How to debug MMU on powerpc?

Gong Zhuo gongzhuo at
Sat Jul 1 13:05:57 EST 2000

>> Gong Zhuo wrote:
> >   I am using MVME2600 and debug my program through serial port. I must
> > MMU for some reasons and the virtual address and physical address is not
> > consistent. That means virtual address 0 maybe physical address 0xc000.
> > have used BAT method. All the mapping is finished in assembly language
and I
> > have make the cross-compile , gdb for powerpc-eabi. But how can I link
> > debug my program?
> From your descriptiojn your program is not running under Linux, but is
> a stand-alone program. If this is not true and you intend to run it
> under Linux, then you must let Linux manage the MMU.
> A stand-alone program which is using the MMU must be carefully designed
> so that the MMU can be initialized and then enabled properly. There are
> two basic ways to do this:
>  A) arrange so the startup code will be mapped virtual=physical, so it
>     can start with MMU disabled, setup the BATs (and segments if any),
>     and then enable the MMU. Since this code is mapped virtual=physical
>     it continues to run after the MMU is enabled.
>  B) arrange so the startup code runs with MMU disabled, setup the BATs
>     (and segments if any) and then:
> mtspr SRR1,<desired MSR value with MMU enabled>
> mtspr SRR0,<virtual address to start at>
> rfi
>     Note that the fact that branches are generally PC-relative makes
>     this a lot easier -- you may not need to do anything at all to
>     make it work correctly (initialize the BATs with load immediates,
>     not data accesses). Note also that if the code starts with the
>     MMU enabled you must use much care in modifying the BATs.
> Our proprietary OS uses mthod A (the entire kernel runs
> virtual=physical, only user programs have non-physical virtual
> addresses); Linux's head.S uses method B.
> >   1. If I link my program with virtual address , the code before the mmu
> > opened will be linked with the virtual address but they should run in
> > physical address.
> Sure, but as branches and calls are PC-relative, it may still work.
> You need to be careful about data accesses, however. As you are using
> BATs, there is a simple subtraction/addition to relate virtual and
> physical addresses, and you can explicitly do that for data accesses.
> Look at Linux's head.S (in directory arch/ppc/kernel) for examples of
> this during startup; the value to be subtracted for virt->phys is
> called KERNELBASE (but in the mm code is called PAGE_OFFSET (:-().
> >   2. The powerpc-eabi-gdb will load my program to the entry-point of
> > address, but the mmu has not been opened at that time.
> See above. It depends in detail how your loading of the program arranges
> to jump to its starting point. Note that most boot ROMs and debug ROMS
> do not use the MMU and will load the program where you tell it and
> simply jump to its first location or a physical address you tell it.
> You must take the behavior of your boot/debug ROM into account when
> designing the startup of your program.
> Hmmm. Your paragraph here sort-of implies that you intend to startup
> Linux, run gdb, and have gdb load and execute your program which
> will then take over the MMU (etc.). This is unlikely to work,
> because the execution environment for gdb and the Linux kernel will
> be completely destroyed. I repeat: if you intend to run this program
> under Linux then you must let Linux manage the MMU.
> Note: the version of PPC Linux I have (2.2.15) uses only
> DBAT2,DBAT3,IBAT2,IBAT3, so the other BATs are available
> for system-specific uses. My custom drivers use DBAT0 and
> DBAT1. Before doing this you must make sure that the
> system-level code for your board does not already use
> these BATs (write a module to dump the BATs, or inspect
> the Linux startup code VERY carefully). This may be what
> you are looking for.

Thank you very much for your help . I have another question. In fact , I am
developing a stand-alone program and the virtual address is not the same as
physical address. The mothed B has been used . I use the cross-compile of
gcc for powerpc-eabi and want to use the psim (powerpc simulator) to debug
my program . If the virtual=phiscal , everything will be ok. But the virtual
!= physical , can the psim still be used? If it can , how to compile my
program and load it into the psim?

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