RTEMS 4.6.1 On-Line Library

Interrupt Processing CPU Models Without VBR and RAM at 0

RTEMS Motorola MC68xxx Applications Supplement

4.3: CPU Models Without VBR and RAM at 0

This is from a post by Zoltan Kocsi <zoltan@bendor.com.au> and is a nice trick in certain situations. In his words:

I think somebody on this list asked about the interupt vector handling w/o VBR and RAM at 0. The usual trick is to initialise the vector table (except the first 2 two entries, of course) to point to the same location BUT you also add the vector number times 0x1000000 to them. That is, bits 31-24 contain the vector number and 23-0 the address of the common handler. Since the PC is 32 bit wide but the actual address bus is only 24, the top byte will be in the PC but will be ignored when jumping onto your routine.

Then your common interrupt routine gets this info by loading the PC into some register and based on that info, you can jump to a vector in a vector table pointed by a virtual VBR:

//
//  Real vector table at 0
//

    .long   initial_sp
    .long   initial_pc
    .long   myhandler+0x02000000
    .long   myhandler+0x03000000
    .long   myhandler+0x04000000
    ...
    .long   myhandler+0xff000000


//
// This handler will jump to the interrupt routine   of which
// the address is stored at VBR[ vector_no ]
// The registers and stackframe will be intact, the interrupt
// routine will see exactly what it would see if it was called
// directly from the HW vector table at 0.
//

    .comm    VBR,4,2        // This defines the 'virtual' VBR
                            // From C: extern void *VBR;

myhandler:                  // At entry, PC contains the full vector
    move.l  %d0,-(%sp)      // Save d0
    move.l  %a0,-(%sp)      // Save a0
    lea     0(%pc),%a0      // Get the value of the PC
    move.l  %a0,%d0         // Copy it to a data reg, d0 is VV??????
    swap    %d0             // Now d0 is ????VV??
    and.w   #0xff00,%d0     // Now d0 is ????VV00 (1)
    lsr.w   #6,%d0          // Now d0.w contains the VBR table offset
    move.l  VBR,%a0         // Get the address from VBR to a0
    move.l  (%a0,%d0.w),%a0 // Fetch the vector
    move.l  4(%sp),%d0      // Restore d0
    move.l  %a0,4(%sp)      // Place target address to the stack
    move.l  (%sp)+,%a0      // Restore a0, target address is on TOS
    ret                     // This will jump to the handler and
                            // restore the stack

(1) If 'myhandler' is guaranteed to be in the first 64K, e.g. just
    after the vector table then that insn is not needed.

There are probably shorter ways to do this, but it I believe is enough to illustrate the trick. Optimisation is left as an exercise to the reader :-)

RTEMS Motorola MC68xxx Applications Supplement