What are registers?

Registers are temporary memory units that store words. The registers are located in the processor, instead of in RAM, so data can be accessed and stored faster. The eight registers in our example are called PC, AC, IR, TIR, +1, AMASK, MAR, and MBR, and are used as follows:

Registers:
• PC: Program Counter. Stores the address of the macro-instruction currently being executed.
• AC Accumulator. Stores a previously calculated value or a value loaded from the main memory.
• IR Instruction Register. Stores a copy of the instruction loaded from main memory.
• TIR Temporary Instruction Register. As the CPU evaluates exactly what an instruction is supposed to do, it stores the edited instruction in the TIR.
• 1 A constant that represents the number one. The CPU cannot access a number unless it is in a register or loaded from main memory, or somehow computed. Therefore this register is set aside to represent this often used number,
• AMASK Address Mask. When the CPU needs to know the address of a target word that an instruction is using, the AMASK is AND'ed with the instruction to eliminate the opcode, leaving only the desired address. If that didn't make sense, leave it to the discussion of macro-instructions later on.
• MAR Memory Address Register. This register contains the address of the place the CPU wants to work with in the main memory. It is directly connected to the RAM chips on the motherboard.
• MBR Memory Buffer Register. This register contains the word that was either loaded from main memory or that is going to be stored in main memory. It is also directly connected to the RAM chips on the motherboard.



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