System Architecture

System Architecture

1. FDE CYCLE

   • Fetch instructions
     • Essentially, all a CPU does is carry out instructions, one after another, billions of times a seconds. The Fetch-Decode-Execute cycle describe how it does it.
       • 1)Copy memory address from the program counter to the MAR
         • 2)Copy the instructions stored in the MAR address to the MDR.
           • 3)Increment (increase) the program counter to the point to the address of the address of the next instructions, ready for the next cycle
             • Harvard architecture Stores programs and data in separate memory units. Entering or altering programs is harder with this method
               • Peripheral Also known as a device. A piece of hardware connected to the central processing unit / computer to provide input, output or backing storage
                 • BIOS Basic Input and Output System. This is the ROM chip on the motherboard that takes care of booting the OS into memory
               • RAM (Random Access Memory) is a device that can plug into the motherboard. RAM contains programs, instructions and data that are currently being worked on (in use)
               • ROM (Read Only Memory) is usually a flash memory chip on the motherboard that contains instructions needed to start up (boot) the operating system
     • Decode instructions
       • The instruction in the MDR is decoded by the CU. The CU may then prepare for the next step, e.g by loading values into the MAR or MDR
         • CPU Central processing Unit - This processes all of the data and instructions that make a computer system work
           • Bootstrap Loader Responsible for loading (Booting) the Operating System into memory
     • Execute instruction
       • The instructions is performed. This could be: Load data from memory, write data to memory, do a calculation or logic operation (using the ALU), change the address in the PC, or halt the program
         • Von Neumann architecture Traditional computer architecture that forms the basis of most digital computer systems. Programs and data can be stored in the same memory unit.
           • MAR (Memory address register) Holds memory address about to be used by the CPU
             • Embedded systems A dedicated computer system built into another device (usually designed for a specific purpose) e.g. Dishwasher or microwave. Cheaper to produce and more efficient than general purpose computer.

2. Registers

   • Accumulator - The part of the ALU that stores the intermediates results when doing a calculation
     • ALU (Arithmetic Logic Unit) Performs logical operations (AND, OR, NOT), Shift operations and Arithmetic operations (+,-, *, / )
       • FDE (Fetch Decode Execute) cycle Fetches instuctions and data from memory and brings them back to the CPU to be decoded and executed (run)
         • Data Capacity A characteristic of secondary storage which means - "How much can the storage device hold?" (usually measured in kb, Mb, Gb, Tb)
           • Cost A characteristic of secondary storage which means - "How expensive is the storage device (per GB)?"

3. Hardware The physical (you can touch them) components that make up a computer system (e.g. RAM, fan, mouse)

   • Cores How many individual processors (cores) are on a chip. The more cores, the more instructions can be done simultaneously.

4. Software The virtual (you can't touch them) programs or applications that a computer can run (e.g word processor, browser, file manager)

   • Cache size Refers to how many frequently used instructions can be stored in the cache on the CPU.

5. Input device Device that transmits data into the computer so that it can be processed (e.g. buttons, gamepad, keyboard, microphone)

   • CU (Control Unit) Controls activities in the CPU (Such as flow of data between CPU and other parts) and makes FDE cycle happen in correct orde
     • Non volatile Means that when power is removed, data is retained/kept
     • Virtual memory This type of memory is for when the computer runs out of available RAM and so uses part of the HDD (Hard Disk Drive) instead. This is not desirable however as the HDD is slower than just using RAM.
   • Cache Sits between the CPU and RAM. Very fast. Holds frequently used data so that the processor doesn't have to keep going back and forth to the RAM.
     • Flash memory This type of memory is solid state (i.e. No moving parts) that retains data even when power is removed. It is used in SSDs and memory cards, for example.
     • Primary storage Sometimes called Main Memory. Refers to the memory areas that the CPU can access very quickly (Cache, RAM and ROM). Primary storage has the fastest read/write speeds and is mostly volatile.
       • Optical storage A form of storage that uses light/lasers to read and write data (e.g. CD ROM, CD-RW, DVD ROM, Blu-ray)
         • Speed A characteristic of secondary storage which means - "How fast can data be read/written?" (usually measured in MB/sec)
         • Portability A characteristic of secondary storage which means - "Can you unplug/carry it around easily?"
         • Durability A characteristic of secondary storage which means - "How long is the product expected to function for before it is no longer useable?"
     • Secondary storage Also known as backing storage. This is non-volatile - it's where all data (operating systems, applications and user files) are stored when not in use. Read/write speeds are much slower compared to primary storage.
       • Magnetic Storage A form of storage that uses magnetism to read and write data(e.g HDD, Floppy Disc, Cassette Tape, Magnetic tape)
       • Solid state storage A form of storage that uses flash memory chips to store data(e.g. SSD (Solid State Drive), USB flash memory, SD card). It has no moving parts.
         • Reliability A characteristic of secondary storage which means - "How likely is it that the product will fail in its lifespan?"
   • Volatile Means that when the power is removed then all data is lost.

6. Output device Device that presents the results of processing to the user (e.g. screen, speakers, LED, Motor)

   • MDR (Memory Data Register) Holds data or instructions that have been fetched from memory
     • Accumulator Stores the results of the calculations done by the ALU
       • Clock speed Number of instructions that can be carried out in a second (Hz) - These days in Ghz (Billion)
   • Program Counter Holds the memory address of the instruction for each cycle