physical memory and physical addressing
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Physical Memory and Physical Addressing
By:Preeti Mudda
Prof: Dr. Sin-Min LeeCS147 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
http://www.cs.duke.edu/courses/fall98/cps104/lectures/week10-l2/sld024.htm
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
http://www.cs.duke.edu/courses/fall98/cps104/lectures/week10-l2/sld024.htm
http://www.cs.duke.edu/courses/fall98/cps104/lectures/week10-l2/sld024.htm
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
http://www.cs.duke.edu/courses/fall98/cps104/lectures/week10-l2/sld016.htm
SRAM
http://www.cs.duke.edu/courses/fall98/cps104/lectures/week10-l2/sld018.htm
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: http://www.eecs.wsu.edu/~hauser/teaching/Arch-
F07/handouts/Chapter10.pdf Wikipedia