Describe each of the following? SRAM, Pseudo-SRAM, DRAM, ROM, PROM, EPROM, EEPROM, MRAM, FRAM, ...

SRAM (Static RAM) — Volatile read/write memory built from a 6-transistor (6T) bistable latch per cell. It holds its value as long as power is applied and needs no refresh. It is very fast and has uniform, deterministic access time, but is low-density and expensive (6 transistors per bit) and consumes static leakage power. Used for caches, register files, tightly-coupled memory, and small fast on-chip buffers.

Pseudo-SRAM / PSRAM — A DRAM storage core (so it is dense and cheap like DRAM) wrapped in on-chip control logic that performs refresh automatically and presents an SRAM-like asynchronous interface to the system. The designer gets large capacity at low cost without having to manage refresh in software. Common in cost- and pin-constrained embedded systems (e.g., cellphones, microcontroller external memory). Self-refresh is hidden from the host.

DRAM (Dynamic RAM) — Volatile memory storing each bit as charge on a tiny capacitor with one access transistor (1T1C). Extremely dense and cheap per bit, but the charge leaks, so it must be periodically refreshed (every cell read/rewritten within tREF, typically ~64 ms). Access involves row activation (RAS), column access (CAS), and precharge, giving it non-uniform latency. Used as main/bulk system memory.

ROM (Mask ROM) — Non-volatile, read-only memory whose contents are fixed at fabrication time by a photomask. Cannot be changed after manufacture. Cheapest per bit at high volume; used for fixed firmware/boot code and constant tables in mass-produced products.

PROM (Programmable ROM) — One-time programmable (OTP) non-volatile memory. Shipped blank and programmed once by the user (e.g., by blowing fuses/antifuses). Cannot be erased or reprogrammed afterward.

EPROM (Erasable PROM) — Non-volatile, electrically programmed but erased by UV light through a quartz window on the package. Erase is bulk (whole chip) and takes minutes of UV exposure. Reusable but inconvenient; largely obsolete, superseded by EEPROM/Flash.

EEPROM (Electrically Erasable PROM) — Non-volatile, electrically programmable and erasable byte-by-byte in-circuit. Convenient for storing small amounts of configuration/calibration data. Has limited write/erase endurance (typically ~10^4–10^6 cycles per cell) and slow writes relative to reads. Available as parallel and serial (I²C/SPI/Microwire) parts.

MRAM (Magnetoresistive RAM) — Non-volatile memory that stores bits in the magnetic orientation of a magnetic tunnel junction rather than as charge. It is fast (approaching SRAM/DRAM speeds), effectively unlimited (very high) endurance, byte-writable, and non-volatile — making it attractive as a unified memory. Higher cost and lower density than DRAM/Flash today.

FRAM / FeRAM (Ferroelectric RAM) — Non-volatile memory using a ferroelectric capacitor whose polarization state stores the bit. It offers very fast, low-power writes, byte-level write access, and very high endurance (often 10^10–10^14 cycles), far exceeding EEPROM/Flash. Lower density and higher cost than Flash, and traditional read is destructive (requires write-back). Excellent for frequently-updated logs, counters, and metering data.

(Flash, for completeness) — Non-volatile memory that is electrically programmable but erased in blocks/sectors (not per byte). Comes in NOR (random-access, code storage, execute-in-place) and NAND (high-density, page-oriented, mass storage). Limited endurance; requires erase-before-write. See questions 7 and 8.