The memory system can be characterised with their Location, Capacity, Unit of transfer, Access method, Performance, Physical type, Physical characteristics, Organisation.
- Processor memory: The memory like registers is included within the processor and termed as processor memory.
- Internal memory: It is often termed as main memory and resides within the CPU.
- External memory: It consists of peripheral storage devices such as disk and magnetic tape that are accessible to processor via i/o controllers.
- Word size: Capacity is expressed in terms of words or bytes.
- The natural unit of organisation
- Number of words: Common word lengths are 8, 16, 32 bits etc.
- or Bytes
Unit of Transfer
- Internal: For internal memory, the unit of transfer is equal to the number of data lines into and out of the memory module.
- External: For external memory, they are transferred in block which is larger than a word.
- Addressable unit
- Smallest location which can be uniquely addressed
- Word internally
- Cluster on Magnetic disks
- Sequential access: In this access, it must start with beginning and read through a specific linear sequence. This means access time of data unit depends on position of records (unit of data) and previous location.
- eg. tape
- Direct Access: Individual blocks of records have unique address based on location. Access is accomplished by jumping (direct access) to general vicinity plus a sequential search to reach the final location.
- eg. disk
- Random access: The time to access a given location is independent of the sequence of prior accesses and is constant. Thus any location can be selected out randomly and directly addressed and accessed.
- eg. RAM
- Associative access: This is random access type of memory that enables one to make a comparison of desired bit locations within a word for a specified match, and to do this for all words simultaneously.
- eg. cache
- Access time: For random access memory, access time is the time it takes to perform a read or write operation i.e. time taken to address a memory plus to read / write from addressed memory location. Whereas for non-random access, it is the time needed to position read / write mechanism at desired location.
- Time between presenting the address and getting the valid data
- Memory Cycle time: It is the total time that is required to store next memory access operation from the previous memory access operation.
Memory cycle time = access time plus transient time (any additional time required before a second access can commence).
- Time may be required for the memory to “recover” before next access
- Cycle time is access + recovery
- Transfer Rate: This is the rate at which data can be transferred in and out of a memory unit.
- Rate at which data can be moved
- For random access, R = 1 / cycle time
- For non-random access, Tn = Ta + N / R; where Tn – average time to read or write N bits, Ta – average access time, N – number of bits, R – Transfer rate in bits per second (bps).
- Disk & Tape
- CD & DVD
- Decay: Information decays mean data loss.
- Volatility: Information decays when electrical power is switched off.
- Erasable: Erasable means permission to erase.
- Power consumption: how much power consumes?
- Physical arrangement of bits into words
- Not always obvious