Quote:
Originally Posted by Tanaris4  where is the extra data stored then that is outside of the 32-bits? If you look @ the 2nd line with dword_11D76CC, it only shows the 011D to the left of it. Where is the 76CC stored? |
You really should read that link I gave you, it contains some basic stuff on how ppc assembly works.
Quote:
|
Originally Posted by IBM Code: --------------------------------------------------------------------------
| opcode | src register | dest register | immediate value |
| 6 bits | 5 bits | 5 bits | 16 bits |
--------------------------------------------------------------------------
The number of fields and their sizes will vary by instruction, but the important point here is that these fields take up space in the instruction. In the case of addi, after just those three fields are placed into the instruction, there are only 16 bits left for the immediate value you're adding!
That means that li can only load 16-bit immediates. You cannot load a 32-bit pointer into a GPR with just one instruction. You must use two instructions, loading first the top 16 bits and then the bottom. That is exactly the purpose of the @ha ("high") and @l ("low") suffixes. (The "a" part of @ha takes care of sign extension.) Conveniently, lis (meaning "load immediate shifted") will load directly into the high 16 bits of the GPR. Then all that's left to do is add in the lower bits.
This trick must be used whenever you load an absolute address (or any 32-bit immediate value). The most common use is in referencing globals. |
Look at the following addi instruction for the rest of the 32-bit address.
The reason you're getting
Code:
lis %r29, dword_11D76CC@h
addi %r29, %r29, dword_11D76CC@l
(guess what the @h and @l means
)
and not the actual instructions used
Code:
addis r29,0,0x11D
addi r29,r29,0x76CC
is because IDA is smart enough to know what that instruction combo does and lists it in a more human-readable format. Although there's probably a setting somewhere to show real instructions if you prefer it, I haven't looked.
Now as to the reason those 2 "instructions" (lis isn't a real intruction, it's a mnemonic) aren't immediately following each other is probably because of compiler optimization.
