[Vm-dev] Re: localIP, instructionPointer, currentBytecode data size

[David T. Lewis]

On Tue, Mar 02, 2010 at 12:41:34AM -0800, Ang BeePeng wrote:
> 
> I saw the following in win32 VM interp.c, 
> 
>      # define pointerForOop(oop)		((char *)(sqMemoryBase + ((usqInt)(oop))))
>    
>      ...
>    
>      char* localSP;
>      char* localIP;
>      sqInt currentBytecode;
> 
>      browserPluginInitialiseIfNeeded();
>      /* begin internalizeIPandSP */
>      localIP = pointerForOop(instructionPointer);   
> //foo->instructionPointer
>      localSP = pointerForOop(stackPointer);
> 
> 
> Is it true that char* in bold explain difference in both version? 
> localIP = pointerForOop(instructionPointer); instead of 
> localIP = pointerForOop(foo->instructionPointer);

Yes. You can think of an oop as a 32 bit unsigned-int (for 32-bit
object memories, the usual case). Its value represents an offset
into the object memory, and it is defined as an usqInt so that an
oop value can be stored into a "slot" in the object memory.

The actual addressing of object memory is done by bytes, hence the
cast to (char *) in converting to memory addresses. This allows
addressing individual bytes (of course) and also supports the
trick used for small integers; if an "oop" has a 1 in the low
order bit, then it cannot be pointing to a valid 32-bit location
in the object memory, so it is not really an oop. These "oop values"
are used to directly represent small integers.

Dave