Finding a CPU Design Bug in the Xbox 360 | Random ASCII
https://randomascii.wordpress.com/2018/01/07/finding-a-cpu-design-bug-in-the-xbox-360/
january 2018 ‧ IFTTT Pocket programming reddit imported ‧ open in karakeep
From Reddit saved post — r/programming, u/ketralnis, Mar 17 2026
https://www.reddit.com/r/programming/comments/1rwhdaw/finding_a_cpu_design_bug_in_the_xbox_360/
In 2005, a design bug was discovered in the Xbox 360 CPU involving a new instruction called xdcbt, which prefetches data directly to L1 cache, bypassing L2 and breaking memory coherency. This led to elusive crashes due to inconsistent memory views across cores, especially when xdcbt was speculatively executed by the CPU's branch predictor. The discovery highlighted how speculatively executed instructions, even if not retired, could cause real side effects—foreshadowing issues similar to Meltdown and Spectre. Ultimately, the xdcbt instruction was deemed too hazardous to safely include in game code due to unpredictable speculative execution.
The recent reveal of Meltdown and Spectre reminded me of the time I found a related design bug in the Xbox 360 CPU – a newly added instruction whose mere existence was dangerous. Back in 2005 I was…
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