digging through the IBM BIOS for the PS/2 Model 77, i found this interesting bit of code that tests if it is running on a 16-bit 8086/8088 or a 32-bit 286+. 
while running in real mode, the BIOS tries to execute the MOV EAX,0xffaa0055 instruction. even though EAX is a 32-bit register, this is possible to run in real mode using the segment override prefix, which in this case is the 0x66 byte.
unless of course you happen to be running on a 16-bit 8088/8086, in which case the 0x66 op code aliases to the 0x76 op code, which is JBE (jump if below or equal). since a previous instruction left the zero flag set, it branches to the target.
and that target? it's an error handling routine which just prints an error code out to the diagnostic port.
(i made a mistake in the original tweet, it's a 386 and up that has the prefix. not sure what this code would do on a 286.)
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