vbr

Short:        A util to change the VBR
Author:       blacky@caiw.nl
Uploader:     blacky caiw nl
Type:         util/misc
Requires:     68010+ CPU
Architecture: m68k-amigaos


VBR displacer for any amiga with a 68010+ CPU.


The VBR (Vector Base Register) of the 68010+ CPU'S is used by the CPU
to find out where the interrupt-vectors are placed.

In the 'old' 68000 the interrupt-vectors were placed at a fixed position,
at the start of CHIP-mem. This memory is not always directly available to the
CPU (due to DMA activity from the CUSTOM-CHIPS). By using the VBR, these
interrupt-vectors can be placed in FAST-mem or 32BIT-mem.


There are just two problems with this scheme :

1 - Very many old programs written for the 68000 expect the interrupt vectors
    to be at the beginning of CHIP-mem. therefor these programs don't work
    if the VBR is pointing to a different memory-position.

2-  The Operating System doesn't alter the VBR (for compatibilty-reasons),
    but this means that the computer is not running as fast as it can.



With this program you can : 

turn the VBR on - setting it to a point in FAST-mem or 32BIT-mem.

turn the VBR off - setting it to the default position for a 68000.






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