On Thu, 5 Oct 2006, Kota, Ramgopal wrote:
> I am new to MIPS and uclinux port.I am getting familiarised with 2.6.18
> linux code.
> I see the following code in asm/mips/kernel/genex.S &
> asm/mips/kernel/traps.c
>
> ++++++++++++ Genex.S +++++++++++++
> NESTED(except_vec3_generic, 0, sp)
> .set push
> .set noat
> #if R5432_CP0_INTERRUPT_WAR
> mfc0 k0, CP0_INDEX
> #endif
> mfc0 k1, CP0_CAUSE
> andi k1, k1, 0x7c
> #ifdef CONFIG_64BIT
> dsll k1, k1, 1
> #endif
> PTR_L k0, exception_handlers(k1)
> jr k0
> .set pop
> END(except_vec3_generic)
>
> +++++++++++++ traps.c +++++++++++++++
> set_except_vector(0, handle_int);
> set_except_vector(1, handle_tlbm);
> set_except_vector(2, handle_tlbl);
> set_except_vector(3, handle_tlbs);
>
> set_except_vector(4, handle_adel);
> set_except_vector(5, handle_ades);
>
> set_except_vector(6, handle_ibe);
> set_except_vector(7, handle_dbe);
>
> set_except_vector(8, handle_sys);
> set_except_vector(9, handle_bp);
> set_except_vector(10, handle_ri);
> set_except_vector(11, handle_cpu);
> set_except_vector(12, handle_ov);
> set_except_vector(13, handle_tr);
>
> In R3000 manual , bits 2-6 indicate exception code value. In genex.S ,
> the cause register is anded with 0x7c to extract 2-6 bits.
> I am not able to understand why it is not shifted the last 2 bits.
>
> Please try to educate me why this is not done.
The exception code value is used to obtain a pointer from the
exception_handlers array. Since pointers are 32 bit, you have to convert the
index into the array to an offset for the array by multiplying by 4. This
nullifies the shift.
On 64-bit platforms, pointers are 64 bit and the index is converted to an
offset by multiplying by 8. Since the value is already shifted 2 positions to
the left, only 1 shift remains to be done (cfr. the dsll).
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
