Computer memory Organization

Computer Memory Organization

• A memory unit is the collection of storage units or devices together. The memory unit stores the binary information in the form of bits. Generally, memory/storage is classified into 2 categories.

• Volatile Memory: This loses its data when power is switched off. 

• Non-Volatile Memory: This is permanent storage and does not lose any data when power is switched off.

Memory Hierachy

• In the Computer System Design, Memory Hierarchy is an enhancement to organize the memory such that it can minimize the access time. The Memory Hierarchy was developed based on a program behaviour known as locality of references. The figure below clearly demonstrates the different levels of the memory hierarchy

Characteristics of Memory Hierarchy 1.Capacity: It is the global volume of information the memory can store. As we move from top to bottom in the Hierarchy, the capacity increases. 2.Access Time: It is the time interval between the read/write request and the availability of the data. As we move from top to bottom in the Hierarchy, the access time increases. 3.Performance: Earlier when the computer system was designed without Memory Hierarchy design, the speed gap increases between the CPU registers and Main Memory due to large difference in access time. This results in lower performance of the system and thus, enhancement was required. This enhancement was made in the form of Memory Hierarchy Design because of which the performance of the system increases. One of the most significant ways to increase system performance is minimizing how far down the memory hierarchy one has to go to manipulate data. 4.Cost per bit: As we move from bottom to top in the Hierarchy, the cost per bit increases i.e. Internal Memory is costlier than External Memory.

Types of Memory

• External Memory or Secondary Memory – Comprising of Magnetic Disk, Optical Disk, Magnetic Tape i.e. peripheral storage devices which are accessible by the processor via I/O Module.

• Internal Memory or Primary Memory – Comprising of Main Memory, Cache Memory & CPU registers. This is directly accessible by the processor.

Memory Access Methods

• Each memory type is a collection of numerous memory locations. To access data from any memory, first, it must be located and then the data is read from the memory location. Following are the methods to access information from memory locations.

• Random Access: Main memories are random access memories, in which each memory location has a unique address. Using this unique address any memory location can be reached in the same amount of time in any order. 

• Sequential Access: This method allows memory access in a sequence or in order. 

• Direct Access: In this mode, information is stored in tracks, with each track having a separate read/write head.

Main Memory 

• The memory unit that communicates directly within the CPU, Auxillary memory and Cache memory, is called main memory. It is the central storage unit of the computer system. It is a large and fast memory used to store data during computer operations. Main memory is made up of RAM and ROM, with RAM integrated circuit chips holing the major share. 

• RAM: Random Access Memory 

• DRAM: Dynamic RAM, is made of capacitors and transistors, and must be refreshed every 10~100 ms. It is slower and cheaper than SRAM. 

• SRAM: Static RAM, has a six transistor circuit in each cell and retains data until powered off.

• NVRAM: Non-Volatile RAM, retains its data, even when turned off. Example: Flash memory. 

• ROM: Read Only Memory, is non-volatile and is more like permanent storage for information. It also stores the bootstrap loader program, to load and start the operating system when the computer is turned on. PROM(Programmable ROM), EPROM(Erasable PROM) and EEPROM(Electrically Erasable PROM) are some commonly used ROMs.

Auxiliary Memory

• Devices that provide backup storage are called auxiliary memory. For example, Magnetic disks and tapes are commonly used as auxiliary devices. Other devices used as auxiliary memory are magnetic drums, magnetic bubble memory and optical disks.

• It is not directly accessible to the CPU and is accessed using the Input/Output channels.

Cache Memory

• The data or contents of the main memory that are used again and again by CPU, are stored in the cache memory so that we can easily access that data in a shorter time.

 • Whenever the CPU needs to access memory, it first checks the cache memory. If the data is not found in cache memory then the CPU moves onto the main memory. It also transfers a block of recent data into the cache and keeps on deleting the old data in the cache to accommodate the new one.