Semiconductor Memory
- RAM
— Misnamed as all semiconductor memory is random access
— Read/Write
— Volatile
— Temporary storage
— Static or dynamic
Dynamic RAM
- Bits stored as charge in capacitors
- Charges leak
- Need refreshing even when powered
- Simpler construction
- Smaller per bit
- Less expensive
- Need refresh circuits
- Slower
- Main memory
Dynamic RAM Structure
DRAM Operation
- Address line active when bit read or written
— Transistor switch closed (current flows)
- Write
— Voltage to bit line
– High for 1 low for 0
— Then signal address line
– Transfers charge to capacitor
- Read
— Address line selected
– transistor turns on
— Charge from capacitor fed via bit line to sense amplifier
– Compares with reference value to determine 0 or 1
— Capacitor charge must be restored
Static RAM
- Bits stored as on/off switches
- No charges to leak
- No refreshing needed when powered
- More complex construction
- Larger per bit
- More expensive
- Does not need refresh circuits
- Faster
- Cache
Static RAM Structure
SRAM v DRAM
- Both volatile
— Power needed to preserve data
- Dynamic cell
— Simpler to build, smaller
— More dense
— Less expensive
— Needs refresh
— Larger memory units
- Static
— Faster
— Cache
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