Interrupts

Two routines GetIntVec and SetIntVec are dedicated to managed the standard vectors for interrupts. You can use them to setup you own interrupt handlers and to restore the previous interrupts handlers.

The presence of the OS ROM during interrupts handling is required for proper operation of the following routines and code examples. So if you want disable the OS ROM, you have to use $DEFINE ROMOFF, to ensure properly setup OS interrupts vectors and handlers.

If we have disabled the OS ROM using {$define romoff} and are calling routines placed in the memory area $C000..$FFFF we must take care to set PORTB properly.

procedure NewInterrupt; interrupt; assembler;
asm

    dec portb                  // Disable the OS ROM
    jsr user_proc_c000_ffff    // Call routine in RAM under the OS ROM
    inc portb                  // Enable the OS ROM

    ...                        // Interrupt-specific return logic, see below
end;

NMIs - VBL, DLI

GetIntVec

GetIntVec(iVBLI, pointer);  // Get the address of the immediate vertical blank interrupt handler (VVBLKI, $0222)
GetIntVec(iVBLD, pointer);  // Get the address of the deferred vertical blank interrupt handler (VVBLKD, $0224)
GetIntVec(iDLI, pointer);   // Get the address of the display list interrupt handler (VDSLST, $0200)

var oldVBL: pointer;

begin
    GetIntVec(iVBLD, oldVBL);   // Remeber old vector to restore it later
end.

SetIntVec

SetIntVec(iVBLI, pointer);  // Set the address of the immediate vertical blank interrupt handler (VVBLKI, $0222)
SetIntVec(iVBLD, pointer);  // Set the address of the deferred vertical blank interrupt handler (VVBLKD, $0224)
SetIntVec(iDLI, pointer);   // Set the address of the display list interrupt handler (VDSLST, $0200)

Immediate Vertical Blank Interrupt

The VBLI (vertical blank immediate) interrupt handler is terminated by jumping to the SYSVBV address ($E45F). It will perform the standard tasks of the immediate vertical blank interrupt and restore the values of the A, X, Y CPU6502 registers from the stack before it returns.

procedure NewVBLI; interrupt; assembler;
asm
    jmp sysvbv
end;


begin
    SetIntVec(iVBLI, @NewVBLI);
end.

Deferred Vertical Blank Interrupt

The VBLD (vertical blank deferred) interrupt handler is terminated by jumping to the XITVBV address ($E462) which will restore the value of the A, X, Y CPU6502 registers from the stack before it returns.

procedure newVBLD; interrupt; assembler;
asm
    // Your code
    jmp xitvbv
end;


begin
    SetIntVec(iVBLD, @newVBLD);
end.

Display List Interrupt

As opposed to the vertical blank interrupt handler, which save the values of the A, X, Y CPU6502 registers before the interupt vector is called, the display list interrupt handler only saves the A CPU6502 register to the stack. Therefore the iDLI (display list) interrupt handle is termianted with single PLA instruction.

procedure NewDLI; interrupt; assembler;
asm
    // Your code
    pla
end; // Will become RTI


begin
    SetIntVec(iDLI, @NewDLI);
end.

IRQs - TIMER1, TIMER2, TIMER4

GetIntVec

GetIntVec(iTIM1, pointer);  // Get the address of the TIMER 1 interrupt handler (VTIMR1, $0210)
GetIntVec(iTIM2, pointer);  // Get the address of the TIMER 2 interrupt handler (VTIMR2, $0212)
GetIntVec(iTIM4, pointer);  // Get the address of the TIMER 4 interrupt handler (VTIMR4, $0214)

var oldIRQ: pointer;

begin
    GetIntVec(iTIM4, oldIRQ);
end.

SetIntVec

SetIntVec(iTIM1, pointer);  // Set the address of the TIMER 1 interrupt handler (VTIMR1, $0210)
SetIntVec(iTIM2, pointer);  // Set the address of the TIMER 2 interrupt handler (VTIMR2, $0212)
SetIntVec(iTIM4, pointer);  // Set the address of the TIMER 4 interrupt handler (VTIMR4, $0214)

procedure TimerIRQ; assembler; interrupt;
asm
    // Your code
    pla
end; // Becomes RTI

begin
    SetIntVec(iTIM4, @TimerIRQ, 0, 28);
    repeat until keypressed;

    SetIntVec(iTIM4, oldIRQ);
end.

When the system executes a jump to an IRQ interrupt handler, it puts the contents of the accumulator on the stack beforehand. Keep this in mind and end the interrupt handler with a single PLA instruction.

Additional parameters are required to trigger a new IRQ interrupt, such as the choice of base clock clock_base = [0,1] and frequency rate = [6.255]. Values of rate less than 6 will cause the system to slow down severely, up to a possible suspension.

SetIntVec(iTIM1, pointer, clock_base, rate);
SetIntVec(iTIM2, pointer, clock_base, rate);
SetIntVec(iTIM4, pointer, clock_base, rate);