Problems with the Cyrix 6x86's cache under Windows NT have forced Microsoft to more or less high-handedly reconfigure the processor. Then it works just ticking over, at least in current versions.
There's no doubt that the Cyrix 6x86 processor still has considerable difficulties in a lot of board designs. Its enormous energy consumption causes a lot of trouble in many boards. Furthermore, it has bigger problems than the Pentium with reflections on the CPU bus, especially if such big trouble makers like a Coast socket come into play. In the last issue of c't we figured out with which CPU steppings and clock rates it sometimes begins to flounder, on account of the Chaintec IFM 586 board. Cyrix has distributed appropriate application notes to the board makers in order to suppress the disturbing reflections with help of suitable damping resistors.
In a board designed for the Cyrix processor we nevertheless had to realise that - other than the Pentium in the same board - it didn't make it through several days of continuous operation running Windows 95 without crashing.
Cyrix has been unable to give a reason for the system crashes we experienced. Both the Cyrix vice president Steve Tobak and applications engineer Graham Jackson, responsible for European operations, declared that they don't know about a processor bug - there are rumours of infrequent cache allocation errors.
The American computer magazine Byte reports that the processor instabilities have lead Microsoft to simply switch off the 6x86 cache's write back mode in the final release of Windows NT 4.0. According to Byte the result is a 30% decrease in the performance of Windows NT. More or less, this means a knockout.After all, it is supposed to earn its merits above all under Windows 95, but who'll trust it there when it can only proceed at half speed under NT? In addition to that, an American survey showed that a good 25% of 6x86 users want to use Windows NT.
It's quite possible, that the problems at Microsoft may have been caused only by the reflections mentionend above, since they may be noticeable only with write back cache enabled. With version 2.7 of the processor, coming on to the market now, Cyrix doesn't just promise a few extra percent of performance (the return stack was switched off previously), but also fewer reflection problems. According to Cyrix version 2.7 can run properly under Windows NT without being slowed down. The Texan "midget", fighting for survival with its three percent market share, obviously couldn't achieve any more from Microsoft.