Using address and operand overrides in real address mode

jj1984

Member
In modern Intel systems, when in real address mode (the 16-bit mode that the old 8086 ran in) what happens when you use the 66h operand override and/or the 67h address override?

I know that in real mode there are no descriptor tables and no concept of protection, and, so there are no default operand/address bits in segment descriptors to be overridden.

So, in real mode do the override bytes do nothing, change the sizes to 32 bits, change the sizes to 8 bits, or throw an exception?

What happens?
 

Cromewell

Administrator
Staff member
Turns out, does exactly what you'd expect. Switches you to 32bit or 16bit as the case may be.
 
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