Instruction Set Architecture and Design - Fundamentals of Computer Organization and Architecture - page 17

Three basic steps are needed in order for the CPU to perform a write operation
into a specified memory location:

1. The word to be stored into the memory location is first loaded by the CPU
into a specified register, called the memory data register (MDR).
2. The address of the location into which the word is to be stored is loaded by
the CPU into a specified register, called the memory address register (MAR).
3. A signal, called write, is issued by the CPU indicating that the word stored in
the MDR is to be stored in the memory location whose address in loaded in
the MAR.

Figure 2.2 illustrates the operation of writing the word given by 7E (in hex) into the
memory location whose address is 2005. Part a of the figure shows the status of the reg-
isters and memory locations involved in the write operation before the execution of the
operation. Part b of the figure shows the status after the execution of the operation.
It is worth mentioning that the MDR and the MAR are registers used exclusively
by the CPU and are not accessible to the programmer.
Similar to the write operation, three basic steps are needed in order to perform a
memory read operation:

1. The address of the location from which the word is to be read is loaded into
the MAR.
2. A signal, called read, is issued by the CPU indicating that the word whose
address is in the MAR is to be read into the MDR.
3. After some time, corresponding to the memory delay in reading the specified
word, the required word will be loaded by the memory into the MDR ready
for use by the CPU.

Before execution After execution

Figure 2.2 Illustration of the memory write operation