Computer System Fundamentals

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  • Created on: 25-11-20 12:16
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  • Computer Systems Fundamentals
    • Functional Components
      • Motherboard
        • RAM chips sit apart from the CPU on the motherboard.  For the data values to transfer from one place to the other, there must be a physical connection between the two. We call this connection a BUS.
          • The CPU accesses each RAM cell in turn, retrieves the fragment of code and processes (runs) it.i  This arrangement is called the Fetch-Decode – Execute cycle.
            • Cache
              • Cache is memory which sits between the main RAM and the CPU.
                • When the processor needs to read from or write to a location in main memory, it first checks whether a copy of that data is in the cache. If so, the processor immediately reads from or writes to the cache, which is much faster than reading from or writing to main memory.i
                  • Cache Memory can be located in the Processor itself, on the Motherboard, or both.  The closer the cache is to the processor, the higher the level – and the faster it can be accessed!
                    • Connected to the CPU by a special data bus called the Front Side Bus. 
          • Buses
            • Data bus - The RAM is connected to the CPU by copper circuit wires, which can carry electrical signals.  Data values are passed along this circuit, the data bus Bi-directions meaning data can flow between the MDR & RAM.
            • Address bus - The address bus carries binary signals which are then interpreted as address numbers, preparing the correct cell to send or receive data.  Uni-directional – it only ever points from the MAR to the RAM.
            • Control bus - connects the CU to all other components.  Caries timing and control signals.  Bi-directional as the CU sends out control signals and receives acknowledgements.
          • Whenever a program is running, it must first be loaded into RAM.  RAM is divided up into cells, each with a unique number. Each cell contains part of the program code, stored in a binary format.
            • The RAM capacitors can only hold their values for a very small period of time before they discharge.  To retain the values, the capacitors must be constantly refreshed with electrical current. We therefore refer to RAM as dynamic memory.
              • RAM is volatile – it loses all the stored information if the power is cut off.
              • RAM is cheap and has fast access time
              • Synchronous RAM (SDRAM) Sychronised to system busses.  Standard on most systems
              • Double Datarate RAM (DDRAM) - modern motherboards and processors use this RAM, which effectively doubles transfer rates by using both phases of the clock cycle.
              • DDR2/DDR3 -  have now replaced the original DDR specification.   These can transfer data at 4x the normal rate per clock cycle and use less power.  They use 240 pins instead of the original 168. 
              • Programmable Read Only Memory (PROM.  PROM is manufactured blank.  It has its program burned onto it at a later point using a PROM burner.  programmed once – it cannot then be wiped clean and re-used. Slow access & high cost
              • Eraseable Programmable Read Only Memory (EPROM) and Electrically Erasable Read Only Memory (EEPROM)  Reprogrammable under certain conditions.  EPROM usis UV light  EEPROMS using electrical current.. Both oftern used as firmware.Non-volatile.  Epensive and slow access.
              • Static Random Access Memory (S-RAM). Do not confuse this with SDRAM! Modern computers ship with S-RAM installed.     Bi-polar electric cells which behave like small batteries and do not need to be constantly refreshed with current.  Volatile.  Faster than DRAM, stable but needs more pwoer and is expensive so onlye ever uses as memory cache.
        • ROM - ROM chip role is to start the computer, perform the Power On Self Test (POST), load basic input and output services, then load the operating system. 
          • Sometime known as Basic Input Output System (BIOS) chip
          • Non-volatile - retains programming when power is switched off.
            • ROM chips have programs burnt onto them.  Cannot be reprogrammed.
              • They are comparatively expensive to produce and their access times are very slow compared to RAM.
      • CPU - Central Processing Unit (Processor)
        • Registers, store info in binary.  Made of capacitors (can be charged or discharged
        • Classic Diagram - 7 components Control Unit, ALU, Accumulator, Instruction Register, Program Counter, Memory Address, Memory Data
          • ALU - performs calculations or Boolean logic, data is sent to the Arithmetic and Logic Unit. The ALU also consists of capacitors and transistors
            • Accumulator - Like the CU, the ALU needs a “workspace” to manipulate values.  This is called the Accumulator




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