The Alpha 21264 CPU: NT's Greatest RISC (1998)
This vintage article from 1998 offers a deep dive into Digital's Alpha 21264, a formidable 64-bit RISC processor designed to power Windows NT and challenge Intel's looming IA-64 Merced. It details the CPU's groundbreaking out-of-order execution, innovative cache hierarchy, and aggressive clock speed targets, making it a compelling read for hardware enthusiasts. The story's popularity on HN stems from its nostalgic look at a bygone era of intense CPU architecture competition and a what-if scenario where RISC could have dominated the enterprise market.
The Lowdown
Published just as BYTE magazine folded in 1998, this article introduces the Digital Alpha 21264, a high-performance 64-bit RISC processor positioned as a leading contender for Windows NT workloads. At a time when the computing world awaited Intel's 64-bit Merced chip, the 21264 was already available, promising blazing speeds and significant technical advancements designed to outperform its x86 rivals.
- Performance Prowess: The Alpha 21264 was projected to reach clock speeds of 700MHz, with ambitions for 1GHz by 2000, aiming to be substantially faster than the anticipated Merced processor, especially in floating-point tasks.
- RISC's Last Stand: The article frames the 21264 as the last major RISC architecture supported by Microsoft for future NT versions, highlighting a general retreat of RISC chips in the broader market against x86 dominance and the emerging IA-64.
- Architectural Innovations: Key features included a sophisticated out-of-order execution engine capable of juggling 80 instructions simultaneously, a unique clustered register scheme, and enhanced branch prediction achieving about 95% accuracy.
- Memory and Bus Design: It featured significantly larger Level 1 caches (64KB instruction and data), an off-chip Level 2 cache with high bandwidth, and a point-to-point I/O channel. Notably, its bus protocols were licensed by AMD for its K7 processor, suggesting potential for lower-cost Alpha systems.
- Industry Flux: The Alpha's future was uncertain due to Digital's sale of its chip foundry to Intel and its acquisition by Compaq, an Intel-centric company. Despite these shifts, Compaq expressed commitment to the Alpha, with future generations (21364, 21464) already planned.
Ultimately, the Alpha 21264 stood out as a marvel of 1990s microprocessor engineering, embodying a bold challenge to Intel's evolving supremacy. Its detailed technical description provides a fascinating glimpse into the cutting-edge design choices and market pressures that defined the high-performance CPU landscape at the turn of the millennium.