What are the characteristics of Memory?
- Location
- Capacity
- Unit of transfer
- Access method
- Performance
- Organisation
What is Capacity normally expressed as?
Bytes
What are three ways of expressing memory performance?
- Access Time
- Memory Cycle time
- Transfer Rate
What is Access Time?
Time between presenting the address and getting the valid data (Stated as N clock) typically nanoseconds
What is Memory Cycle time
Time may be required for the memory to recover before next access
What is Transfer Rate?
Rate at which data can be moved - typically gigabytes
What are the characteristics of physical memory?
- Decay
- Volatility
- Erasable
- Power consumption
What type of physical memory use semiconductors?
- SRAM
- DRAM
- Flash
How did Core memory work?
- Magnetic ring for each bit
- Used high currents
- 1-6 µs cycle time
How does storage hierarchy looklike?
- Registers
- L1 Cache
- L2 Cache
- Main Memory
- Disk cache
- Disk, Flash/SSD
- Optical
- Tape
What is DRAM and how does it work?
Dynamic RAM
- Bits stored as charge in capacitors
- Charges leak so need refreshing even when powered
- Simpler construction
- Smaller per bit
- Less expensive
- Slower (6-60ns)
- Used in main RAM
- “switch” connects it to the read or write circuit
How does DRAM refresh work?
- Each bit discharges over time and is boosted back by the refresh
- This is a disadvantage of DRAM - but density is very high
What are the specs and characteristics of DDR5?
- Approx. 2 x DDR4
- Faster clocks (up to 4Ghz) -> more bandwidth
- Two independent 32 bit channels
- Some error correction built-in
- Lower voltage (1.1V) -> 20% lower power use
- Can burst read 64 bytes
- Capacity up to 96GB DIMM
What is SRAM and its characteristics?
Static RAM
- Bits sorted as on/off gates (using 4-6 transistors)
- No charges to leak, no refreshing needed when powered
- More complex construction - Larger per bit
- More expensive per MiB (100x)
- Faster (0.5 to 10ns)
- Good for Cache and embedded RAM
- Only used as main RAM on microcontrollers
What is ROM?
Read Only Memory
- Permanent storage (Non-volatile)
- Hardware support library subroutines
- Systems programs (BIOS)
What do CPUs use for DIMMs?
Two 64 bit channels
How can increasing DDR4 DIMMs help?
4 can increase bandwidth and decrease latency to/form cache as accesses can be interleaved
What type of errors can occur in memory?
- DRAM can loose data (e.g. 25k failures per Mbit per billion hours)
- Hard Failure (Permanent defect - most common)
- Soft Error (Random, non-destructive as there is no permanent damage to memory)
What are Caches?
- Caches are small blocks of fast SRAM located on CPU chip
- The CPU needs data faster than DRAM can provide so Memory requests go there, not DRAM
How does a cache operation work?
- CPU reads a memory location
- address goes to cache
- If present, cache provides data (cache hit)
- If not present, block read required from main RAM to cache (cache miss)
- Then deliver data request by CPU
What are the Cache Design parameters?
- Size
- Mapping function needed
- Replacement Algorithm
- Write Policy
- Block Size
- Number of Caches
Why is it preferable to use smaller L1 cache than a bigger L2 cache?
Big caches have longer latency
What is latency?
The time it takes for data to travel from one place to another
How does RAM get shielded from the cores with cache?
Multiple cores usually share a large level 3 cache to shield RAM from the cores
