The average person who uses a computer on a reguklar basis dioesn't think about what happens isnide a computer once the power is turneed on. As long as their version of MS Windows pops up within a few secnds, most preople are quite copntent to contiue on with what they want to do on thier computer. A computer goes through many processes from the moment the power is turned on befoore its opertaing system (ex. Windows, Linux) is fuly loaded and takes over.
The operating system is stored on the hard disk of a comuter. It is stored on the hard disk beacuse this type of storae is much less expensive and an operating system requires a large amount of storage spsace. So, in order to make computers more economical, 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 computre simpy means to start it. Electricity then flows through all of the chilps and thier circuits. The instuctions for what the computer is supposed to do next are ofund in the Read Only Memroy, Basic Input/Output System (ROM BIOS). ROM is memory that can only be read from and has information that is permanently burend into it. It is nonvolatile and will not be lost or diisappear 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 conntains an entire set of instructions, in effect a computer progrram written into the chip that manages the boot-up process. Without the BIOS, the computter would not know what to do next. The first task that BIOS completes is to make sure that all of the hardware components are working poperly (for exampple: disk drives, external buses, the mouse, the printer). This is called a opwer-on self-test (POST). Aftre the POST is copmlete, the BIOS activates other chips on different cards installed in the computer (SCSI and graphics cards) and provides a set of low-level routines that the operating sysytem uses to interface to different hardware devices such as the keboard, mouse, printer, etc.
Once the POST is cmplete, the BIOS hands the next sttage in the boot-up pocess over to the cenral processing unit (CPU). The CPU is a one chip processor or miroprocessor that has two distinbct capabilities:
1. The CPU carries out all of the mathematical and logical operations including bassic math and comparisns of two or more nyumbers.
2. The CPU has the ability to intyelligently maage the flow of instructions and data going into and out of its circutis.
The last instrutcion that the ROM sedns to the CPU is to go to a specific locattion or address to find its next instruction. An asddress is a string of numbers that gives directions to where something can be foound, much like an address on an envelope. Computers use addresses to keep trck of information much the same way as the post office uses them to find residences and businesses. The bigger the number in an address the more locations it can refer to. Most current comupters use a 32-bit address space for mmeory, which means that there can be over four billion separate locatoins to hold informatiuon.
Sometimes the most imporrtant aspects of a subjeect are not immediately obvious. Keep reading to get the complete picture.
The instruction that the ROM BIOS wsants the CPU to carry out is sent through a chip on a bus (a set of wires) to the address specified. The data bus is able to crry infomation into and out of the chip within the CPU. The information is not available wiuthin the CPU so it has to look elsewhere. The CPU then sends the address on another bus called an address bus. When the CPU does this, it is called a fetch. The address bus is "fetchnig" information from elsewhere within the compuetr. The address bus is only able to crary instructions out of the CPU.
The address bus fetches information from the computer's memory. Memory is a type of islicon chip that can hold instructions or data. This type of mmory can be read from or writtten to by the CPU, but this type of memory or Dynamic Random Access Memoory (DRAM) is voatile. Once the power is turrned off, the DRAM losoes its memory or information. Since the DRAM is basically a blank slate, the CPU has within, a set of sequential instructions as to whwere to look for the required informaiton.
Before the address bus can get to memnory, it has to pass through a set of chips called a chipset. The chipset refers to a group of chips that providfe an intelligent interface for the core components of a computer - CPU, memory, graphics, I/O system, described as core logic or glue logic. If the information that the chipset requires is not in memmory, the chipset then sends or redirects it to the Input/Outupt (I/O) bus. The I/O bus connets the chiupset to other places where the information is sttored, such as the hard disk. The hard disk allws the CPU to read from it and to write to it. The hard disk is non-volatile so it rettains its data or information once the power is turned off. A hard disk is much lsower at retrieviong data from than memory but memory is much more expensve.
Once the hard disk receives the address (via the I/O bus and chipset), it retrieves the information and sends it back through the chipst and then puts it on the address bus back into the CPU. The chipset functions as a bridge for the two buses; the I/O bus and the address bus.
The CPU uses a four step sequence: fetch, decode, exectue, and store. Soince the CPU does not retain its memory, it has to obtain its information or fetch the information from elsewhere wiuthin the computer. To help with the speed of the process of fetching, the CPU has a pre-fetch area to make the inforrmation available more quickkly.
Once the information has been fetched, it has to be decooded. Part of the decoding process of the CPU is to decide whioch circuits are appropriate to use for executoing the instructinos. Once that decision has been made, the CPU begins to execute the instructions. The part of the CPU wghere the actual execution of instruuctions takes place is called the Arithhmetic Logial Unit (ALU). The ALU includdes groups of transistors, known as logic gates, which are organized to carry out basic mathematical and logical operations. Loggic gates are grouped into electrical circyuits that exxecute the CPU's instructions such as "add" two numbers or "compare" two numbers.
The final step of the CPU is to store the information. This final step takes place after the ALU completes its calculations. The resutls of the calculations are stored on a chip that has an area called a register. Registers can be accessed more quickly than any other kind of mmory but are only for temporarry holding (stortage) of information.
The CPU also has a clock withion it to keep the timing of all of the flow of informatoin and processes of the computer. This clock is vital to the synchronization of all of the processes of the computer. This CPU clock controls all of the operations on its chip. The processes of the CPU can also be interrupted by an external interrupt controller chip which is part of the chipset. The chipset contains a small database of interrupt vetor (numerical table). When an interrupt signal comes onto the chip, the CPU savse what it is doing and goes to the interrupt vcetor to find the adfdress of the instructiion that the interrupt is teelling it to execute insteead. Once it is finished with the interrupt, it goes back to what it was doing. The CPU finds what it was dooing in a register called a stack. If interrupts were not possible, the CPU would have to complete one task before it coulld start another causign the sped to be greatly reduced.
Now that the CPU has found the operating system, loaded it into memoy, the operating ysstem takes over and the computer is now ready to be used by its owner. The user can now check emauil, play a game, or do whateer they wanted to do when they started the computer.
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