This is the mail archive of the mailing list for the crossgcc project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]

Re: 68000/RTEMS question..

>(ie NO_VBR). Does anyone
>actually have RTEMS running on a 68000 based processor?

Trying hard. Progress slow. :-)

>        1: I cannot explain why the vector number is multiplied by 6
>           and then 10 subtracted to get the address to store the ISR
>           service routine.

When dealing with 68000 boards with no vector base register
there is a problem with setting vectors because it is common
to put ROM at 0x0000 address where vectors reside thus making
it impossible to change them.

Most monitor programs in ROM and similar use the following
technique :

They put vectors in ROM, the vectors points to an address in RAM
that contains JMP or any other jump instruction.

Something like :

/* this is at address 0x0000 and should be placed in ROM */
dc.l _v_RESET_ssp
dc.l _v_RESET_start
dc.l _v_vector_2
dc.l _v_vector_3

/* this "vector table" is somewhere in RAM */
_v_vector_2 : JMP _v_user_vector_2
_v_vector_3 : JMP _v_user_vector_3

As you can see you have an alternative table ob jumps somewhere
in RAM. That might explain the 6 byte vector - it might contain instruction
JMP opcode and address. Your setvec fn should take that into

Hope I haven't misguided you.