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RAM and ROM

RAM is volatile memory used to hold instructions and data of currently running programs. It loses integrity after loss of power. RAM memory modules are installed into slots on the computer motherboard. RAM is also becoming increasingly embedded in computer motherboards, making upgrading difficult, if not impossible.

ROM (Read Only Memory) is non-volatile: data stored in ROM maintains integrity after loss of power. A computer Basic Input Output System (BIOS) Firmware is stored in ROM. While ROM is “read only,” some types of ROM may be written to via flashing, as we will see shortly in the “Flash Memory” section.

Note
The volatility of RAM is a subject of ongoing research. Historically, it was believed that DRAM lost integrity after loss of power. The “cold boot” attack has shown that RAM has remanence: it may maintain integrity seconds or even minutes after power loss. This has security ramifications: encryption keys usually exist in plaintext in RAM and may be recovered by “cold booting” a computer off a small OS installed on DVD or USB key, and then quickly dumping the contents of memory. A video on the implications of cold boot called “Lest We Remember: Cold Boot Attacks on Encryption Keys” is available at https://citp.princeton.edu/our-work/memory/.

Remember that the exam sometimes simplifies complex matters. For the exam, simply remember that RAM is volatile (though not as volatile as we once believed).

DRAM and SRAM

Static Random Access Memory (SRAM) is fast, expensive memory that uses small latches called “flip-flops” to store bits. Dynamic Random Access Memory (DRAM) stores bits in small capacitors (like small batteries), and is slower and cheaper than SRAM. The capacitors used by DRAM leak charge, and must be continually refreshed to maintain integrity, typically every few to a few hundred milliseconds, depending on the type of DRAM. Refreshing reads and writes the bits back to memory. SRAM does not require refreshing, and maintains integrity as long as power is supplied.

Firmware

Firmware stores small programs that do not change frequently, such as a computer’s BIOS (discussed below), or a router’s operating system and saved configuration. Various types of ROM chips may store firmware, including PROM, EPROM, and EEPROM.

PROM (Programmable Read Only Memory) can be written to once, typically at the factory. EPROM (Erasable Programmable Read Only Memory) and EEPROM (Electrically Erasable Programmable Read Only Memory) may be “flashed,” or erased and written to multiple times. The term “flashing” derives from the use of EPROMs: flashing ultraviolet light on a small window on the chip erased the EPROM. The window was usually covered with foil to avoid accidental erasure due to exposure to light. EEPROMs are the modern type of ROM, electrically erasable via the use of flashing programs.

A Programmable Logic Device (PLD) is a field-programmable device, which means it is programmed after it leaves the factory. EPROMs, EEPROMs, and Flash Memory are examples of PLDs.

Flash Memory

Flash memory (such as USB thumb drives) is a specific type of EEPROM, used for small portable storage drives. The difference is any byte of an EEPROM may be written, while flash drives are written by (larger) sectors. This makes flash memory faster than EEPROMs, but still slower than RAM.

Note
Firmware is chip-based, unlike magnetic disks. The term “flash drive” may lead some to think that flash memory drives are “disk drives.” They are physically quite different and have different remanence properties.

A simple magnetic field will not erase flash memory. Secure destruction methods used for magnetic drives, such as degaussing (which we will discuss shortly), will not work with flash drives.