Apollo PRISM

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Apollo PRISM

What it was
PRISM was Apollo Computer’s high‑performance CPU for its DN10000 workstation line. It used a VLIW (very long instruction word) design, meaning the compiler decided which instructions to group together so several operations could run in parallel in one clock.

How it worked
- The processor could dispatch one integer instruction or one integer plus one floating‑point instruction per clock.
- It included a combined floating‑point add/multiply in a single instruction.
- It used a register set of 64 words: thirty-two 32‑bit integer registers and thirty-two 64‑bit floating‑point registers (overlayed as sixty‑four 32‑bit registers).
- This setup let the CPU keep its functional units busy and aimed to maximize throughput.

Architecture and design ideas
- PRISM was a pure 32‑bit design and was often described as two CPUs in one because it could operate on integer and floating‑point data in parallel.
- It relied heavily on the compiler to select safe instruction pairs and to pack them into wide instruction words, shifting the burden of scheduling from hardware to software.
- The design aimed to maximize performance by moving the scheduling work into the compiler, but that also meant software had to be tightly matched to the hardware.

The compilers
- Apollo’s compilers were among the first commercial tools to use single static assignment (SSA) techniques, an approach that helped optimize how instructions were arranged for the wide words PRISM could execute.

History and today’s context
- The PRISM project began in 1986 and the first announcements came in 1988, with initial systems delivered in October 1988.
- About a thousand DN10000 systems were sold. PRISM was marketed as the fastest CPU available at the time and helped make the DN10000 a powerful workstation.
- In 1989, Hewlett-Packard bought Apollo, ending further PRISM development. Some ideas from PRISM influenced later HP architectures (HP-PA and efforts that contributed to Itanium).
- Competitors of the era included Intel’s i860; DEC’s own PRISM project was canceled, and PRISM’s approach stood out for its compiler-driven design.
- PRISM’s approach—pushing more of the instruction scheduling to the compiler and tightly linking software to hardware—was influential in later RISC and compiler-based design trends, even as the specific PRISM hardware itself did not continue.