Setting gp on pic code

Setting gp on pic code

[Alex Gonzalez]

Hi,

I am trying to set the gp register on pic code as follows:

"la gp,_gp"

Disassembling the resulting code,

"lw gp,0(gp)"

I am confused as to why it uses a gp-relative address instead of an
absolute address for _gp. How I am supposed to reset gp then?

A bit of background. I am working on a dual core chip, with each core
running independently. They both boot from the same startup code, then
the second core waits until the first one sets all the hardware and
loads a pic binary into a shared memory region.

The second core is then released and should reset gp and jump to the
new binary (which is compiled separately from the startup code).

This arrangement seems to work OK if the binary is compiled non-pic,
but for pic code  the gp register is not set to the correct value.

Any insight into this appreciated.

Thanks,
Alex

Re: Setting gp on pic code

[Brett Foster]

Alex Gonzalez wrote:

>Hi,
>
>I am trying to set the gp register on pic code as follows:
>
>"la gp,_gp"
>
>Disassembling the resulting code,
>
>"lw gp,0(gp)"
>  
>
Yes, this is normal... Use the following to set up the GP:
        //init GP
        lui     gp,%HI(_gp)
        ori     gp,%LO(_gp)

Re: Setting gp on pic code

[Alex Gonzalez]

Thanks, that works.

Just for completeness, it needed to be addiu instead of ori as follows:

    lui     $28,%HI(_gp)
    addiu   $28,%LO(_gp)

because %LO is a signed 16 bit number.

Alex

On 1/16/06, Brett Foster <fosterb@uoguelph.ca> wrote:
> Alex Gonzalez wrote:
>
> >Hi,
> >
> >I am trying to set the gp register on pic code as follows:
> >
> >"la gp,_gp"
> >
> >Disassembling the resulting code,
> >
> >"lw gp,0(gp)"
> >
> >
> Yes, this is normal... Use the following to set up the GP:
>         //init GP
>         lui     gp,%HI(_gp)
>         ori     gp,%LO(_gp)
>