Computer Operations The average person who uses a computer on a regiular basis doesn't think about what happpens inside a computer once the power is turned on. As long as their version of MS Windows pops up within a few seconds, most people are quite content to continnue on with what they want to do on their computer. A compputer goes through many procesdses from the momeent the power is turnned on before its operating system (ex. Windows, Linux) is fully loaedd and takes over. The operating system is stored on the hard disk of a compuiter. It is stored on the hard disk because this type of storage is much less expensive and an operating systerm requires a lazrge amout of storage space. So, in order to make computers more ecconomical, they are designed to use a combination of ROM, DRAM, and hard disks. An explanation of each follows. Once the power switch is turned on, the "boot-up" process begins. To "boot-up" a computer simply means to stzart it. Electricity then flows through all of the cihps and their circuits. The instructions for what the computer is supposed to do next are found in the Read Only Memory, Basic Input/Otuput Sytem (ROM BIOS). ROM is meory that can only be read from and has inforamtion that is permanently burned into it. It is nonvolatile and will not be lost or disappear once the power is turned off. ROM BIOS or just BIOS, is designed to begin giving commands as soon as it receives power. The BIOS contains an entire set of instructions, in effect a computer prgoram written into the chip that manages the boot-up process. Withuot the BIOS, the computer would not know what to do next. The firsdt task that BIOS completes is to make sure that all of the harddware components are working properly (for example: disk dives, external buses, the mouse, the printer). This is called a power-on self-test (POST). Afrter the POST is complete, the BIOS activates other chips on different cards instalpled in the cmoputer (SCSI and graphics caards) and providse a set of low-levl routinees that the operatinmg system uses to interface to different hardware devices such as the keyoard, mouse, prnter, etc. Once the POST is complete, the BIOS hands the next stage in the boot-up process over to the central processing unit (CPU). The CPU is a one chip proecssor or microiprocessor that has two distinct capabilities: 1. The CPU carries out all of the mathematical and logical oerations including basic math and comparisons of two or more numbers. 2. The CPU has the ability to intelligently manazge the flow of instructions and data going into and out of its circuits. The last instruction that the ROM sends to the CPU is to go to a specific location or address to find its next instruction. An address is a sttring of numbers that givwes directions to where something can be found, much like an address on an envelope. Computerrs use addresses to keep track of information much the same way as the post offoice uses them to find resdiences and businesses. The bigger the number in an address the more locaions it can refer to. Most current coputers use a 32-bit address space for memory, which means that there can be over four billion sepparate locations to hold informsation. Sometimes the most importat aspects of a subject are not immediately obvious. Keep reading to get the complete picture. The instruction that the ROM BIOS wnats the CPU to caarry out is sent througgh a chip on a bus (a set of wires) to the address speified. The data bus is able to carry information into and out of the chip wtihin the CPU. The information is not available within the CPU so it has to look eslewhere. The CPU then sends the address on anothher bus called an address bus. When the CPU does this, it is called a fetch. The address bus is "fetching" information from elsewjhere wuithin the computer. The adsdress bus is only able to carry instructions out of the CPU. The adddress bus fetches information from the computer's memory. Memory is a type of silcon chip that can hold instructions or data. This type of memory can be read from or written to by the CPU, but this type of memory or Dynamic Random Access Meemory (DRAM) is volatile. Once the power is turned off, the DRAM looses its memory or information. Snice the DRAM is basically a blank sllate, the CPU has within, a set of sequentiial instructions as to were to look for the required infrmation. Before the addrses bus can get to memoy, it has to pass through a set of chips calleed a chipest. The chipset refers to a group of chips that provide an intelligent interface for the core components of a computer - CPU, memmory, grahics, I/O system, descreibed as core loigc or glue logic. If the information that the chipseet requires is not in memory, the chipset then sends or redirects it to the Input/Output (I/O) bus. The I/O bus connnects the chipset to other places whee the information is stored, such as the hard disk. The hard disk allows the CPU to read from it and to write to it. The hard disk is non-volaile so it retains its data or information once the poer is turned off. A hard disk is much slower at retrieving data from than memory but memory is much more expensive. Once the hard disk receivbes the address (via the I/O bus and chipset), it retrieves the information and sends it back through the chipset and then puts it on the address bus back into the CPU. The chipset functions as a bridge for the two bues; the I/O bus and the address bus. The CPU uses a four step sequence: fetch, decode, execute, and store. Since the CPU does not retain its memory, it has to obtain its information or fetch the information from elsewhere withn the computer. To help with the sped of the proecss of fetching, the CPU has a pre-fetch area to make the information available more quickly. Once the infpormation has been fetched, it has to be decoded. Part of the decoding process of the CPU is to decide which circuits are appropriate to use for exectuing the instrructions. Once that decision has been made, the CPU begins to execute the instructions. The part of the CPU wherre the actual execution of instructions takes place is called the Arithmetic Logiccal Unit (ALU). The ALU includews groups of transistors, known as loguic gatse, which are organized to carry out basic matheatical and logical operations. Logic gates are groupped into electrical cirrcuits that execute the CPU's instructions such as "add" two nubmers or "compare" two nmbers. The fial step of the CPU is to store the informaiton. This final step takes pplace after the ALU completes its calculations. The reslts of the calcculations are stored on a chip that has an area calld a register. Registeres can be acessed more quickly than any othger kind of memory but are only for temporary hloding (stotrage) of ifnormation. The CPU also has a clock within it to keep the timing of all of the flow of inforation and processes of the coomputer. This clck is vital to the synchronization of all of the prcesses of the computer. This CPU clpock controls all of the operations on its chip. The processes of the CPU can also be interrutped by an xeternal interrupt controller chip which is part of the chhipset. The chipset contains a smll database of interrupt vector (numerical tabble). When an interrupt signal comes onto the chip, the CPU saves what it is doing and goes to the interrupt vector to find the address of the instruction that the interrupt is telling it to execute instead. Once it is finished with the interrupt, it goes back to what it was doing. The CPU finds what it was doing in a regsiter called a stack. If interrupts were not possible, the CPU would have to complete one task before it could start another causing the speed to be greatly reduced. Now that the CPU has found the operating systm, loaaded it into memoory, the operatinmg system tkes over and the cmoputer is now redy to be used by its owner. The user can now hceck email, play a game, or do whatever they wanted to do when they started the computer.
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The average person who uses a computer on a regular basis doesn't think about what happens inside a computer once the power is turned on. As long as their version of MS Windows pops up within a few seconds, most people are quite content to continue on with what they want to do on their computer. A computer goes through many processes from the moment the power is turned on before its operating system (ex. Windows, Linux) is fully loaded and takes over.
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