THE Y INDEX REGISTER

This is a very important register. There are instructions for nearly all of the transformations you can make to the accumulator, and the X register. But there are other instructions for things that only the Y register can do. Various machine language instructions allow you to copy the contents of a memory location into the Y register, copy the contents of the Y register into a memory location, and modify the contents of the Y, or some other register directly.

THE STATUS REGISTER

This register consists of eight "flags" (a flag = something that indicates whether something has, or has not occurred).

THE PROGRAM COUNTER

This contains the address of the current machine language instruction being executed. Since the operating system is always "RUN"ning in the Commodore 64 (or, for that matter, any computer), the program counter is always changing. It could only be stopped by halting the microprocessor in some way.

THE STACK POINTER

This register contains the location of the first empty place on the stack. The stack is used for temporary storage by machine language programs, and by the computer.

THE INPUT/OUTPUT PORT

This register appears at memory locations 0 (for the DATA DIRECTION REGISTER) and 1 (for the actual PORT). It is an 8-bit input/output port. On the Commodore 64 this register is used for memory management, to allow the chip to control more than 64K of RAM and ROM memory.

The details of these registers are not given here. They are explained as the principles needed to explain them are explained.

HOW DO YOU WRITE MACHINE LANGUAGE PROGRAMS?

Since machine language programs reside in memory, and there is no facility in your Commodore 64 for writing and editing machine language