Clipper architecture

Lore & Background

The first processors using the Clipper architecture were designed and sold by Fairchild, but the division responsible for them was subsequently sold to Intergraph in 1987; Intergraph continued work on Clipper processors for use in its own systems. The initial Clipper microprocessor produced by Fairchild was the C100, which became available in 1986. This was followed by the faster C300 from Intergraph in 1988. The final model was the C400, released in 1990, which was extensively redesigned to be faster and added more floating-point arithmetic registers. The C400 combined superscalar instruction dispatch and superpipelined operation, making it the first processor to use both techniques simultaneously.

Unlike many other microprocessors, the Clipper processors were actually sets of several distinct chips. The C100 and C300 consist of three chips: one central processing unit containing both an integer unit and a floating point unit, and two cache and memory management units (CAMMUs), one for data and one for instructions. The C400 consists of four basic units: an integer CPU, an FPU, an MMU, and a cache unit. The initial version used one chip each for the CPU and FPU and discrete elements for the MMU and cache unit, but in later versions the MMU and cache unit were combined into one CAMMU chip.

The Clipper has 16 integer registers (R15 is used as the stack pointer), 16 floating-point registers (limited to 8 in early implementations), plus a program counter, a processor status word, and a system status word. User and supervisor modes have separate banks of integer registers. The Clipper is a load/store architecture, where arithmetic operations could only specify register or immediate operands. The basic instruction parcel is 16 bits. Addressing modes include register relative, PC-relative, absolute, and indexed. The processor supports operations such as 32×32→32-bit multiply, divide, and remainder, 64-bit shifts and rotates, 32×32→64-bit extended multiplies, integer register push/pop, subroutine call and return, atomic memory load and set msbit, supervisor trap, and more complex macro instructions for push/pop multiple registers, return from interrupt, and string operations.

Reader's Guide

Intergraph sold several generations of Clipper systems, including both servers and workstations, under the InterAct, InterServe, and InterPro product lines, targeted largely at the computer-aided design (CAD) market. Fairchild promoted the CLIX operating system, a version of UNIX System V, for use with the Clipper; Intergraph adopted CLIX for its Clipper-based systems and continued to develop it, making it the only operating system available for those systems. Intergraph did work on a version of Microsoft Windows NT for Clipper systems and publicly demonstrated it, but this effort was canceled before release. Intergraph decided to discontinue the Clipper line and instead began selling x86 systems with Windows NT.

Intergraph started work on a subsequent Clipper processor design known as the C5, but this was never completed or released. Nonetheless, some advanced processor design techniques were devised for the C5, and Intergraph was granted patents on these. These patents, along with the original Clipper patents, have been the basis of patent-infringement lawsuits by Intergraph against Intel and other companies. The Clipper architecture's legacy lies primarily in its technical innovations—particularly the C400's pioneering combination of superscalar and superpipelined techniques—and in the patent litigation that followed its discontinuation.

Did You Know?

• The Clipper processors were sets of multiple chips: the C100 and C300 used three chips, while the C400 used four basic units.
• Intergraph sold Clipper-based systems under the InterAct, InterServe, and InterPro product lines, mainly for the CAD market.
• Intergraph's patents from the unfinished C5 design, along with original Clipper patents, were used in patent-infringement lawsuits against Intel and other companies.