Memory Data Register

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  eBook - ePub

  Computer Architecture and Security

  Fundamentals of Designing Secure Computer Systems

  • Shuangbao Paul Wang, Robert S. Ledley(Authors)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  Registers are used in many different ways in a computer. One can be used to hold the address of current instruction being executed. It is called program counter register (PC). The instruction register (IR) holds the actual instruction being executed currently by the computer. The status register holds several 1-bit registers and each one keeps track of special conditions of the CPU. The flags contain information such as arithmetic carry, negative sign, overflow and other critical information that is monitored by the control unit.

  Memory, or random access memory (RAM) can be individually accessed in a random manner. A computer memory system usually contains billions of single 1-bit memory units. In order to access those memory units, a processor uses two registers: memory address register (MAR) and Memory Data Register (MDR) (Foster and Iberall, 1985). Sometimes a Memory Data Register is referred to as a memory buffer register as it holds the data that is being stored or retrieved from the memory location currently addressed by the memory address register.

  Figure 3.5 shows the relationship between the MAR, MDR and memory. An address decoder is used to reduce the number of addresses required for the large number of memory units. For example, a 32-bit address bus can address 232 = 4G memory.

  Figure 3.5 The relationship between the MAR, MDR, and memory

  In MAR, the highest bit called most significant bit (MSB) and the lowest bit called the least significant bit (LSB) (Randell, 1982). In MDR, the number of bits that can be accessed at the same time is determined by the width of the register that is connected to the processor. For a 32-bit data bus, one instruction can process 32-bit data. While 64-bit data can be processed within one instruction for a 64-bit data bus. So generally speaking a 64-bit computer is faster.

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  eBook - ePub

  Computer Fundamentals - 8th Edition

  Concepts, Systems & Applications

  • Pradeep K.Sinha(Author)
  • 2004(Publication Date)
  • BPB Publications

    (Publisher)

  Memory Address Register (MAR). It holds the address of the active memory location. It is loaded from program control register when the system reads an instruction from memory.
• Memory Buffer Register (MBR). It holds the contents of the accessed (read/written) memory word. The system transfers an instruction word placed in this register to instruction register. A data word placed in this register is accessible for operation with accumulator register or for transfer to I/O register. To store a word in a memory location, the system first transfers it to MBR and then writes it in memory from MBR.
• Program Control register (PC). It holds the address of the next instruction for execution. Normally, a system stores instructions of a program in consecutive memory locations, and executes them in sequence unless it encounters a branch instruction. A branch instruction is an operation that causes a transfer to a non-consecutive instruction. The system transfers address part of a branch instruction to PC register so that it becomes the address of the next instruction.
• Accumulator register (A). It holds the data on which the system has to operate, intermediate results, and results of operations performed. The system uses it during execution of most instructions. The system returns the result of an arithmetic operation to accumulator register for transfer to main memory through memory buffer register. In many computers, there are more than one accumulator registers.
• Instruction register (I). It holds the current instruction under execution. As soon as the instruction is stored in this register, its operation and address parts (see Chapter 12 ) are separated. The system sends the address part of the instruction to MAR, while it sends operation part to control unit, where it is decoded and interpreted. Finally, the control unit generates and sends command signals to the appropriate unit for carrying out the task specified in the instruction.
• Input/Output register (I/O).