with the addressing of memory locations in page zero. These addresses, therefore, ALWAYS have a high order byte of zero. The zero page mode of addressing only expects one byte to describe the address, rather than two when using an absolute address. The zero page addressing mode tells the microprocessor to assume that the high order address is zero. Therefore zero page addressing can reference memory locations whose addresses are between $0000 and $00FF. This may not seem too important at the moment, but you'll need the principles of zero page addressing soon.

THE STACK

The 6510 microprocessor has what is known as a stack. This is used by both the programmer and the microprocessor to temporarily remember things, and to remember, for example, an order of events. The GOSUB statement in BASIC, which allows the programmer to call a subroutine, must remember where it is being called from, so that when the RETURN statement is executed in the subroutine, the BASIC interpreter "knows" where to go back to continue executing. When a GOSUB statement is encountered in a program by the BASIC interpreter, the BASIC interpreter "pushes" its current position onto the stack before going to do the subroutine, and when a RETURN is executed, the interpreter "pulls" off the stack the information that tells it where it was before the subroutine call was made. The interpreter uses instructions like PHA, which pushes the contents of the accumulator onto the stack, and PLA (the reverse) which pulls a value off the stack and into the accumulator. The status register can also be pushed and pulled with the PHP and PLP, respectively.

The stack is 256 bytes long, and is located in page one of memory. It is therefore from $01 00 to $01 FF. It is organized backwards in memory. In other words, the first position in the stack is at $01 FF, and the last is at $0100. Another register in the 651 0 microprocessor is called the stack pointer, and it always points to the next available location in the stack. When something is pushed onto the stack, it is placed where the stack pointer points to, and the stack pointer is moved down to the next position (decremented). When something is pulled off the stack, the stack pointer is incremented, and the byte pointed to by the stack pointer is placed into the specified register.