Basic Memory Operations
The memory unit supports two basic operations: read and write. The read operation reads previously stored data and the write operation stores a new value in memory. Both of these operations require a memory address. In addition, the write operation requires specification of the data to be written. The address and data of the memory unit are connected to the address and data buses of the system bus, respectively. The read and write signals come from the control bus. Two metrics are used to characterize memory. Access time refers to the amount of time required by the memory to retrieve the data at the addressed location. The other metric is the memory cycle time, which refers to the minimum time between successive memory operations.
The read operation is nondestructive in the sense that one can read a location of the memory as many times as one wishes without destroying the contents of that location. The write operation, however, is destructive, as writing a value into a location destroys the old contents of that memory location.
Steps in a Typical Read Cycle:
1. Place the address of the location to be read on the address bus,
2. Activate the memory read control signal on the control bus,
3. Wait for the memory to retrieve the data from the addressed memory location and place
them on the data bus,
4. Read the data from the data bus,
5. Drop the memory read control signal to terminate the read cycle.
Steps in a Typical Write Cycle:
1. Place the address of the location to be written on the address bus,
2. Place the data to be written on the data bus,
3. Activate the memory write control signal on the control bus,
4. Wait for the memory to store the data at the addressed location,
5. Drop the memory write signal to terminate the write cycle.
18-The two major types of memory are:
1- Read-write or random access memory( RAM)
It has a variable content , also it is generally used to store the variable data. RAM can also be used to store frequently changed programs and other information. The term “random” means that any memory location can be accessed in the same amount of time, regardless of its position in the memory.
RAM allows the computer to store information quickly for later reference:
1- The active part of the operating system, the fundamental program that control the operation of the computer.
2- The application program being executed (for example a letter being written with the word processing program).
3- A representation of the data being presented on the video display
In RAM, the stored information will be lost when computer power supply is removed (even a short interruption). That is RAM, is volatile memory. Some form of RAM, called flash memory, retain their electrical charges when a computer is turned off When program instruction, reads the data in memory address, it gets a copy of the data. This is called a nondestructive read, because the contents of the memory address are not changed. Sending data to a RAM memory address is called a destructive write, because the new data erases whatever was there before.
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