Cyrix 6x86

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The Cyrix 6x86 (codename M1) is a sixth-generation, 32-bit 80x86-compatible microprocessor designed by Cyrix and manufactured by IBM and SGS-Thomson. It was originally released in 1996.



The 6x86 is superscalar and superpipelined and performs register renaming, speculative execution, out-of-order execution, and data dependency removal. However, it continued to use native x86 execution and ordinary microcode only, like Centaur's Winchip, unlike competitors Intel and AMD which introduced the method of dynamic translation to micro-operations with Pentium Pro and K5.

With regards to internal caches, it has a 16-kB primary cache and is socket-compatible with the Intel P54C Pentium. It was also unique in that it was the only x86 design to incorporate a 256-byte Level 0 scratchpad cache. It has six performance levels: PR 90+, PR 120+, PR 133+, PR 150+, PR 166+ and PR 200+. These performance levels do not map to the clock speed of the chip itself (for example, a PR 133+ ran at 110 MHz, a PR 166+ ran at 133 MHz, etc).

Note that the 6x86 and 6x86L weren't completely compatible with the Intel P5 Pentium instruction set and is not multi-processor capable. For this reason the chip identified itself as a 80486 and disabled the CPUID instruction by default. CPUID support could be enabled by first enabling extended CCR registers then setting bit 7 in CCR4. The lack of full P5 Pentium compatibility caused problems with some applications because programmers had begun to use P5 Pentium-specific instructions. Some companies released patches for their products to make them function on the 6x86.

The first generation of 6x86 had heat problems. This was primarily caused by their higher heat output than other x86 CPUs of the day and, as such, computer builders sometimes did not equip them with adequate cooling. The CPUs topped out at around 25 W heat output (like the AMD K6), whereas the P5 Pentium produced around 15 W of waste heat at its peak. However, both numbers would be a fraction of the heat generated by many high performance Netburst Pentium 4 versions, some years later.

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