On 24-01-08 11:31, sahlot arvind wrote:
I compiled kernel for ARM processor.
I am trying to trace kernel control flow. I am looking at file
"arch/arm/kernel/head.S" . Code starting with
-------------------
__INIT
.type stext, #function
ENTRY(stext)
mov r12, r0
mov r0, #PSR_F_BIT | PSR_I_BIT | MODE_SVC @ make sure svc mode
msr cpsr_c, r0 @ and all irqs disabled
bl __lookup_processor_type
-------------------
Question comes up at last line (bl __lookup_processor_type) of this
code snippet.
If I look into System.map file I find the address of symbol
"__lookup_processor_type" as c0008168 and as per my understanding kernel
image is loaded in the memory starting after first 1 MB.
On x86 that's the normal/classic arrangement at least yes. Don't know about
ARM actually, but that sounds likely.
Here I assume that I am correctly interpreting System.map file. Please
let me know if I am misinterpreting this file. I assume that first column
contains address, second I dont know (please tell) and third symbol.
Right?
Right. It's a generic symbol file, for which "man nm" will provide more detail.
Now since MMU is disabled at this point of time how come we can branch
to "__lookup_processor_type" whose address is after 3 GB??? What I mean
is - at this stage page tables have not been setup then how can we
access a symbol which has been assigned address as c0008168 and lying in
some memory region???
Basically -- you get to ignore the symbol file in this case. The symbol file
is generated _assuming_ the kernel runs at 3G since that's where it's at in
virtual terms after paging has been enabled but it's not being told that for
the bit that runs pre-paging such isn't in fact correct.
If you look at the comment preceding that bit, you'll see it say that the
code is "mostly" (?) position independent and although I don't know ARM
assembly nor have an ARM toolchain installed, I bet that if you look at the
actual generated code, you'll see it generating the branch target as a
relative target -- that is, as a "jump program_counter +/- offset", where
"offset" is the only thing that is encoded in the instruction stream. In
that case, it's only the _difference_ "target - program_counter" that is
relevant and not any absolute values.
Rene.
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