Chapter 3

• what does memory refer to?

  Memory is usually referred to as the internal devices which the computer can access directly. This memory can be the user's workspace, temporary data or data that is key to running the computer.
• what does storage refer to?

  Storage devices allow users to store applications, data and files. The user's data is stored permanently and they can change it or read it as they wish. Storage needs to be larger than internal memory since the user may wish to store large files (such as music files or photographic images).

  
  

  Storage devices can also be removable to allow data, for example, to be transferred between computers. Removable devices allow a user to store important data in a different building in case of data loss.
• what is the primary memory?

  Primary memory is the part of computer memory which can be accessed directly from the CPU and, contains the random access memory (RAM) and read-only memory (ROM) memory chips. Primary memory allows the processor to access applications and services temporarily stored in memory locations.
• Is RAM really random?

  All computer systems come with some form of RAM. These memory devicesare not really random, it refers to the fact that any memory location can be accessed independent of which memory location was last used. Access time to locate data is much faster in RAM than in secondary devices.
• what can RAM do?

  RAM can also be

  » written to or read from, and the data stored can be changed by the user or by the computer

  » used to store data, files, part of an application or part of the operating system currently in use

  » volatile (memory contents are lost on powering off the computer).
• What happens as the RAM gets larger?

  In general, the larger the RAM, the faster the computer will operate. In reality, RAM never runs out of memory, it continues to operate but just becomes slower and slower as more data is stored. As RAM becomes 'full', the processor has to continually access the secondary data storage devices to overwrite old data on RAM with new data. By increasing the RAM size, the number of times this has to be done is considerably reduced, thus making the computer operate more quickly.
• how many types of RAM are there and what are they?

  dynamic RAM (DRAM)

  static RAM (SRAM).
• What is Dynamic RAM (DRAM)?

  Each DRAM chip consists of a number of transistors and capacitors. Each of these parts is tiny since a single RAM chip will contain millions of capacitors and transistors.

  » Capacitors hold the bits of information (0 or 1).

  » Transistors act like switches; they allow the chip control circuitry to read the

  capacitor or change the capacitor's value.
• why does RAM need to be constantly refreshed?

  This type of RAM needs to be constantly refreshed (that is, the capacitor needs to be re-charged every 15 microseconds otherwise it would lose its value). If it is not refreshed, the capacitor's charge will leak away very quickly, leaving every capacitor with the value 0.
• What are the advantages of DRAMs over SRAMs?

  DRAMs have a number of advantages over SRAMs.

  They:

  » are much less expensive to manufacture than SRAMs

  » consume less power than SRAMs

  » have a higher memory capacity than SRAMs.
• what is Static RAM (SRAM)?

  A major difference between SRAM and DRAM is that SRAM does not need to be constantly refreshed.

  It makes use of flip flops which hold each bit of memory.

  SRAM is much faster than DRAM when it comes to data access (typically, access time for SRAM is 25 nanoseconds and for DRAM is 60 nanoseconds).

  DRAM is the most common type of RAM used in computers, but where absolute speed is essential, for example in the processor's memory cache, SRAM is the preferred technology. Memory cache is a high speed portion of the memory.It is effective because most programs access the same data or instructions many times. By keeping as much of this information as possible in SRAM, the computer avoids having to access the slower DRAM.
• When asked about the differences between the differences of DRAMs and SRAMs, these are the bullet points to write about DRAMs.

  - consists of a number of transistors and capacitors

  - needs to be constantly refreshed

  - less expensive to manufacture than SRAM

  - has a higher memory capacity than SRAM

  - main memory is constructed from DRAM

  - consumes more power than SRAM under

  reasonable levels of access, as it needs to be constantly refreshed
• When asked about the differences between DRAMs and SRAMs, these are the bullet points to write about SRAMs.

  - uses flip-flops to hold each bit of memory

  - does not need to be constantly refreshed

  - has a faster data access time than DRAM

  - processor memory cache makes use

  of SRAM

  - if accessed at a high frequency, power

  usage can exceed that of DRAM
• what is ROM?

  Another form of primary memory is the read-only memory (ROM). This is similar to RAM in that it shares the same random access properties, but it cannotbe written to or changed. As the name suggests, ROM is a read-only memory device.
• what can ROM do?

  ROMs are:

  » non-volatile (the contents are not lost after powering off the computer)

  » permanent memory devices (the contents cannot be changed)

  » often used to store data which the computer needs to access when powering up for the first time for example, the basic input/output system (BIOS).
• main differences between RAM and ROM? (RAM)

  - temporary memory device

  - volatile memory

  - can be written to and read from

  - used to store data, files, programs, part

  of OS currently in use

  - can be increased in size to improve

  operational speed of a computer
• main differences between RAM and ROM? (ROM)

  - permanent memory device

  -non-volatile memory device

  - data stored cannot be altered

  -sometimes used to store BIOS and other

  data needed at start up
• what is PROM?

  A programmable read-only memory (PROM) is a type of ROM chip that can be altered once. A PROM is made up of a matrix of fuses. Programming a PROM requires the use of a PROM writer which uses an electric current to alter specific cells by 'burning' fuses in the matrix. Due to the method of programming (writing), a PROM can only be written to once. They are often used in mobile phones and in RFID tags.
• What is EPROM?

  An erasable programmable read-only memory (EPROM) is different to a PROM because they use floating gate transistors and capacitors rather than fuses. Ultra violet (UV) light is used to program an EPROM through a quartz window. They are used in applications which are under development, such as the programming of new games consoles.
• What are embedded systems?

  Embedded systems involve installing microprocessors into devices to enable operations to be controlled in a more efficient way. Devices such as cookers, refrigerators and central heating systems can now all be activated by a web-enabled device (such as a mobile phone or tablet). The time a central heating system switches on or off and the temperature can all be set from an app on a mobile phone from anywhere in the world.
• what are the pros of embedded systems?

  - small in size and therefore easy to fit into devices

  - relatively low cost to make

  - usually dedicated to one task, making for simple interfaces and often no requirement of an operating system

  - consume very little power

  - very fast reaction to changing input (operate in real time)

  - with mass production comes

  reliability
• what are the cons of embedded systems?

  - difficult to upgrade devices to take advantage of new technology

  - troubleshooting faults in the device becomes a specialist task

  - although the interface can appear to be simple, in reality it can be more confusing (changing the time on a cooker clock can require several steps, for example)

  - any device that can be accessed over the internet is also open to hackers, viruses, and so on

  - due to the difficulty in upgrading and fault finding, devices are often just thrown away rather than being repaired (wasteful)
• what are secondary storage devices?

  Secondary storage includes storage devices that are not directly accessible by the CPU. They are non-volatile devices which allow data to be stored as long as required by the user. This type of storage is much larger than primary memory, but data access time is considerably slower than RAM and ROM. All applications, the operating system, device drivers and general files (for example, documents, photos and music) are stored on secondary storage. The following section discusses the various types of secondary storage that can be found on the majority of computers. Secondary storage devices fall into three categories: magnetic, solid state and optical.
• what are hard disk drives (HDD)?

  type of magnetic storage device that uses spinning disks.
• How do you store data on the computer using HDD?

  Hard disk drives (HDD) are still one of the most common methods used to store data on a computer.

  Data is stored in a digital format on the magnetic surfaces of the disks(or platters, as they are frequently called). The hard disk drive will have a number of platters which can spin at about 7000 times a second. A number of read-write heads can access all of the surfaces in the disk drive. Normally each platter will have two surfaces which can be used to store the data. These read-write heads can move very quickly - typically they can move from the centre of the disk to the edge of the disk (and back again) 50 times a second.

  Data is stored on the surface in sectors and tracks.A sector on a given track will contain a fixed number of bytes.
• how do HDDs read data?

  To read the data, the head sweeps across the tracks on the spinning platters. A small magnetometer (a sensor that reads magnetic forces) reads the states of the magnetised dots underneath the read-write head. The magnetised states of the dots on the platters are then translated to 1s and 0s.
• how do HDDs write data?

  In order to write data onto the platters, the magnet on the end of the read-write head is used to generate a smallmagnetic field. The field causes the dots to switch state, becoming either magnetised or not-magnetised. When the magnetic field is taken away the dots keep their new state, allowing data to be stored until another magnetic field is applied.
• What is latency? (explanation) ( more than 2 marks)

  Unfortunately, hard disk drives have very slow data access when compared to, for example, RAM. Many applications require the read-write heads to constantly seek for the correct blocks of data; this means a large number of head movements. The effects of latency then become very significant. Latency is defined as the time it takes for a specific block of data on a data track to rotate around to the read-write head.

  Users will sometimes notice the effect of latency when they see messages such as, 'Please wait' or, at its worst, 'not responding'.
• what is latency? (short answer)

  the lag in a system; for example, the time to find a track on a hard disk, which depends on the time taken for the disk to rotate around to its read-write head.
• how does the data become fragmented in HDD?

  When a file or data is stored on an HDD, the required number of sectors needed to store the data will be allocated. However, the sectors allocated may not be adjacent to each other. Through time, the HDD will undergo numerous deletions and editing, which leads to sectors becoming increasingly fragmented, resulting in a gradual deterioration of the HDD performance (in other words, it takes longer and longer to access data). Defragmentation software can improve on this situation by 'tidying up' the disk sectors.

  An HDD is a direct access device; however, data in a given sector will be read sequentially.
• Definition of fragmentation?

  storage of data in non-consecutive sectors; for example, due to editing and deletion of old data.
• what are removable hard disk drives?

  Removable hard disk drives are essentially HDDs that are external to the computer and can be connected to the computer using one of the USB ports. In this way, they can be used as back-up devices or as another way of transferring files between computers.
• what are solid state drives?

  Latency is an issue in HDDs, as discussed earlier. Solid state drives (SSD) reduce this issue considerably. They have no moving parts and all data is retrieved at the same rate. They do not rely on magnetic properties. The most common type of solid state storage devices store data by controlling the movement of electrons within NAND chips. The data is stored as 0s and 1s in millions of tiny transistors (at each junction one transistor is called a floating gate and the other is called a control gate) within the chip. This effectively produces a non-volatile rewritable memory.
• what is the main difference between using NAND chips and EEPROM?

  However, a number of solid state storage devices sometimes use electronically erasable PROM (EEPROM) technology. The main difference is the use of NOR chips rather than NAND. This makes them faster in operation but devices using EEPROM are considerably more expensive than those that use NAND technology. EEPROM also allows data to be read or erased in single bytes at a time. Use of NAND only allows blocks of data to be read or erased. This makes EEPROM technology more useful in certain applications where data needs to be accessed or erased in byte-size chunks.
• What do the majority of SSD devices use?

  Because of the cost implications, the majority of solid state storage devices use NAND technology. The two are usually distinguished by the terms flash memory (use NAND) and EEPROM (use NOR).
• So, what are the main benefits of using an SSD rather than an HDD?

  Solid state drives:

  » are more reliable (no moving parts to go wrong)

  » are considerably lighter (which makes them suitable for laptops)

  » do not have to 'get up to speed' before they work properly

  » have a lower power consumption

  » run much cooler than HDDs (both these points again make them very

  suitable for laptop computers)

  » are very thin (because they have no moving parts)

  » access data considerably faster.
• what is the main draw back of SSD?

  The main drawback of SSD is the still unknown longevity of the technology. Most solid state storage devices are conservatively rated at only 20GB write operations per day over a three year period - this is known as SSD endurance. For this reason, SSD technology is not commonly used in servers, for example, where a huge number of write operations take place every day. However, this issue is being addressed by a number of manufacturers to improve the durability of these solid state systems and they are rapidly becoming more common in applications such as servers and cloud storage devices.
• What is one thing to note about when it comes to over-writing existing data on flash memory?

  Note that it is also not possible to over-write existing data on a flash memory device; it is necessary to first erase the old data and then write the new data at the same location.
• what technology do memory sticks use?

  Memory sticks/flash memories (also known as pen drives) use solid state technology. They usually connect to the computer through the USB port. Their main advantage is that they are very small, lightweight devices which make them suitable for transferring files between computers. They can also be used as small back-up devices for music or photo files, for example.
• why do complex or expensive software, such as an expert system, will often use a memory stick as a dongle?

  Complex or expensive software, such as an expert system, will often use a memory stick as a dongle. The dongle contains additional files which are needed to run the software. Without this dongle, the software will not work properly. It therefore prevents illegal or unauthorised use of the software, and also prevents copying of the software since, without the dongle, it is useless.