This document provides an overview of RAM (random access memory). It describes RAM as volatile memory that does not retain data when power is turned off. The document then discusses RAM components like SRAM and DRAM. SRAM stores bits using a flip-flop circuit that retains data as long as power is applied, while DRAM uses a capacitor and transistor that must be regularly refreshed to maintain its charge and data. The document concludes with a comparison of SRAM and DRAM, noting key differences in their data volatility, refresh needs, cell structures, speeds and costs.

2. CONTENTS
What is RAM ?
Block diagram of Ram.
Types of RAM.
SRAM design & Operation.
DRAM operation.
Summary.

3. What is RAM?
Ram is volatile memory, meaning it does not
retain data, when the electric power is turned
off or fails, so if you turn off your computer,
all memory stored in RAM is lost.
When your computer is turned on again, the
BIOS reads your operating system and related
files from the hard disk and loads them back
into RAM.
Contd….

4. RAM stores its data in a series of memory
cells that can be accessed in any order, thus
why it is called Random.
SAM (serial access memory) is much the
same as RAM, but can only access its
memory cells in a specific order.

5. Block Diagram Of RAM
CS WR Memory operation
0 x None
1 0 Read selected word
1 1 Write selected word
2k x n memory
ADRS OUT
DATA
CS
WR
k
n
n
• A Chip Select, CS, enables or disables the RAM.
• ADRS specifies the address or location to read from or write to.
• WR selects between reading from or writing to the memory.
To read from memory, WR should be set to 0.
OUT will be the n-bit value stored at ADRS.
To write to memory, we set WR = 1.
DATA is the n-bit value to save in memory.

6. Memory Sizes
We refer to this as a 2k x n memory.
 There are k address lines, which can specify one of 2k addresses.
 Each address contains an n-bit word.
 For example
2k x n memory
ADRS OUT
DATA
CS
WR
k
n
n
A 224 x 16 RAM contains 224 = 16M words, each 16 bits long.
 The RAM would need 24 address lines.
 The total storage capacity is 224 x 16 = 228 bits.

7. Reading RAM
To read from this RAM, the controlling circuit must:
 Enable the chip by ensuring CS = 1.
 Select the read operation, by setting WR = 0.
 Send the desired address to the ADRS input.
 Te contents of that address appear on OUT after a little while.
Notice that the DATA input is unused for read operations.
2k x n memory
ADRS OUT
DATA
CS
WR
k
n
n

8. Writing RAM
To write to this RAM, you need to:
 Enable the chip by setting CS = 1.
 Select the write operation, by setting WR = 1.
 Send the desired address to the ADRS input.
 Send the word to store to the DATA input.
 The output OUT is not needed for memory write operations.
2k x n memory
ADRS OUT
DATA
CS
WR
k
n
n

9. Types of RAM
1. Dynamic Random Access Memory (DRAM)
• Contents are constantly refreshed 1000 times per second
• Access time 60 – 70 nanoseconds
2. Synchronous Dynamic Random Access Memory
(SDRAM)
• Quicker than DRAM
• Access time less than 60 nanoseconds
3. Direct Rambus Dynamic Random Access Memory
(DRDRAM)
• New type of RAM architecture
• Access time 20 times faster than DRAM
• More expensive

10. Types of RAM
4. Static Random Access Memory (SRAM)
• Doesn’t need refreshing
• Retains contents as long as power applied to the chip
• Access time around 10 nanoseconds
• Used for cache memory
• Also for date and time settings as powered by small battery
5. Video Random Access memory (VRAM)
•Holds data to be displayed on computer screen
•Has two data paths allowing READ and WRITE to occur at the same
time
•A graphics card may have its own VRAM chip on board
EXAMPLE: NVDIA

11. Types of RAM
6. Virtual memory
• Uses backing storage e.g. hard disk as a temporary location
for programs and data where insufficient RAM available
• Swaps programs and data between the hard-disk and RAM as
the CPU requires them for processing
• A cheap method of running large or many programs on a
computer system
• Cost is speed: the CPU can access RAM in nanoseconds but
hard-disk in milliseconds (Note: a millisecond is a
thousandth of a second)
• Virtual memory is much slower than RAM

12. SRAM(Static Random Access Memory)
 SRAM is a type of semiconductor memory that uses
Bistable circuitry to store each bit.
 Doesn’t need refreshing
Retains contents as long as power applied to the chip
Access time around 10 nanoseconds
Used for cache memory
Also for date and time settings as powered by small battery

13. Design Of SRAM
 A typical SRAM cell is made up of
six MOSFETs. Each bit in an SRAM
is stored on four transistors (M1,
M2, M3, M4) that form two cross-coupled
inverters.
 This storage cell has two stable
states which are used to denote 0
and 1.
 Two additional access transistors
serve to control the access to a
storage cell during read and write
operations.

14. SRAM Operations
 An SRAM cell has three different states. It can be
in: standby (the circuit is idle), reading (the data
has been requested) and writing (updating the
contents).

15.  Standby
 If the word line is not asserted, the
access transistors M5 and M6
disconnect the cell from the bit lines.
The two cross-coupled inverters
formed by M1 – M4 will continue to
reinforce each other as long as they are
connected to the supply.

16.  Reading
 Assume that the content of memory is 1 stored at
Q. The read cycle is started by precharging both
the bit lines to a logic 1,
 The word line WL, enabling M5 AND M6 .
 The second step occurs when the value stored in Q
& Q’ are transferred to the bit line by leaving BL at
its precharged value and discharging BL’ through
M1 and M5 to a logical 0
 On the BL side, the transistors M4 and M6 pull the
bit line toward VDD, a logical 1 (M4 as it is turned
on because Q’ is logically set to 0).
 Bit lines reach a sense amplifier, which will sense
which line has higher voltage and thus will tell
whether there was 1 stored or 0. The higher the
sensitivity of sense amplifier, the faster the speed
of read operation is.

17. DRAM
Dynamic Random Access Memory
 Basic storage device is not a flip flop but a MOS and a capacitor
 Charge determine the stored bit (0,1)
 Data stored as a charge not remain infinitely due to leakage current,
therefore periodic refresh cycle is required to maintain stored data.

18. OPERATION
VDD
O/P
T1 C
T3
I/P
A
 When A= 1 T1 will be ON then
capacitor charges to I/P value during
interval A=1
When T1 is OFF the invertor will
remember the sample data because of
stored charge on capacitor.
 Charge on capacitor eventually leak
OFF. Because of leakage through
insulation which supports the gate on
T2.
 To larger extent it is due to leakage
through reverse biased junction formed
b/w sub. And drain of T1.

19. 4 Transistor DRAM cell

20.  Capacitor C1 AND C2 become accessible to data terminal
when gate T7, T8 , T5 AND T6 are made to conduct by
making X=Y=1
 When Vg > Vt(T1), T1 is ON and correspondingly capacitor
has no charge and T2 is OFF.
 If any operation is not performed for a long time the charge
of capacitor is lost due to leakage therefore refreshing is
needed.
 Refreshing is done be brief access from Vdd to cell this is
done by making T11 and T22 ON.
 Suppose initially T1 is ON and T2 is OFF, refresh is applied
through T12, T6 and C1 charges to previous value.
 note that since T2 is OFF all current goes to C1 allowing it
to replenish any charge due to leakage.

21. Comparison between SRAM and DRAM
SRAM DRAM
1. Data Volatility Y Y
2. Data refresh Operation N required
3. Cell structure 6T 1T-1C
4. Power consumption high/low high
5. Read Speed (latency) ~10/70 ns ~50ns
6. Write Speed ~5/40ns ~40ns
7. Cost high low
8. Power supply single single
9. Application ex. Cache Memory Main
Memory

22. Thank you