Download What*s a RAM?

What’s a RAM?
• Random Access Memory
• Two main types: Static RAM (SRAM) and
Dynamic RAM (DRAM)
– Differences lie in how bits are stored
– Other types: Flash RAM, SDRAM, Video RAM,
FERAM
Basic RAM Architecture
Word Lines
Bit Cell
Bit Lines
High
Sense Amplifier
Low
Address
Data
Static RAM (SRAM)
Word
Line
Read: Drive word line, sense
value on bit lines
Write: Drive word line, drive
new value (strongly) on bit lines
!Bit
Bit
Accessing a Static RAM
CE
Addr
Data
Read
Write
Note: CE signal is often active-low as opposed to how shown here. SRAMs also
generally have a write enable signal
Dynamic RAM (DRAM)
Word Line
Read: Drive word line, sense
value on bit line (destroys saved
value)
Write: Drive word line, drive new
value on bit line.
Bit Line
Dynamic RAM Timing (Read)
RAS
CAS
Addr
Again, control signals are often active-low
Static vs. Dynamic RAM
• Static RAM
– Fast (active drive)
– Less dense (4-6 transistors/bit)
– Stable (holds value as long as power applied)
• Dynamic RAM
– Slower
– High density (1 transistor/bit)
– Unstable (needs refresh)
Neither device holds data if power removed