[Info-vax] x86S Specification

[Camiel Vanderhoeven]

Arne Vajhøj wrote:
> On 11/3/2024 9:06 AM, Camiel Vanderhoeven wrote:
>> Arne Vajhøj wrote:
>>> x86-64 in long mode only support 2 modes in PTE's, so
>>> VMS x86-64 is a hardware 2 mode OS 4 mode OS - U in ring 3,
>>> S, E and K in ring 0.
>>
>> Not exactly.
>>
>> Ring 3 is used for Exec, Super, and User
>>
>> Ring 0 is used for kernel and for transitions between modes (SWIS)
>>
>> Running Exec and Super in ring 0 would blow away the separation 
>> (which, I might add, is there more for stability than for security, 
>> before I unintentionally re-start that debate)
> 
> You are more afraid that DCL or RMS would step on VMS than
> applications would step on DCL or RMS?

No, certainly not. That is why we have a separate set of page tables for 
each mode. For instance, a page that has kernel write / exec read 
protections is represented by the following PTEs in these 4 sets of page 
tables:

kernel mode: S(upervisor) W(riteable)
exec mode: U(ser) R(eadable)
super mode: not present
user mode: not present

Camiel