20Mhz Motorola 68881 floating-point processor
The 115 had 4MB memory ON the processor card, available only on the desktop 6151.
Has a *lot* of "yellow wires" on the back
HMS has it as P/N 08F3639

 This puppy has the same layout as the 4MB 08F3369, but the upper left is only solder pads for the little metal cans. The solder pads are for the pins from the memory cubes.

  This board has the same layout, but the area for the memory cubes was never meant to take them. The Green resist covered solder pads are in a rectangular pattern. I think that it's more of a ground plane than anything...

The Advanced Processor of the 115 models have the standard features of:
* 100 Nanosecond processor cycle time. (032 had 170nS)
* 16  32-bit general purpose registers
* 16  32-bit system control registers, including:
  - countdown timer register
  - exception control registers
  - interrupt request buffer register
  - instruction address register
* 118  2 and 4 byte instructions with storage, branch, control, and integer math, including multiply and divide step instruction.
  * Memory Managment Unit with translation look-aside buffers, and address translation including:
 - 40-bit virtual address.
 - 256 Megabyte memory segments.
 - Multiple independent virtual address spaces.
 - address space size of 4 gigabytes.
 - demand paging.
 - page size of 2048 or 4096 bytes.
 - Memory protection.
 - Real memory addressability of up to 16 Megabytes.
 - Hardware assist for load real address 

    The Advanced processor (APC) has these enhancements to the original processor. 

* Overlapped Load and Store operations.
* Several instructions are serialized to allow for better memory and I/O operations.
* Additional exception control register information.
* Instruction prefetch, and 16 byte loop detection.
* MMU allows multiple outstanding load and store requests
* On board 20 Mhz MC68881 floating point processor.
  - Trigonometrice and transcendental functions.
  - Seven different data types and conversions.
* I/O interface enhancements.

The second method, called peripheral processor, is so that the 68881 can interface to earlier 68xxx models, like the 68000 or 68008. In this mode, communication between the 68881 and CPU takes place by reading and writing to the 68881's registers, called Coprocessor Interface Registers (CIRS)  These are basically registers that you map into the CPU's address space, much like you map a PIA (6520) registers into the 6502's address space.

Another interesting feature of the 68881 is that it can be configured to work with an 8 bit data bus, with some decrease in performance as compared to its default 32 bit data bus configuration-- of course.

Additionally, the 68881 can have a separate clock attached to it, running at either a faster or slower speed than the rest of the system, due to the asynchronous nature of the chip.

The 68881 can also handle floating point numbers in either IEEE format or a BCD format, much like the format used in the Atari 8 bit FP OS routines.