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In addition it is often desirable to set aside a memory location as a counter. Counters are useful in accessing consecutive memory locations or keeping track of the number of passes through a loop. The increment and decrement instructions are: Increment X-register by one DEX Decrem ent X-register by one INY Increment V-register by one DEY Decrem ent V-register by one INC Inc rement memory by one DEC Dec rement memory by one INX 4. Compare and Branch: Compare instructions are commonly used to determine if a register or memory location that is being used as a counter has reached a certain value.
There is a control unit that decodes instructions and shifts data to and from memory. Of primary importance to us are six special internal data storage locations or registers. Five of these are 8 bit registers . The sixth is a 16 bit register. The five 8 bit registers are the: 1. Accumulator 2. X-Register 3. V-Register 4. Processor Status Register 5. Stack Pointer. The remaining register is the 16 bit program counter. The functions of the registers are as follows: 1. The Accumulator: This is the busiest register in the CPU.
Interrupt Instructions: Interrupts are signals to the processor from another chip or peripheral requesting the processor's attention. There are two types of interrupts; Non-Maskable Interrupts (NMI) and Interrupt Requests (IRQ) . Whether or not the processor responds to an IRQ depends on the IRQ disable bit (I) in the processor status register. If the I bit is clear (ie. equal to zero), then the external interrupt will be serviced. If the I bit is set (ie. equal to 1), the processor will ignore the interrupt request.