"Compatibility mode" is sub-mode of long mode which has limited support for 16/32b i386-style memory segmentation (notably there is no support for vm86 code seg

"Compatibility mode" is sub-mode of long mode which has limited support for 16/32b i386-style memory segmentation (notably there is no support for vm86 code segments and gate descriptors have different format and can only reference 64bit "segments"). It is clear that AMD's intention for this was to implement just enough of segmentation in long mode to allow compatibility with segment-aware user-space code (eg. mixed 16/32b Windows userspace) but there does not seem to be any OS that actually uses 16bit descriptors in long mode (probably due to the mechanism being significantly different than i386's 16/32 compatibility). For compatibility with "normal" flat address space user space code the whole mechanism could be significantly simpler and have same "segmantation" behavior as 64b mode, have 32b EIP pushed on stack by CALL/RET and trap on anything that tries to load new selector anywhere (ie JMP/CALL/RET FAR, MOV whatever, ?S), even REX prefix could be supported in such an "limited compatibility mode" and we would not have to invent things like x32_abi (which is to some extent the same idea, although with different motivations, ie. run new ABI-incompatible 32b code in 64b mode to gain access to new registers and instructions while having 32b pointers and thus reduced memory footprint).