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  • Subject: Re: instruction pointers
  • From: "Leif Svalgaard" <leif@xxxxxxxx>
  • Date: Thu, 7 Jun 2001 06:35:32 -0500

 
Instruction pointers are a bit limited in mi.
 
You cannot add to them or setip to a variable label name.
===> this is on purpose. You can only brabch to an existing static label.
 
Branch to instruction pointer
    B pLbl ;
 
Risc code:
   LQ 26,0XF2E0(31),2                    032   
   BCLA 12,21,0X8020                           
   LD 3,0X1B0(28)                              
   TXER 0,0,42                                 
   LD 2,0X0(27)   <=== R2 now address to jump to                             
   LD 0,0X8(27)                                
   MTSPR 9,2  <=== load counter register                                   
   TD 24,3,0                                   
   BCCTR 20,0 <=== jump (the 20 means unconditionmally)
 
BCCTR is "branch conditional to the address contained in the counter register".
 
It looks like the risc instructions do allow the branching to a variable instruction number. no?
 
===> of course. The PPC is just an ordinary computer
 
Is the counter register 64 bits?
 
===> yes
 
Does the powerpc risc have the ability to branch to code in another pgm?
 
===> yes, but the MI architecture does not support it directly.
(also a security risk)
 
 
===> if you patch the program you can arrange to branch to
a parameter. This gives you the ability to execute RISC
instructions that are part of your data (and therefore are
under your control).
 
 

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