I was interested in operating systems that used segmentation from 32 bit protected mode processors. When AMD added long mode, they didn't the segmentation hardware protection to it. Can any x86-64 processor in 32 bit protected mode use Physical Address extension so it can address more than 4 GB of memory?
Can any x86-64 processor in 32 bit protected mode use Physical Address extension so it can address more than 4 GB of memory?
Yes.
Such an OS would be able to use more than the 64 GiB of memory (physical address space) that PAE was originally limited to; and could also still use segmentation and virtual8086 mode.
Unfortunately, 32-bit code can't use the extra registers that were added, which (depending on software) probably means up to 20% performance loss in most software (compared to 64-bit software on the same CPU).