Memory Types

1. RAM (Random Access Memory)

1.1 What is it?

• Volatile memory for fast, temporary storage.
• Random access: any memory cell can be read/written in constant time.
• Types: SRAM (Static), DRAM (Dynamic), plus advanced like RLDRAM, LPDDR.

1.2 Why we need it

• Enables high-speed data processing by CPU/MCU.
• Acts as a workspace for variables, stacks, program execution, and caching.

1.3 Hardware Details

• SRAM: Each bit stored using 6 transistors (cross-coupled flip-flops). No refresh needed.
  • Fast access, low density, low power in standby, higher power in active mode.
  • Used for: CPU L1/L2/L3 caches, MCU internal RAM.
• DRAM: Each bit stored as a charge in a capacitor, with an access transistor.
  • Requires periodic refresh (every few ms) to prevent data loss.
  • High density, low cost, slower than SRAM.
  • Used as: main memory in PCs, high-end MCUs with external DRAM interfaces.
• LPDDR (Low Power DDR): Special DRAM for mobile devices, with deep power-saving modes.

1.4 Software/Embedded Use

• Stack/Heap Management: Compilers and RTOS allocate program stack, dynamic heap, and static/global data.
• DMA (Direct Memory Access): Peripheral devices access RAM directly, offloading CPU.
• Double-buffering: Used for real-time sensor data (e.g., ADC/DAC), audio streams.
• Caching: Managed by CPU hardware, but software can optimize cache use (e.g., data alignment, cache flush).

1.5 Use Cases

• MCU SRAM for fast I/O buffers.
• PC DRAM for multitasking, large datasets.
• DSPs (Digital Signal Processors) use on-chip RAM for real-time signal processing.

1.6 What more can we do?

• Error correction (ECC RAM) for mission-critical systems.
• Memory protection units (MPU) or MMUs for process isolation.
• Low-power retention RAM for always-on IoT.

2. ROM (Read Only Memory)

2.1 What is it?

• Non-volatile memory for fixed program/data storage.
• Variants: Mask ROM, PROM, OTP (One Time Programmable), EPROM, EEPROM.

2.2 Why we need it

• Secure, unchangeable code storage for bootloaders, root of trust.
• Reliable retention over years/decades.

2.3 Hardware Details

• Mask ROM: Data physically encoded during fabrication by connections/fuses.
• PROM: User-programmable by blowing fuses with high current.
• EPROM: Floating gate transistors, erased by UV light exposure.
  • Requires quartz window in IC package.
• OTP ROM: Special case of PROM/EPROM, programmed only once electrically.

2.4 Software/Embedded Use

• Bootloader or BIOS code runs directly from ROM at power-on.
• Interrupt vectors and reset handlers often stored in ROM.
• Security: ROM-stored cryptographic keys, root certificates.

2.5 Use Cases

• Game console cartridges (classic Nintendo, Sega, etc.).
• Factory-programmed firmware for smartcards.
• Security chips for anti-tamper applications.

2.6 What more can we do?

• Combine with Flash/EEROM for field upgrades.
• Use ROM for trusted computing base in secure IoT.

3. Flash Memory

3.1 What is it?

• Non-volatile, solid-state memory: reprogrammable, block/sector-erasable.
• Two main architectures: NOR and NAND.

3.2 Why we need it

• Cheap, high-density, reliable for program/data storage.
• Enables field updates (firmware, OS), mass storage (filesystems).

3.3 Hardware Details

• Cell: Floating-gate MOSFET, stores electrons to represent data (charge = 0/1).

• NOR Flash: Parallel access, allows direct code execution (XIP - Execute In Place).

  • Fast random read, slower writes/erase, used for code storage in MCUs.

• NAND Flash: Serial/block access, higher density, better for bulk storage.

  • Used in SSDs, SD cards, eMMC/Universal Flash Storage (UFS).

• Endurance: Limited program/erase cycles (10k–1M cycles/sector for NOR, ~1k–100k for NAND).

• Wear Leveling: Algorithms to spread wear evenly, crucial for file systems.

3.4 Software/Embedded Use

• Firmware Updates (OTA): Store new firmware, use bootloader to switch/rollback.
• File systems: FAT, YAFFS, JFFS2, LittleFS designed for Flash’s block/erase constraints.
• Bad Block Management: ECC and remapping in drivers or hardware.

3.5 Use Cases

• Code/data storage in MCUs (e.g., STM32, ESP32 internal flash).
• SD card for data logging in IoT.
• SSDs in consumer laptops/servers.

3.6 What more can we do?

• SLC (Single-Level Cell) for endurance, MLC/TLC/QLC for density.
• Integrate with secure enclaves for encrypted storage.
• 3D NAND: stacking layers for even greater density.

4. EEPROM (Electrically Erasable Programmable ROM)

4.1 What is it?

• Non-volatile memory, byte-addressable, electrically erasable/programmed.
• Retains data for 10–100+ years.

4.2 Why we need it

• Store configuration, calibration, IDs, security keys that may change infrequently.
• Data survives power loss.

4.3 Hardware Details

• Similar cell to Flash (floating-gate transistor), but individual bytes can be written/erased.

• Write/erase speeds: ~1-10 ms/byte, compared to microseconds for RAM.

• Endurance: 1M+ cycles per byte (less than RAM, more than Flash in some cases).

• Common interfaces: I2C, SPI, parallel.

4.4 Software/Embedded Use

• Store user settings, device serials, error logs.
• Implement “wear leveling” in firmware to avoid frequent rewrites of same cell.
• Debounce writes: accumulate changes in RAM, commit infrequently.

4.5 Use Cases

• I2C EEPROM (e.g., 24Cxx chips) for external MCU storage.
• Storing device MAC addresses, production calibration.
• RFID tags, smartcards.

4.6 What more can we do?

• Use secure EEPROM for cryptographic elements (smartcards, TPM).
• Implement rolling logs/circular buffers in EEPROM for fail-safe logs.

5. Additional Advanced Types

5.1 NVRAM (Non-Volatile RAM)

• Battery-backed SRAM or modern MRAM/FRAM.
• Data retained after power loss.
• Used for RTCs, last-state retention in critical apps.

5.2 MRAM (Magnetoresistive RAM)

• Uses magnetic tunneling junctions to store bits (not electric charge).
• Non-volatile, unlimited endurance, SRAM-like speed.
• Still expensive, used in aerospace, automotive, and critical IoT.

5.3 FRAM (Ferroelectric RAM)

• Uses ferroelectric layer to store bits, non-volatile, extremely high write endurance.
• Used for fast, reliable log/parameter storage in low-power MCUs.

5.4 Software integration for all types:

• Memory-mapped I/O: Peripherals/flash mapped to CPU address space for direct access.
• RTOS/bootloader: Partition memory regions for code/data/upgrade slots.
• Memory drivers: HAL (Hardware Abstraction Layer) provides unified APIs for multiple memory chips.

6. System Integration and Security

Hardware

• Memory buses (parallel/serial): Impact bandwidth and latency.
• Address decoding: Maps physical memory devices to logical address spaces.
• Power domains: Isolate/retain critical memory across power states.

Software

• Memory allocators: Heap/stack control, dynamic allocation.
• Filesystem drivers: Flash-friendly algorithms, error recovery.
• Security: Isolate sensitive code/data in ROM/secure Flash; implement access control, encryption.

7. Comparison Table (Deeper Metrics)

*Battery-backed NVRAM loses data if battery dies.