Analysis
What causes the bug is not an interrupt mask, nor are interrupts being explicitly disabled. Instead, an anomaly in the Cyrix's instruction pipeline prevents interrupts from being serviced for the duration of the loop; since the loop never ends, interrupts will never be serviced. The xchg instruction is atomic, meaning that other instructions are not allowed to change the state of the system while it is executed. In order to ensure this atomicity, the designers at Cyrix made the xchg uninterruptible. However, because of pipelining and branch predicting, another xchg enters the pipeline before the previous one completes, leaving the processor in this uninterruptible state forever.
Read more about this topic: Cyrix Coma Bug
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