Motorola 68881
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The Motorola 68881 was a floating-point coprocessor chip that was utilized in some computer systems that used the 68020 or 68030 CPU. The addition of the 68881 chip added substantial cost to the computer, but added a floating point unit that could rapidly perform floating point math calculations. At the time, this was useful mostly for scientific and mathematical software.
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[edit] Overview
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" were "traps" which would throw an interrupt, handing control to the computer's operating system. If a 68881 were present in the system, the CPU would allow it to execute the instruction. If not, the OS would either call an FPU emulator to execute the instruction using 68020 integer-based software code, or would return an error code to the program.
The 68881 had eight 80-bit data registers. It allowed seven different modes of numeric representation, including single-precision, double-precision, and extended-precision, as defined by the IEEE floating-point standard, or "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.
The Motorola 68882 was an improved version of the 68881, with better pipelining, and eventually available at higher clock speeds. Its instruction set was exactly the same as that of the 68881. Motorola claimed in some marketing literature that it executed some instructions 40% faster than a 68881 at the same clock speed, though this did not reflect typical performance at all.
Notable computers including 68881 or 68882 FPUs included the Sun 3 from Sun Microsystems, the Macintosh II family of computers from Apple Computer, and the Commodore Amiga 3000. Some third-party Amiga products used the 68881/2 as a memory-mapped peripheral to the 68000.
When the Motorola 68040 processor was introduced, it included the FPU internally. Most instructions and numeric representation modes from the 68881 were supported in hardware, but some were not, and were emulated through a software package.
[edit] Selected statistics
[edit] 68881
- 155,000 transistors on-chip
- 16MHz version ran at 160 KFLOPS
- 20MHz version ran at 192 KFLOPS
- 25MHz version ran at 240 KFLOPS
[edit] 68882
- 176,000 transistors on-chip
- 25MHz version ran at 264 KFLOPS
- 33MHz version ran at 352 KFLOPS
- 40MHz version ran at 422 KFLOPS
- 50MHz version ran at 528 KFLOPS
[edit] 68040
- 25MHz FPU ran at 3.500 MFLOPS
- 33MHz FPU ran at 4.662 MFLOPS
- 40MHz FPU ran at 5.600 MFLOPS
These statistics came from the comp.sys.m68k FAQ. No statistics are listed for the 16MHz and 20MHz 68882, though these chips were indeed produced.
[edit] References
| List of Motorola/Freescale microprocessors | |
|---|---|
| The 6800 family : | 6809 (see also: Hitachi 6309) |
| 68000 family : | 68000 | 68008 | 68010 | 68012 | 68020 | 68030 | 68040 | 68060 | ColdFire | DragonBall |
| Low-cost variants: | 68EC000 | 68EC020 | 68EC030 | 68EC040 | 68LC040 |
| Pre-PowerPC RISC : | 88000 |
| Floating-point coprocessors : | 68881, 68882 |
| PowerPC family : | PPC 75x ("PowerPC G3") | PPC 74xx ("PowerPC G4") | PowerQUICC |
