[OpenBIOS] static and global variable problem

[Mark Cave-Ayland]

On 21/12/12 14:45, Programmingkid wrote:

> How to reproduce the problem:
> 1. Start QEMU for debugging: qemu-system-ppc -m 128 -bios openbios-qemu.elf.nostrip -cdrom darwin8.iso -s -S
>
> 2. Start up gdb to read the firmware file: powerpc-linux-gdb openbios-qemu.elf.nostrip
>
> 3. Connect to QEMU using this command inside gdb: target remote localhost:1234
>
> 4. Set a breakpoint at this place: b ofmem.c:414
>
> 5. Have OpenBIOS start the boot process by entering this in QEMU: boot cd:,\\:tbxi
>
> 6. When the breakpoint in tripped, type this in gdb: print&next_grab_slot.
>
> next_grab_slot will have an address that isn't available to QEMU. The value I see is 0xfffda060. QEMU would have to have over 4 GB of ram in order to access this address.

In short, this is actually fine. You have to remember that OpenBIOS uses 
the MMU (emulated by QEMU) and hence physical pages can be mapped to any 
virtual address within the 32-bit address space of the PPC CPU.

Once the MMU is enabled in the OpenBIOS startup code, all addresses 
(including those used by gdb) are virtual addresses. If you want to view 
the current virtual to physical mappings, use the "info tlb" command 
from the QEMU monitor.

Note that if you try and access an unmapped address then you will 
normally obtain some kind of "page fault" or "TLB miss" processor 
exception. So if you can read and write from the variable, then 
everything is working correctly.


ATB,

Mark.