$0318-$0319/792-793:   Vector: Hardware NMI Interrupt Address

This vector points to the address of the routine that will be executed when a Non-Maskable Interrupt (NMI) occurs).

There are two possible sources for an NMI interrupt. The first is the RESTORE key, which is connected directly to the 6510 NMI line. The second is CIA #2, the interrupt line of which is connected to the 6510 NMI line.

When an NMI interrupt occurs, a ROM routine sets the Interrupt disable flag, and then jumps through this RAM vector. The default vector points to an interrupt routine which checks to see what the cause of the NMI was.

If the cause was CIA #2, the routine checks to see if one of the RS-232 routines should be called. If the source was the RESTORE key, it checks for a cartridge, and if present, the cartridge is entered at the warm start entry point. If there is no cartridge, the STOP key is tested. If the STOP key was pressed at the same time as the RESTORE key, several of the Kernal initialization routines such as RESTOR, IOINIT and part of CINT are executed, and BASIC is entered through its warm start vector at 40962. If the STOP key was not pressed simultaneously with the RESTORE, the interrupt will end without letting the user know that anything happened at all when the RESTORE key was pressed.

Since this vector controls the outcome of pressing the RESTORE key, it can be used to disable the STOP/RESTORE sequence. A simple way to do this is to change this vector to point to the RTI instruction. A simple POKE 792,193 will accomplish this. To set the vector back, POKE 792,71. Note that this will cut out all NMIs, including those required for RS-232 I/O.

   Points to $FE47

Kernal-Reference:

 JMP ($0318) : $FE44

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