision, as defined by the IEEE floating-point standard, IEEE 754. It was designed specifically for floating-point math and was not a general-purpose CPU. For example, when an instruction required any address calculations, the main CPU would handle them before the 68881 took control.

The CPU/FPU pair were designed such that both could run at the same time. When the CPU encountered a 68881 instruction, it would hand the FPU all operands needed for that instruction, and then the FPU would release the CPU to go on and execute the next instruction.

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