I am currently learning ARM assembly by trying to reverse a broken printer. I retrieved the firmware from the on board NAND flash and I am now trying to make some sense of it.
However, I get stuck at the very beginning: the interrupt table. From documentation I read on the web, the interrupt table is a table that contains instructions (most often LDR PC instructions) that load a direct memory address into the PC. The interrupt table is a sequence of these instructions all directly next to each other (this might not be the most technical correct explanation, but that is how I understand it, please correct me if I am wrong). In the underneath table these instructions would be:
e5 9f f0 18 which I believe is a
ldr pc, [pc, #24] instruction.
In the firmware I retrieved, after every two vectors, there is a byte with value
89. I do not know why it is there and what it does. I couldn't find it anything on the web that shows something similar (or I am looking for the wrong thing). What would be the function of this byte?
From how I understand it, the address 0x8e0 will be loaded into PC and the CPU will run from there, however, this means ignoring the four
89 bytes. If the bytes should be taking into account, than the CPU would jump to
9f f0 18 89, but this is outside of the binary.
I believe the CPU is big endian and probably an Armv7, but it is an unknown Marvel chip, so I am not 100% sure.
This is the hexdump of the interrupt table:
00000000 e5 9f f0 18 e5 9f f0 18 89 e5 9f f0 18 e5 9f f0 |................| 00000010 18 89 e5 9f f0 18 e5 9f f0 18 89 e5 9f f0 18 e5 |................| 00000020 9f f0 18 89 00 00 08 e0 00 00 00 04 ac 00 00 00 |................| 00000030 08 00 00 00 0c db 00 00 00 10 00 00 00 14 db 00 |................|
Why are the four
89 bytes there and is my assumption that the CPU will jump to