Memory Devices 1. Memory concepts 2. RAMs 3. ROMs 4. Memory expansion & address decoding applications 5. Magnetic and Optical Storage

Memory Concepts (Units of Binary Data)+(Basic Memory Array) As a rule, memory store data in units: 1.Bit – smallest unit of binary data 2.Byte – 8 bit unit 3.Nibble – two 4-bit units 4.Word – one or more bytes Memory Array: 1.Each storage element called cell (1 or 0) 2.Memory made up of arrays of cells 3.Each block in memory array represents one storage cell, identified by row and column

Memory Concepts (Memory Address and Capacity) Memory Address: (2D or 3D array) 1.The location of a unit data in a memory array 2.The address of a bit is specified by row and column, or only by a row 3.Depends on how the memory is organized 4.Nibble – two 4-bit units 5.Word – one or more bytes Capacity: 1.The total number of data units that can be stored

Memory Concepts (Memory Operation-Write and Read) Memory stores binary data: 1.The write operation puts data into specified address in the memory when needed. 2.The read operation copies data out of a specified address in the memory. 3.Data units go into the memory and came out of memory on a set of line called the data bus. 4.For write and read operation, an address is selected by placing a binary code representing the desired address on a set of lines called address bus. 5.Word – one or more bytes

Memory Concepts (Memory Operation-Write and Read) The WRITE operation: 1.To store a byte of data in the memory, a code held in address register is placed in address bus. 2.The address decoder decodes the address and select the specified location in the memory. 3.The memory get the WRITE command and the data byte held in the data register is placed in the data bus and stored in selected memory address, thus write operation completed.

Memory Concepts (Memory Operation-Write and Read) The READ operation: 1.To read a byte of data in the memory, a code held in address register is placed in address bus. 2.The address decoder decodes the address and select the specified location in the memory. 3.The memory get the READ command and a ‘copy’ of the data byte that is stored in the selected memory address is placed on the data bus and loaded into data register, thus completing read operation

RAMs & ROMs (Memory Operation-Write and Read) Memories RAMROM 1.Read & Write capabilities 2.Lose the data when power is off 3.Volatile memories 1.Data stored permanently or semi-permanently. 2.Data can be read but no write 3.Retain stored data even if power is off 4.Non volatile memories

Random-Access Memories (RAMs) Static RAM (SRAM): 1.Use latches as storage elements and can therefore store data infinitely as long as the power is applied. Dynamic RAM (DRAM): 1.Use capacitors as storage elements and cannot retain data very long without the capacitors being recharge by a process called refreshing. Both SRAMs and DRAMs will lose stored data when dc power is removed (volatile memories) 1.Data can be read much faster from SRAMs than from DRAMs. 2.DRAMS can store more data than SRAMs for a given physical size and cost because DRAM much simpler and chip area can be cramp into given chip area.

Static RAMs Memory Cell: 1.All static RAMs are characterized by latch memory cells. 2.The cell is selected by an active level on the Select line and a data bit (1 or 0) is written into cell by placing it on the Data in line. A data bit is read by taking it off the Data out line. Memory Cell Array: 1.Organized in rows and columns 2.Each set of Data in and Data out lines go to each cell in a given column and are connected to a single data line that serves a both an input and output (Data I/O) through data input and data output buffer.

1.Dynamic memory cells store a data bit small capacitor rather than in a latch. 2.The advantages: Very simple Allow very large memory arrays to be constructed on a chip at a lower cost per bit. 3.The disadvantages: Storage capacity cannot hold its charge over an extended period of time and will lose the stored data bit unless its charge is refreshed periodically. To refresh requires additional memory circuitry and complicates the operational of DRAM. Dynamic RAMs

ROMs (READ ONLY MEMORIES) Memories RAMROM 1.Read & Write capabilities 2.Lose the data when power is off 3.Volatile memories 1.Data stored permanently or semi-permanently. 2.Data can be read but no write 3.Retain stored data even if power is off 4.Non volatile memories