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Candidates should be able to:
 describe the difference between RAM and ROM
 explain the need for ROM in a computer system
 describe the purpose of RAM in a computer system
 explain how the amount of RAM in a personal computer affects the performance of the computer
 explain the need for virtual memory
 describe cache memory
 describe flash memory
 discuss how changes in memory technologies are leading to innovative computer designs.
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 Memory is made up of semi-conductor switches. The position of the switch (open or closed) is used to represent the binary numbers 0 or 1.
 A single unit of memory is called a BIT and can store 0 or 1.
 Two BITS together allow 4 different binary numbers to be stored: 00 , 01 , 10 or 11.
 Eight BITS can store 256 different numbers:
00000000 through to 11111111
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RAM – Random Access Memory
 The contents of RAM can be altered so a computer can both read from and write to memory addresses in RAM.
 RAM is described as volatile meaning that if the power is switched off or the battery removed then the contents will be lost .
ROM - Read only Memory
 The contents of ROM cannot be altered so a computer can only read from memory addresses in ROM.
 ROM is described as non-volatile meaning that if the power is switched off or the battery removed then the contents are not lost .
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 ROM is needed for devices where programs or data must not be lost when the power is turned off or batteries are removed etc.
 Embedded computers such as those in microprocessor controlled devices use
ROM to store the software to control the hardware, as well as data such as cooking/washing times etc.
 A typical computer uses special ROM called the BIOS ( Basic
Input/Output System ) which permanently stores the software needed to access computer hardware such as the hard drive and then load an operating system into RAM and start to execute it.
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 When a personal computer is in use the following are copied into RAM from the backing storage:
 The Operating System (OS)
 All the other programs that are running
 Any data files that are in use.
 RAM is needed for this task because it would be far too slow for the CPU to directly access this data from the secondary storage .
 New files or changes made to files while the computer system is in use need to be saved before is switched off. This is done by copying the data from RAM to the secondary storage.
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 Memory is linked to the CPU by two different types of BUS, the address bus and the data bus ( a BUS is a set of very thin parallel wires that transmit binary data ).
 The ADDRESS BUS identifies the location of the data ( a bit like a house number/postcode does ).
 The DATA BUS is then used to transfer the contents of the memory address ( the data ) between the memory location and the CPU ( and visa-versa with RAM ).
MEMORY ADDRESS
1 0 0 1 1
1 0 1 0 0
1 0 1 0 1
MEMORY CONTENTS
1 1 0 0 0 1 1 0
1 0 0 0 1 1 0 0
1 0 0 0 0 0 1 1
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 When a computer is running, the operating system , all the other running programs plus any files in use are all stored in RAM.
 This is because secondary storage ( such as a hard drive ) would be far too slow for the CPU.
 If there is insufficient RAM then the CPU has to constantly transfer blocks of data between RAM and virtual memory .
 This will significantly slow the rate that the computer can process instructions and data.
 SUMMARY : The more RAM installed in a personal computer, the higher the performance.
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 Because there is only a limited amount of RAM in a computer it is possible for it to run low if too
many programs are running at one time OR too many large files have been loaded.
 However, at any one time only some blocks of the data in RAM are actively being used by the CPU so it is possible for the OS to copy blocks of inactive data from RAM to the hard drive and copy active blocks of data into RAM to be accessed by the CPU.
 To the CPU, it therefore appears that all the programs are running from RAM and each program behaves as if it had as much RAM as it needed, hence the term virtual memory.
 This solution is a very cheap but does have its limitations.
 Because the hard drive is so much slower than RAM, this swapping of inactive and active data blocks will cause a slight delay as the computer switches tasks.
 If a computer is very low on RAM or accessing many different areas of memory then it will slow down dramatically because too much time will be spent swapping blocks of data between real and virtual memory.
 The area of the hard disk that acts as the virtual memory is called a page file and the size can set using the
Operating System. It is usually set to be around twice the size of the RAM.
 SUMMARY: Virtual memory is memory that uses secondary storage but to a program it appears as if the whole program is loaded and running from RAM .
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 Cache memory is a small amount of very fast memory that is built into the CPU. Blocks of instructions and data that are in use by the
CPU are copied from RAM into cache memory, along with the associated memory addresses.
 If the CPU needs to access a memory address it first checks the cache memory to see if there is a match. If there is then it access the contents of the cache version.
A CPU with a larger cache memory and more levels of cache memory will have a higher performance than one without cache memory.
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 Flash memory is a special type of RAM that, unlike normal RAM, is non-volatile ( it does not need a power supply to preserve the memory contents ). Flash memory cannot however replace RAM in a computer as the read/write speeds are too slow.
 Flash memory can be described as a solid state storage device because the contents are non-volatile and there are no moving parts. Typical storage sizes range from 1GB to 16GB and higher.
 Examples of flash memory in use :
 Compact Flash™ and Secure Digital™ ( SD ) memory cards in digital cameras.
 Mini and Micro SD cards in smart phones.
 Memory cards for video game consoles.
 USB memory sticks.
 SUMMARY : Flash memory combines the permanence of ROM with the flexibility of
RAM, but not the speed.
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How are changes in memory technologies leading to innovative computer designs?
SUMMARY : Memory technology has developed significantly over the years. Changes include:
 Greater storage density; ( the number of bits per chip has increased from almost 1 kilobit (Kb) to 2 gigabit (GB) per chip ) meaning computer systems, in particular portable ones, can be much smaller.
 Faster read/write times; allowing increases in the performance of computer systems, although these speed increases still lag behind the advances in processor speeds.
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Less power consumption; allowing mains computers to consume less electricity and portable computing devices to last longer on battery power or use smaller batteries and be even more portable.
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Lower costs per gigabit; making it affordable to install significant amounts of RAM in quite basic computer systems.
 Flash memory capacity has risen to the point where solid state storage becomes a realistic alternative to the hard drive in smaller systems. This means that a computer can start operating far more quickly when it is started up as the data can be copied into RAM so much faster.
 Cache memory built into the CPU has made data transfer between the processor and RAM far more efficient, allowing significant increases in performance at a reasonable cost.
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