The key characteristics of computer memory systems are listed below.
- Location
- Capacity
- Unit of Transfer
- Performance
- Access method
- Physical type & characteristics
The internal memory capacity is expressed in terms of bytes or words whereas the external memory capacity is expressed in bytes.
The size of a word is equal to the number of bits used to represent an integer and the instruction length.
The relationship between the length in bits A of an address and the number N of addressable units is 2A = N.
The Unit of transfer is the number of bits read out of or written into memory at a time for main memory.
The important performance parameters are access time, memory cycle time and transfer rate.
The access time is the time it takes to perform a read or write operation for random access memory and time it takes to position the read–write mechanism at the desired location for non random access memory.
The access time plus any additional time required before a second access can commence is memory cycle time. The memory cycle time is concerned with the system bus, not the processor.
The transfer rate is the rate at which data can be transferred into or out of a memory unit.
For random access memory,
Transfer time = 1/Cycle Time
For non random access memory,
where
Tn - Average time to read or write n bits
TA - Average access time
n - No of bits
R - Transfer rate in bps
The relation between capacity, access time and cost of a memory is
- Faster access time, High cost per bit
- Larger capacity, Low cost per bit
- Larger capacity, slower access time