CENTRAL PROCESSING UNIT Fig. 2-1 Programming model of 65C02 The program counter "PC" is an 16-bit counter which keeps track of the address of the next instruction to be fetched and executed. The stack pointer "S" is an 8-bit register which keeps track of the low-order eight bits of the address of the "top" of a 256-byte RAM stack maintained in $0100 to $01FF of the address space. The stack is primarily used for storing the subroutine call or interrupt return addresses. Use of the stack allows nested subroutine calls and multiple level interrupts. 2-2