From: muta...@gmail.com   
      
   On Sunday, March 19, 2023 at 9:49:35 AM UTC+8, muta...@gmail.com wrote:   
   > On Sunday, March 19, 2023 at 9:20:58 AM UTC+8, Dan Cross wrote:    
   >    
   > > >> Yes. That was one of the important design criteria for 64-bit    
   > > >> x86, in fact. However, you're generally going to do so by    
   > > >> entering 32-bit mode.    
   > > >    
   > > >I didn't want to enter 32-bit mode.    
   >    
   > > Huh.   
   > I could switch between 64 bit mode and 32 bit mode if    
   > boot services allowed that.    
   >    
   > But probably best to not rely on that.    
   >    
   > Basically the intention is to run PDOS-generic as 32-bit code.    
   >    
   > It can be entered by a pseudo BIOS layered on top a real    
   > BIOS, so that it can run on a real 80386.    
   >    
   > Or EFI can boot a bootx64.efi and it can run PDOS-generic.    
   >    
   > PDOS-generic and all applications would all be a.out 80386    
   > code.    
   >    
   > And that would suffice as a starter system that should allow    
   > further development of both 16-bit and 64-bit systems.    
   >    
   > There are two things still on my mind:    
   >    
   > 1. Is there anything that would prevent 80386 code from    
   > running in x64 mode? ie my original question hasn't been    
   > answered.    
   >    
   > 2. Can you bootstrap a 64-bit compiler from a 32-bit    
   > compiler? I thought Alex said that wasn't possible, but    
   > on pdos.org there is cc32.c and cc32n.c which I believe    
   > allow you to get to cc64.c.    
   >    
   > BFN. Paul.   
      
   Someone has replied offline and said that call and ret won't   
   do 32-bit anymore, but it is probably possible to do that   
   manually.   
      
   I assume the syntax would be:   
      
   mov eax, after   
   mov [esp + 0], eax   
   mov eax, sub   
   jmp *eax   
   after:   
      
   sub:   
   ...   
   mov eax, [esp + 0]   
   jmp *eax   
      
      
   And some instructions no longer exist, e.g. short form INC 0x40   
      
   But again, it sounds like this is a minority and there are no show-stoppers.   
      
      
   Another thing is that there is apparently no EFI call to get   
   into 32-bit mode, so the above technique is the only   
   solution I have.   
      
   BFN. Paul.   
      
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