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This means that a CPU cannot simultaneously read an instruction and read or write data from or to the memory. In a computer using the Harvard architecture, the CPU can both read an instruction and perform a data memory access at the same time, [7] even without a cache.
This addressing mode, which always fetches data from memory or stores data to memory and then sequentially falls through to execute the next instruction (the effective address points to data), should not be confused with "PC-relative branch" which does not fetch data from or store data to memory, but instead branches to some other instruction ...
The most common modification builds a memory hierarchy with separate CPU caches for instructions and data at lower levels of the hierarchy. There is a single address space for instructions and data, providing the von Neumann model, but the CPU fetches instructions from the instruction cache and fetches data from the data cache.
Copy data from a memory location or a register to a memory location or a register (a machine instruction is often called move; however, the term is misleading). They are used to store the contents of a register, the contents of another memory location or the result of a computation, or to retrieve stored data to perform a computation on it later.
During the fetch stage, the address stored in the PC is copied into the memory address register (MAR) and then the PC is incremented in order to "point" to the memory address of the next instruction to be executed. The CPU then takes the instruction at the memory address described by the MAR and copies it into the memory data register (MDR ...
The final important element in the CPU is the program counter (PC) register. The PC is a 16-bit binary counter whose low 15 bits specify the address in instruction memory of the next instruction for execution. Unless directed otherwise by a branching instruction, the PC increments its value at the end of each clock cycle.
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In other words, this register is used to access data and instructions from memory during the execution phase of instruction. MAR holds the memory location of data that needs to be accessed. When reading from memory, data addressed by MAR is fed into the MDR (memory data register) and then used by the CPU. When writing to memory, the CPU writes ...