GU1: 3. Memory

Memory: What is the definition of main memory?

Memory that is directly accessed by the CPU.

Memory: Where is main memory located?

Main memory sits directly on the motherboard and allows the CPU to read and write data almost instantly.

Memory: What are the three types of main memory?

RAM, ROM, cache

Memory: What is the role of ROM within a computer?

ROM stores firmware such as BIOS/UEFI instructions needed to start the computer and begin the boot process. ROM also contains low level instructions that help the CPU communicate with critical hardware components before main drivers are loaded.

Memory: What happens during boot?

After POST, a small program called the bootloader is loaded into RAM, then the bootloader loads the OS to RAM.

Memory: Why is RAM called "Random Access Memory"?

Because memory locations can be accessed directly and in any order, so reading the data takes the same amount of time regardless of where it is stored. (as opposed to ROM, which may be faster or slower depending how close to the circumference of the disk it is stored).

Memory: What is the main purpose of RAM?

To hold data and instructions temporarily.

Memory: How does SRAM store data?

Using flipflop circuits made from transistors.

Memory: Does SRAM need to be refreshed?

No - each bit is held in a stable state and does not need to be refreshed. This allows data to be accessed quickly without being lost when read.

Memory: How does SRAM compare to DRAM?

SRAM is faster, but more expensive with a lower capacity.

Memory: How does DRAM store data?

DRAM stores data using capacitors that hold electrical charge. Each bit is stored as either a charged or uncharged capacitor.

Memory: Does DRAM need to be refreshed?

Yes - DRAM must be constantly refreshed by reading and rewriting the data. This makes DRAM slower (than SRAM) but with higher capacity.

Memory: What is SDRAM and how is it different from DRAM?

SDRAM is Synchronous Dynamic RAM. DRAM is asynchronous.

Memory: How does SDRAM work?

SDRAM operates synchronously with the system clock, whereas DRAM operates asynchronously (slower) than the system clock. All signals are tied to the system clock for better controlled timing. SDRAM is what is used in more modern RAM DIMMs.

Memory: What does DDR stand for?

Double Data Rate

Memory: Why is DDR faster than SDR?

DDR Memory sends double the amount of data by using the rise and fall of the clock signal to send data (compared to SDR which only sends data on the rise of the clock signal).

Memory: What is "DIMM" and why is it called that?

Dual Inline Memory Module

This is the form of most types of RAM. It has two independent rows on pins (hence "dual").

Memory: How many pins can a DIMM module have?

168, 184, 240, or 288 pins

Memory: How do you calculate the bandwidth of RAM?

Multiply the bytes by the MHz:

A 64 bit DIMM with a speed of 200 MHz - 64 bits = 8 bytes

8 bytes x 200 MHz = 1600 MB/s bandwidth

Memory: What is ECC?

Error Correcting Code

Memory: What does ECC do?

Detects if the data was correctly processed by the memory module and makes a correction if needed.

Memory: Where is ECC commonly used?

Servers - as they have to be up and running at all times.

Memory: What is secondary storage?

Any memory not directly connected to the CPU.

Memory: What are some examples of secondary storage?

Solid-state drives

Flash drives

Magnetic storage

Optical storage

Memory: How does optical storage work?

Binary data is stored as variations of height on the disks surface - pits + lands. To read the data, light shines on the disk. If the light shines on a flat bit (land), it is reflected back up. If the light hits a divot (pit), the light bounces off at an angle. Data is written by a laser "burning" the pits into the disk.

Memory: How does electromagnetic storage work?

Binary data is written on the disk with electromagnets. It is either magnetised or demagnetised. The data is written and read with magnetic read/write heads. Not very portable and can be noisy and generates heat.

Memory: How does solid state memory work?

Solid state storage consists of digital circuits that retain data, which can then be changed.

Memory: What are the advantages of solid state memory?

Solid state storage is fast, but expensive, and is quieter and more reliable as it is non-mechanical.

Memory: What is cloud storage and where is the data stored?

Where data is stored on multiple servers in a remote location.

Memory: What type of storage do cloud storage servers use?

Servers use magnetic storage, although now also use solid state storage.

Memory: Are there any disadvantages to using cloud storage?

It can be costly depending on the storage required and the providers fees. It also requires internet access which may not be suitable for certain needs.

Memory: What is cache memory?

Cache is a type of memory that stores copies of frequently used and expected data.

Cache is much faster than RAM, but has a much lower capacity.

Cache is checked before RAM by the CPU.

Memory: Why is cache important?

By pre-loading required information, it speeds up the CPU.

Memory: What are the different levels if cache and where are they?

Level 1 - on the core - this is the cache the computer will check first and is often referred to as primary cache.

Level 2 - on the core - but was previously separate, so is sometimes still referred to as “external” cache. Bigger and slower than level 1.

Level 3 - on the processor - catches extra data from all the cores, so is referred to as “shared” cache. Bigger and slower than level 2.

Memory: What is masked ROM?

A type of non-volatile memory where data is permanently programmed directly into the chip during the manufacturing process. Because the data is physically "masked" or hardwired into the silicon, it cannot be altered, erased, or reprogrammed by the end user.

Memory: What is PROM?

Programmable Read-Only Memory — a type of ROM that is manufactured blank and can be programmed once by the user.

Memory: How is PROM programmed?

Special hardware is used to permanently write data by blowing tiny internal fuses. Because the fuses can only be destroyed once, this means that it can only be programmed once.

Memory: Can PROM be changed after programming?

No — it is permanent.

Memory: When is PROM used?

When data needs to be written once and never changed (e.g. firmware).

Memory: What is EPROM?

Erasable Programmable Read-Only Memory — a type of ROM that can be erased and reused.

Memory: How is EPROM erased?

Using ultraviolet (UV) light. Before you can reprogram the EPROM, the entire chip must be wiped.

Memory: How is EPROM reprogrammed?

After erasing, it can be rewritten using special equipment called an EPROM programmer which uses a high voltage electrical pulse.

Memory: What is a limitation of EPROM?

Must be removed from the computer to erase

Erasing takes time

The entire chip must be wiped before reprogramming

Memory: What is EEPROM?

Electrically Erasable Programmable Read-Only Memory — a type of ROM that can be erased and rewritten using electricity.

Memory: How is EEPROM different from EPROM?

It can be erased without removing it from the computer.

Memory: Can EEPROM be updated easily?

Yes — it can be rewritten many times electronically.

Memory: Where is EEPROM commonly used?

BIOS/UEFI firmware

Embedded systems

Plug and play profile data in keyboards and other peripherals

Cache Memory: What is a CPU core?

An individual processing unit.

Cache Memory: What is multi-core processing?

Using multiple cores to run tasks.

Cache Memory: What is cache hit?

When a computer system (CPU, web application etc) requests data and successfully finds it in cache.

Cache Memory: What is cache miss?

When a computer system (CPU, web application etc) requests data and does not find it in cache. The data must be retrieved from a slower main storage location.