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Overview
Memory architecture defines how data is stored, accessed, and managed across different types of memory in a computer system. It directly impacts system performance, latency, and data throughput.
Terminology
| Term | Definition |
|---|
| Latency | Time between a request and data delivery |
| Throughput (Bandwidth) | Rate of data transfer (bits per second) |
| Buffering | Temporary storage of data during transfer |
| Cache Hit | Data found in cache; no need to access main memory |
| Cache Miss | Data not in cache; must be fetched from main memory |
| Temporal Locality | Recently accessed data is likely to be reused |
| Spatial Locality | Data near recently accessed locations is likely accessed next |
Core Concepts
Memory Hierarchy
Memory types are organized by speed, cost, and volatility. More volatile = faster but loses data without power.
| Level | Type | Volatile? |
|---|
| 1 | Registers | Yes |
| 2 | Cache | Yes |
| 3 | RAM | Yes |
| 4 | ROM | No |
| 5 | Secondary Storage (HDD/SSD) | No |
| 6 | External Storage (USB, Optical, Tape) | No |
RAM vs ROM
| Feature | RAM | ROM |
|---|
| Volatility | Volatile | Non-volatile |
| Speed | Fast read/write | Slower writes |
| Use | Active processes | Firmware storage |
SRAM vs DRAM
| Feature | SRAM | DRAM |
|---|
| Speed | Faster | Slower |
| Cost | More expensive | Cheaper |
| Refresh Required | No | Yes |
| Use | Cache memory | Main system memory |
Types of ROM
| Type | Description |
|---|
| PROM | Programmable once |
| EPROM | Erased with UV light; must be physically removed |
| EEPROM | Electrically erasable; reprogrammable in-circuit |
Cache Memory
High-speed memory sitting between the CPU and RAM that reduces access time to frequently used data.
- Cache Hit – Data found in cache; fast retrieval
- Cache Miss – Data not in cache; fetched from main memory
- Temporal Locality – Recently accessed data is reused soon
- Spatial Locality – Nearby data is likely accessed together
- Seek Time – Time to move read/write head to correct track
- Rotational Latency – Time for sector to rotate under the head
- Transfer Time – Time to read/write the data
- Spin-Up Time – Time for disk to reach operating speed
Disk Access Time = Seek Time + Rotational Latency + Transfer Time (+ Spin-Up if starting)
Secondary Storage
Hard Disk Drives (HDD)
- Magnetic storage with spinning platters
- Mechanical components → slower but cheaper
| Structure | Description |
|---|
| Tracks | Concentric circles on a platter |
| Sectors | Smallest unit (typically 512B or 4KB) |
| Clusters | Groups of sectors |
Solid State Drives (SSD)
- No moving parts; uses flash memory
- Faster and more reliable than HDDs
RAID (Redundant Array of Independent Disks)
Combines multiple drives for performance and/or redundancy.
| RAID Level | Strategy | Fault Tolerance |
|---|
| RAID 0 | Striping | None |
| RAID 1 | Mirroring | 1 drive |
| RAID 5 | Striping + Parity | 1 drive |
| RAID 6 | Double Parity | 2 drives |
| RAID 10 | Mirroring + Striping | 1 per mirror pair |
External Storage
Storage outside the main system — major attack vector, especially in air-gapped environments.
| Type | Description |
|---|
| Magnetic Tape | Sequential access, low cost, archival use |
| Optical Media | CD/DVD/Blu-ray; laser-based storage |
| SD Cards / Flash Media | Solid-state portable storage |
⚠︎Security Note
Physical media is a significant attack vector. A U.S. military breach was caused by an infected USB device that spread malware across classified networks.
References / Images
- Memory hierarchy diagram
- Cache structure visuals
- RAID layout diagrams
