Physical Memory and Physical Addressing By: Preeti Mudda Prof: Dr. Sin-Min Lee CS147 Computer Organization and Architecture

Agenda Physical Memory Random Access Memory Two Types of Ram Dynamic RAM Static RAM Physical Address Word Size Byte Alignment

Physical Memory Physical memory is the main memory that has direct/indirect access to CPU Physical memory mainly consists of RAM

Random Access Memory Random Access Memory (RAM): Any data could be accessed in a constant time regardless of its physical location. Two Types of RAM: DRAM – Dynamic Random Access Memory SRAM – Static Random Access Memory

Dynamic Random Access Memory DRAM is an integrated circuit. Millions of transistors and capacitors are paired together to create an array of memory cells Capacitor holds the bit information Transistors behaves like a switch which lets the control circuitry to control the state of the memory chip by reading or changing the capacitor state.

DRAM Architecture DRAM Chips are large and rectangular Arrays of memory cells Support logic- used for reading and writing data in arrays Refresh Circuitry- maintain the integrity of the stored data by periodically refreshing the memory cells

DRAM Memory cell represents single bit data (logic 1/logic 0) Cells are imprinted on silicon wafer Array of columns (bitlines) Array of rows (wordlines) Intersection of bitline and wordline is the address of the memory cell

Support Circuitry Sense amplifiers: Amplifies signal or charge detected on the memory cell Address logic: Select rows and columns Row Address Select(RAS) and Column Address Select(CAS): Latch and resolve the row and column addresses. It initiates or terminates the read/write operation

Support Circuitry Read /Write circuitry: Store or read the information in the memory cell Internals Counters: Keep track of the refresh sequence or initiate refresh cycle Output Enable logic: Prevents the data from displaying it unnecessarily.

DRAM Read timing Row Address Strobe (RAS): Latch row address and initiate the memory cycle. It is required at the beginning of every operation. To enable RAS the voltage transition should be from high to low voltage. RAS is an active low and it should maintain low voltage as long as RAS is required Complete memory cycle: RAS must active for minimum amount time and also inactive for minimum amount of time. Column Address Strobe: Initiate Read/Write Operation CAS must be active before RAS for refresh cycle.

DRAM Timing Write Enable(WE): Write enable signal used to choose for read/write operation. Low voltage-level signifies a write operation High-voltage level signifies a read operation. Output Enable (OE): This control signal is used to prevent displaying the output while read operation. The control signal is grounded when write operation is selected. Data IN/OUT(DQs): DQ pins used for input and output

Steps to access a cell in DRAM

Static RAM Type of semiconductor memory No need to refresh frequently Uses six MOFETS to store each memory bit Size: SRAM has m address lines and n data lines: 2^m words.

SRAM Three different states: Standby: The circuit is idle Reading: Reading the data which has been requested Writing: Writing the data on to the memory

SRAM

SRAM Reading: WE_L is dis- asserted (H), OE_L is asserted (L) D: data output pin Writing: WE_L is asserted (L) where as OE_L is dis-asserted (H). D: Data input pin.

Physical Address Physical address is a memory address that is stored in the form of binary number It is electronically represented on the computer address bus circuitry Through this data bus is enabled to access particular storage from the main memory cell

Physical Memory and Word Size Bits of physical memory are divided into blocks of N bits Terminology – Group of N bits is called a word – N is known as the width of a word or the word size

Physical Memory Addresses Each word of memory is assigned a unique number known as a physical memory address Programmer imagines physical memory to be an array of words Note: entire word must be transferred

Choosing A Word Size Larger word size Implemented with more parallel wires Results in higher performance Higher cost Note: architect usually designs all parts of computer to use one size for: Memory word Integer (general-purpose registers)

Byte Addressing View of memory presented to processor Each byte of memory assigned an address Convenient for programmers Underlying memory can still use word addressing

Translation Between Byte And Word Addresses Performed by intelligent memory controller CPU can use byte addresses (convenient) Physical memory can use word addresses (efficient)

Byte Alignment Refers to integer storage in memory In some architectures – Integer in memory must correspond to word in underlying physical memory In other architectures – Integer can be unaligned, but fetch and store operations are much slower

Memory Size And Address Space Size of address limits maximum memory Example: 32-bit address can represent 2^32 = 4,294,967,296 unique addresses Known as address space Note: word addressing allows larger memory than byte addressing

Resources Physical Memory: IBM article “Understanding DRAM operation” CS-Duke Lecture notes: Memory Systems Wikipedia Howstuffworks website Physical Address: F07/handouts/Chapter10.pdf F07/handouts/Chapter10.pdf Wikipedia