Legal Interrupts ~~~~~~~~~~~~~~~~ A feeble attempt by M.Meany A slightly misleading heading. I am only going to cover adding a server to the vertical blank interrupt chain, not discuss all 15 types of interrupt that can occur. So many programs set up vertical blank interrupts by writing a value straight into the 68000's level 3 interrupt vector that you could be excused for thinking this is the correct method. Well strictly speaking, it is! However, the Amiga is a multitasking machine and several tasks may want to set up interrupts that go bang during the blanking period. For this reason, exec maintains a list of all the interrupt servers to call when an interrupt takes place. This way many interrupt routines get called. For the uninitiated, an interrupt is a signal sent to the processor telling it to suspend current operation and deal with a specific request immediately. For instance, the Blitter can generate an interrupt. The Amiga can be happily multitasking along when the Blitter finishes an operation and send an interrupt to the processor. The processor will switch to Supervisor mode and jump to a predetermined routine that is waiting for just such an event to occur. This routine may just start another Blitter operation in progress and exit, allowing the processor to continue where it left off. Though I've used the Blitter in the above example, there is a more frequently used interrupt. This is the vertical blank interrupt and occurs after each generation of the video display, approx. once every fifteth of a second. This interrupt is used to maintain flicker free displays in games and demos, as well as acting as a general purpose timer for such things as 'Tracker replayers'. So how do you go about adding an interrupt to the above list? This is really quite straight forward. First initialise an Interrupt structure, which is just an extended node structure, then call AddIntServer() passing the address of the Interrupt structure. When you want the interrupt to cease call RemoveIntServer() passing the Interrupt structure. AddIntServer( intNumber, Interrupt ) d0 a1 RemIntServer( intNumber, Interrupt ) d0 a1 In both the above cases, the intNumber should be set to INTB_VERTB ( defined in hardware/intbits.i ) for a vertical blank interrupt server. Preparing The Interrupt Structure ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the Interrupt structure: is_Node Struct Node is_Data LONG is_Code LONG is_Node This is a standard node structure. Note that the nodes priority is used in deciding the order that interrupts are called. The higher the priority, the sooner the interrupt server will be called. You can also give the interrupt a name if you so wish. In this way, multiple copies of the task that started the interrupt will know that the interrupt already exsists. is_Data A pointer that is passed to the interrupt server in address register a1. This allows you to specify a data area that your server can access using a1 ithout having to lea the address. is_Code A pointer to the server routine itself. For example, to set up an Interrupt structure and start the interrupt running you could use the following code: lea MyInt,a1 a1->Interrupt struct move.b #5,LN_PRI(a1) priority = 5 move.l #IntData,is_Data(a1) address of data move.l #IntCode,is_Code(a1) address of server moveq.l #INTB_VERTB,d0 interrupt number CALLEXEC AddIntServer start it! Your server will be installed and called every vertical blank from this point onwards. Interrupt Server Conventions ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Interrupt servers must follow certain conventions, vertical blank servers especially so. Here is an outline of what you should and should not do with a few tips thrown in: On Starting =========== Register a1 will point to the address stored in the is_Data field of your Interrupt structure. Use this! It saves time if you reference any variables from this address. Scratch Registers ================= The following registers can be treated as 'scratch' registers: d0,d1,*a0,a1,a5,a6 a0 can be treated as scratch providing the priority of your server is less than 10. There is a bug in the graphics interrupt server that always expects the address $dff000 to be in a0. The graphics server has priority 10, so if you add a server of higher priority remember to leave this value in a0 when you finish. On Finishing ============ Always leave the processors Z flag set on exiting your server. Failure to do this may result in other important servers not being called. The flag can be set with the following instruction: moveq.l #0,d0 End your server with an RTS, not an RTE. The exec controller is the true handler running in supervisor mode, your server is treated as a subroutine of this handler! An Example ~~~~~~~~~~ By way of an example, how about a NoiseTracker replayer running off a vertical blank interrupt. Just to make sure that the graphics gets priority over the replayer, it will be given a priority of 9. The program will take the following form: 1/ Call mt_init. 2/ Set up interrupt and add it to system. 3/ Wait for LMB to be pressed 4/ Remove interrupt 5/ Exit Here is the code, minus the replayer which you will find in the source file. ********************************* incdir include2.0:include/ include exec/exec_lib.i include hardware/intbits.i ; initialise music data Start jsr _init Initialise data lea MyInt,a1 a1->Interrupt struct move.b #9,LN_PRI(a1) priority = 9 move.l #IntCode,IS_CODE(a1) address of server moveq.l #INTB_VERTB,d0 interrupt number CALLEXEC AddIntServer start it! wait btst #6,$bfe001 LMB pressed? bne.s wait loop if not moveq.l #INTB_VERTB,d0 interrupt number lea MyInt,a1 Interrupt struct CALLEXEC RemIntServer remove interrupt jsr _end stop music rts exit **************************************************************************** * The Interrupt Server * **************************************************************************** IntCode movem.l a0-a4/d2-d7,-(a7) Save all registers jsr _music play routine movem.l (a7)+,a0-a4/d2-d7 Bring back registers moveq.l #0,d0 clear Z flag rts exit **************************************************************************** * Data Section * **************************************************************************** MyInt ds.l IS_SIZE Interrupt structure ************************* To Dave Edwards utter disgust I have used a 'busy wait' loop in the above example. There is a reason for this, it's quick and easy requiring no set up. There is no reason why this method of adding an interrupt server to the vertical blank list is not used in demos. Most demos that replace the interrupt vector at $6c directly usualy call the routine pointed to by this vector any how, so there is no real gain in 'doing the dirty'. To ensure your interrupt is called right at the start of the list, give it a priority of 100 or so. As an extra bonus, you can add more than one server. Each server would then check a flag set by the program proper to see if it is required. If not it would exit immediately allowing other servers in the chain to get in on the act. Infact servers could be added and removed during the progress of a program as required, something you don't see to often in 'doing the dirty' programs. Food for thought.... MM.