Re: instruction pointers

["Leif Svalgaard" <leif@xxxxxxxx>]

From: Steve Richter

 

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).