Memory Arrays: DRAM, SRAM, and ROM

5.5 Memory Arrays

• Memory arrays are used to efficiently store large amounts of data
• The three common types include Dynamic Random Access Memory (DRAM), Static Random Access Memory (SRAM), and Read Only Memory (ROM)
• An M-bit data value can be read or written at each unique N-bit address

Memory Arrays Info

• They consist of a two-dimensional array of bit cells
• Each bit cell stores one bit
• With N address bits and M data bits, an array has 2^N rows and M columns
• Depth: the number of rows or memory location (number of words)
• Width: number of columns (size of word)
• Array size: is calculated by depth × width = 2^N × M

Memory Array Example

• A 2^2 × 3-bit array contains 4 words
• Each word is 3-bits wide
• The 3-bit word stored at address 10 is 100

Memory Array Bit Cells

• Bitline: is related to Data
• Wordline: is related to address
• Each rectangle is a bit cell
• To read a bit cell, the bitline is initially left floating. Then the wordline is turned on, letting the stored value drive the bitline to 0 or 1
• To write a bit cell, the bitline drives strongly to the desired value. Then the wordline turns on, and the bitline overpowers the contents of the bitcell

Memory Array: Wordline

• Wordline acts like an enable
• A single row in the memory array can be read or written via a wordline
• Corresponds to a unique address
• Only one wordline is HIGH at any given time

Memory Ports

• Memories have one or more ports, each giving read and/or write access to one memory address
• Multiported memories access several addresses simultaneously
• A three-ported memory includes two read ports and one write port
  • Port 1 reads data A1 to read data output RD1
  • Port 2 reads data A2 onto RD2
  • Port 3 writes the data input WD3 into address A3 on the rising edge of clock if write enable WE3 is asserted

Types of Memory

• Random Access Memory (RAM): volatile
• Read Only Memory (ROM): nonvolatile

RAM: Random Access Memory

• RAM is volatile, losing its data when power is turned off
• Data can be read and written quickly
• The main memory in computers is RAM, specifically DRAM

ROM: Read Only Memory

• ROM is nonvolatile, retaining data with no power
• Data can be read quickly, but writing is impossible or slow
• Flash memory in cameras, thumb drives, and digital cameras are ROM

Types of RAM

• There are two main types of RAM: DRAM and SRAM
• They differ in how data is stored: DRAM uses a capacitor, while SRAM uses cross-coupled inverters

5.5.2 DRAM

• Data bits are stored in a capacitor
• Is called dynamic because the values need to be refreshed (rewritten) periodically and after being read
• Charge leakage from the capacitor degrades the value
• The operation of reading destroys the stored value
• The logic node at the capacitor will not connect to Vdd or ground, so the logic value is floating

5.5.3 SRAM

• Words are stored in cross-coupled inverters

ROMs

• ROM stores a bit as the presence or absence of a transistor
• To read ROM, the bitline is initially set to be weak HIGH
• If the transistor is present, it pulls the bitline LOW
• If the transistor is absent, the bitline remains High
• A dot notation indicates that the bit is one, e.g., for address 11, the data is 010

5.5.7 Logic with Any Memory Array

• The addresses are laid in a top down order (Like Dada2: the truth table)
• Memory arrays used to perform logic is called Lookup table

Logic with Memory Arrays

• Memory arrays used to perform logic are called lookup tables
• Lookup tables (LUTs): Look up output at each combination (address). A reflection of logic truth table

Multi-ported Memories

• Port: address/data pair
• A 3-ported memory consists of 2 read ports (A1/RD1, A2/RD2) and 1 write port (A3/WD3, with WE3 enabling writing)
• Small multi-ported memories are register files