Lesson Objectives Aims You 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 affects system performance Virtual Memory

RAM We should know by now: The computer consists of a few main components (CPU, Memory, Storage, I/O) All computers need software to do something This is just a list of instructions The CPU carries these instructions out (which is what makes it all “work”)

Definition Quite simply, RAM is: A temporary area of storage for programs and data that are currently being used

This includes the Operating System! Properties of RAM When you run a program: It is copied from storage in to RAM The program is used Data is stored in RAM whilst in use Once the program is closed, any changes/stored data is copied back to storage The memory is erased (marked as free) This includes the Operating System!

Why use RAM? Storage, whilst big, is very slow. Look at the relative speeds of computer components: Operation Access Time Relative time in seconds 1 CPU cycle 0.3 ns 1 s Level 1 cache access 0.9 ns 3 s Level 2 cache access 2.8 ns 9 s Level 3 cache access 12.9 ns 43 s Main memory access 120 ns 6 min Solid-state disk I/O 50-150 μs 2-6 days Rotational disk I/O 1-10 ms 1-12 months Internet: SF to NYC 40 ms 4 years Internet: SF to UK 81 ms 8 years Internet: SF to Australia 183 ms 19 years OS virtualization reboot 4 s 423 years SCSI command time-out 30 s 3000 years Hardware virtualization reboot 40 s 4000 years Physical system reboot 5 m 32 millenia

RAM We could operate a computer with no RAM but it would be sloooooooow. RAM is a happy medium between storage and CPU – running at about half CPU speed or more. RAM provides fast access to programs and data and is stable enough to be reliable in use

How it affects performance - Size The more RAM a system has, the more it can do quickly at any one time More RAM = More space for more OPEN programs and data This means the system SHOULD be faster – why? It does not need to rely on virtual memory (which is slow) It has more space for OPEN programs and data It would NOT give an increase in performance if you weren’t using all your RAM in the first place!

Types of Memory Cache Special type of memory found INSIDE the CPU Very small quantities (1-8mb typically) Runs at (or almost) CPU Speed Enables the CPU to run at full speed rather than idling when waiting for instructions from RAM Stores frequently/recently/about to be executed data/instructions Provides a massive performance increase as data does not have to be fetched from RAM

Flash – called memory but technically a storage technology Uses non-volatile memory chips Very fast permanent storage Limited read/write life Not as fast as RAM so doesn’t replace it in computer systems.

Memory and storage performance is a trade off between: Speed Size Volatility This is the reason why we don’t just get rid of memory and have fast storage instead…!

Performance Compared Cache RAM/ROM Flash Memory/SSD HDD Magnetic Backup Tape Faster, Smaller, More Volatile Slower, Bigger, Less Volatile

Virtual Memory Ask yourself a question: Why do we need storage if we have RAM? Or vice-versa – why do we need RAM if we have storage?

Virtual Memory If a computer runs out of RAM, it shouldn’t just crash/stop Instead we can use a file on the hard drive/secondary storage which pretends to be RAM This is called virtual memory Allows us to remove idle programs out of RAM to free up space Programs can be swapped in and out of physical memory as necessary Gives a solution to lack of RAM Will see a significant slow down in system performance when used

ROM The final type of memory is ROM Read Only Memory This cannot be changed, only read Holds code which is executed at power on (called firmware or BIOS or UEFI) Used to boot the computer – without it, the computer would do nothing at power on Usually the code will look to start an operating system of some sort Also used in games cartridges