I am looking at the following instruction and I can't seem to understand it. I've tried to read a couple of online PowerPC instruction references but they are not very understandable to me.
lwz r4, dword_91C0DB28@l(r28)
Here how the dword shows in IDA
dword_91C0DB28: .long 0
What exactly is going on when lwz is called and why is it useful?
No, the answer above is not correct.
IDA instruction:
lwz r4, dword_91C0DB28@l(r28)
Instruction above access to memory dword_91C0DB28 itself.
This memory address is calculated by IDA's analyse heuristics.
Original instruction:
lwz r4, -12345(r28)
In this moment the real instruction would be like above.
The address0x91C0DB28 you've seen at IDA would be made by $r28 - 12345 during runtime.
(Offset -12345 is just an example).
FYI: You can see original instruction by using dumb disassemblers such as objdump.