From: nowhere@nevernet.at   
      
   On 26/03/2022 07:52, James Harris wrote:   
   > On 25/03/2022 19:46, wolfgang kern wrote:   
   >> On 25/03/2022 10:50, James Harris wrote:   
   >>> On 25/03/2022 07:33, wolfgang kern wrote:   
   >>>> On 23/03/2022 18:50, James Harris wrote:   
   >>>> [agreed..]   
   >>>>> For Intel and probably AMD it looks as though one can get the CPU   
   >>>>> to report the Limit with an LSL instruction and the Attributes with   
   >>>>> LAR but I don't know a way to get the base.   
   >>>>   
   >>>> get_base:        ;selector in eax assume DS=flat   
   >>>>    MOV esi,anybuffer   
   >>>>    SGDT [ds:esi]   ;now you know where the GDT resides   
   >>>>    AND eax,FFF8    ;just in case   
   >>>> ; SHL eax,3       ;mul by 8   
   >>>> ; ADD esi,eax   
   >>>>    LEA esi,[eax*8+esi]   ;same as the two lines above but   
   >>>> faster/shorter   
   >>>>    MOV ecx,[esi+2]       ;low 24 bits of base   
   >>>>    AND ecx,00FFFFFF   
   >>>>    MOV bl,[esi+7]   
   >>>>    SHL ebx,24            ;decimal yet!   
   >>>>    OR ecx,ebx            ;ecx hold 32 bit base of selector   
   eax yet   
   >>>>   
   >>>> it works on both code and data descriptors.   
      
   >>> What if the segreg (and, hence, its Base) had been loaded before   
   >>> switching to Pmode?   
      
   >> As long a GDT is already installed this works with 66 and 67 overrides   
   >> also in RM.   
      
   > That's not what I mean. Rather, let's say selector and base had been   
   > loaded in Real Mode, then you switched to Protected Mode, your SGDT   
   > wouldn't tell you anything useful for such a case because the base   
   > wouldn't have been loaded from the GDT.   
      
   true after power up or hard RESET. If the BIOS may have played with it   
   then there must be valid GDT-entries at least for SS and DS.   
      
   But you can't switch to PM without valid entries in the GDT :)   
   I wont recommend to try this test between set PE and load CS.   
      
   > In short, LSL will report the limit but nothing (AFAIK) will report the   
   > base. It's not a problem, BTW, just an observation that there's no   
   > equivalent of LSL for the base.   
      
   But right, while in RM you can't be sure if a GDT exists or not.   
   Only after RESET you can rely on the default setting.   
      
   Finally I never needed that test because I decided what's where and how   
   all by myself :)   
   __   
   wolfgang   
      
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