Memory Devices-In the digital systems it is the memory that gives the ability to store large quantities of digital information for a short or long periods of time. Ram- Random access memory (temporary storage). Rom- Read only memory (permanent storage). Auxiliary Memory- is another type of storage and it is also called mass storage. It can store large amounts of data that are not currently being used by the CPU and operates at a much slower speed then internal memory.Flash Memory. Memory Terminology Memory Cell- it is a device or an electrical circuit used to store a single bit which is 0 or 1. Memory Word- a group of bits or cells in a memory represents information or data. Byte- 8 Bits Nibble- 4 Bits.Capacity- specifies number of bits that can be stored in a particular memory device or in a complete memory system.Density- another term for capacity.Address- it is a number identifies the location of a word in memory. Each word in a memory location has a unique address.Read Operation- It is the fetch operation performed to transfer data from one address location to another.Access Time- it is the time between the memory receiving a read command signal and the data becoming available at the memory output. In brief it is the time required to perform read operations and the symbol is tACC. Cycle Time- it is the amount of time required for the memory to perform a read or write operation and then return to its original state which is “Ready” status for the next command. Volatile Memory- Memory that requires power in order to store information. Once the power is off the information stored will be lost. Sequential Access Memory (SAM)- Here the access time is not constant but varies depending on the address location. Ex: Magnetic tapes and disc here in order to find particular stored word it has to sequence through all address locations until the desired address is reached. Read/Write Memory (RWM)- it is the memory that can be easily read from or written into.Static Memory Device- here the stored data will remain permanently as long as the power is applied. The data does not have to be rewritten into memory.Dynamic Memory Device- Here stored data will not remain permanently even if the power is applied all the time. The data has to be periodically rewritten into memory which is called “REFRESH” operation. >>General memory operation- even though internal operations are different on each type of memory there are a few operating rules that are the same for all memory systems. That is every memory system requires different types of input and output lines to preform read/write operations, input/output or store operations as well as enable or disable operations. The following diagram shows a memory that stores 32 bit words and four bit word size.
Here the read/write input controls which memory location is to take place the read operation or write operation. A read operation occurs when read R/W =1 and write operation occurs when R/W=0. If the memory capacity is 256 x 8 find the number of data inputs and output lines, also find the number of address line inputs, find the word size, and find the total number of bits stored. Also find this information for 512 x 16. For 256 x 8 there are 8 lines because 256= 28 so the exponent shows the amount of lines 256 shows the word size that the chip can hold and the 8 shows # of bits per word For 256K x 16. 256 x 1024= word size. 16= # of bits per word. Number of address lines= 28=256. Number of input output lines=16. Capacity of the memory= 256K*16. Read only memory holds data that are permanent and will not change frequently. Data can only be read from and cannot be written into. Since it is a nonvolatile memory ROM are widely used in microcomputers to store programs. Type of ROM prom programmable read only memory. This type of ROM are programed by the user and once programed it cannot be erased and reprogramed therefor these devices are often referred to as one time programmable ROM’s. Once