common exception vector:
    -------------------------

    SPRG0 = R4
    OSContext *context = __OSPhysicalContext;
    context->gpr[3] = R3;
    context->gpr[4] = SPRG0;
    context->gpr[5] = R5;
    context->state |= OS_CONTEXT_STATE_EXC;

    context->cr  = CR;
    context->lr  = LR;
    context->ctr = CTR;
    context->xer = XER;

    context->srr0 = SRR0;
    context->srr1 = SRR1;

    SRR1 = MSR | (IR + DR);

    r3 = exception number
    r4 = [800000D4]