HOW TO USE THE KERNAL

When writing machine language programs it is often convenient to use the routines which are already part of the operating system for input/ output, access to the system clock, memory management, and other similar operations. It is an unnecessary duplication of effort to write these routines over and over again, so easy access to the operating system helps speed machine language programming.

As mentioned before, the KERNAL is a jump table. This is just a collection of JMP instructions to many operating system routines.

To use a KERNAL routine you must first make all of the preparations that the routine demands. If one routine says that you must call another KERNAL routine first, then that routine must be called. If the routine expects you to put a number in the accumulator, then that number must be there. Otherwise your routines have little chance of working the way you expect them to work.

After all preparations are made, you must call the routine by means of the JSR instruction. All KERNAL routines you can access are structured as SUBROUTINES, and must end with an RTS instruction. When the KERNAL routine has finished its task, control is returned to your program at the instruction after the JSR.

Many of the KERNAL routines return error codes in the status word or the accumulator if you have problems in the routine. Good programming practice and the success of your machine language programs demand that you handle this properly. If you ignore an error return, the rest of your program might "bomb."

That's all there is to do when you're using the KERNAL. Just these three simple steps:

1) Set up
2) Call the routine
3) Error handling