D0 Scratch.

 D1 Scratch.

 A0 Scratch except in certain cases (see note above).

 A1 Points to the data area specified by the is_Data field of the
    Interrupt structure.  Because this pointer is always fetched
    (regardless of whether you use it), it is to your advantage to make
    some use of it (scratch).

 A5 Points to your interrupt code (scratch).

 A6 Scratch.

In a server chain, the interrupt is cleared automatically by the system.
Having a server clear its interrupt is not recommended and not necessary
(clearing could cause the loss of an interrupt on PORTS or EXTER).

Here is an example of a program to install and remove a low-priority
vertical blank interrupt server:

     vertb.c 

This is the assembler VertBServer installed by the C example:

     vertbserver.asm