Re: jumping to code in RAM in a MPC55xx

I am just going to answer my own questions for future reference.

I did some searching on the internet and came across this note on the
the usenet comp.sys.powerpc.tech

http://newsgroups.derkeiler.com/Archive/Comp/comp.sys.powerpc.tech/2005-11/msg00015.html

In summary, it performs a indirect jump to code via the CTR register.
The following code snippet show how it is done.

	lis	9,0xFC00 /* Function is at 0xFC000000 */
	mtctr	9
	bctrl


On Fri, Jul 18, 2008 at 4:29 PM, Tehn Yit Chin [off-list ref] wrote:

Hi,

This question is not directly related to linux, but a question on how to execute code from.

My test code is as follows..

void ram(unsigned int cat) __attribute__ ((section(".ram_code")));
void ram1(void) __attribute__ ((section(".ram_code")));
void flash(void) __attribute__ ((section(".text")));
void flash1(unsigned int y) __attribute__ ((section(".text")));

void ram(unsigned int cat)
{
    unsigned int abc;

    abc += 12;
    abc += cat;
}

void ram1(void)
{
    ram(123);
}


void flash1(unsigned int y)
{
    unsigned int x;

    x = x + y;

}

void flash(void)
{
    unsigned int def;

    def += 12;

    asm (" bel ram1\n\t");
}

I have the section .ram_code mapped into the internal SRAM of the MPC55xx, which starts at 0x40000000. Code in .text are mapped into MPC55xx's internal FLASH, starting at 0x00000000.

When I attempt to link the code together,  I get the error

U:\src\applications\comms/fatdog.c(35,1): relocation truncated to fit: R_PPC_REL24 against symbol `ram1' defined in .ram_code section in U:\src\applications\comms\fatdog.o

I googled the error and I think it means that the ram1() is too far away and the branch address has been truncated to fit into the binary.

Other than assembler bel instruction, I have also tried bl, b and bea and all of them gave the same link error.

I was wondering if anyone can give some insight on how I can get the MPC55xx to branch between FLASH and SRAM?

Many thanks,
Tehn Yit Chin