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ARM - Assembly GNU Debugger Output Help

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  ARM - Assembly GNU Debugger Output Help Ilias Abrams <iliasabrams@gmail.com> - 2018-03-06 08:28 -0800
    Re: ARM - Assembly GNU Debugger Output Help Wouter Verhelst <w@uter.be> - 2018-03-06 19:04 +0100

#8397 — ARM - Assembly GNU Debugger Output Help

Hi guys, I was wondering, I am running a simple Assembly example with GNU Debugger (GDB) on a RPI (ARM architecture). When I execute the command "info registers" I take the following output. 

(gdb) info registers
r0             0x1      1
r1             0x7efff7b4       2130704308
r2             0x7efff7bc       2130704316
r3             0x10408  66568
r4             0x1042c  66604
r5             0x0      0
r6             0x102e0  66272
r7             0x0      0
r8             0x0      0
r9             0x0      0
r10            0x76fff000       1996484608
r11            0x0      0
r12            0x7efff6e0       2130704096
sp             0x7efff660       0x7efff660
lr             0x76e8f678       1994978936
pc             0x10408  0x10408 <main>
cpsr           0x60000010       1610612752

From the output I can see that some register's values are memory addresses. Am I correct? 
If I type then the following command I take this output:

(gdb) x $r3
0x10408 <main>: 0xe3a00000
(gdb) x $r1
0x7efff7b4:     0x7efff8d3
(gdb) x $r2
0x7efff7bc:     0x7efff8e3

This means that the value of the R3 for example points to another memory address location? 

I am confused, the value of a register is the address that shows at the info field or the address that points to?

Any help to understand this? 
Also, Is any general rule that declares which values of which registers are addresses or actual values like 3,4 and so on?

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#8401

On 06-03-18 17:28, Ilias Abrams wrote:
> Hi guys, I was wondering, I am running a simple Assembly example with GNU Debugger (GDB) on a RPI (ARM architecture). When I execute the command "info registers" I take the following output. 
> 
> (gdb) info registers
> r0             0x1      1
> r1             0x7efff7b4       2130704308
> r2             0x7efff7bc       2130704316
> r3             0x10408  66568
> r4             0x1042c  66604
> r5             0x0      0
> r6             0x102e0  66272
> r7             0x0      0
> r8             0x0      0
> r9             0x0      0
> r10            0x76fff000       1996484608
> r11            0x0      0
> r12            0x7efff6e0       2130704096
> sp             0x7efff660       0x7efff660
> lr             0x76e8f678       1994978936
> pc             0x10408  0x10408 <main>
> cpsr           0x60000010       1610612752
> 
> From the output I can see that some register's values are memory addresses. Am I correct? 

Probably some are, yes, but it's impossible to say.

A general-purpose register holds a number. What the meaning of that
number is, depends on the code that's running on the processor. It may
be a memory address, but it may just as well be a numeric value that the
code running on the processor is doing some calculations on.

There are a few special-purpose registers (such as the stack pointer
(sp) and program counter (pc)) which by virtue of their function must
point to a memory address if the system is going to be doing anything
useful; but beyond that, in order to interpret the value of the
registers you need to inspect the code.

Try using the "disassemble" command. By default, that will give a
disassembled view of the function which the program is "currently"
executing, which should give you some hint as to what the values mean.
Beyond that, your guess is as good as mine.

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