| Electronic computers have changed dramatically over their 50 history, but a few basic principles characterize all computers |
| Computers process information by deterministically following (executing) instructions | ||
| Unlike humans, computers follow instructions exactly | ||
| Computers have no imagination or creativity | ||
| Computers have no intuition | ||
| Computers are literal, with no sense of irony, subtlety, proportion, … | ||
| Computers don’t joke, they’re not vindictive or cruel | ||
| Computers are not purposeful | ||
| … computers only execute instructions | ||
| To perform instructions, computers use a process known as the fetch/execute cycle implemented in their hardware | |
| The F/E Cycle is an unending process, hence the red arrow |
| At the Nenana Ice Classic, where people pay $2 to guess when the ice will break up on the Tananah River, someone processes the guesses |
| A computer is composed five components … | ||
| Arithmetic/Logic Unit (ALU) -- the part that “computes”, e.g. + | ||
| Control -- the part that follows the Fetch/Execute Cycle of the program and tells the ALU what to compute | ||
| Memory -- where data, programs are kept while computing | ||
| Input -- ports to peripheral devices from which data comes | ||
| Output -- ports to peripheral devices to which data goes | ||
| The memory is passive, storing programs and data | |
| Memory is called RAM for “random access memory” because the control can access any random location in the memory | |
| RAM is volatile, meaning it disappears when the power is turned off … how does the computer remember the date? | |
| For the control to execute (run) a program, it must be stored in the RAM. So, one operating system duty is to move programs & data from the disk to the RAM |
| The control follows instructions, telling the other parts what to do | ||
| The instructions come from the program stored in the memory | ||
| Programmers write the instructions (programs) using languages (C, C++, Java, etc.) that are way too complicated for the control to follow … so the programs are translated into a simpler form called machine language that the control can understand. A typical machine instruction is | ||
| add 884, 1004, 6618 | ||
| which means “add the number in memory location 884 to the number in memory location 1004, and put the result in memory location 6618” | ||
| The control keeps track of where it is in the program using a program counter or PC … a better name would be “instruction pointer” | |
| The control also fetches data and returns results to the memory | |
| After the instruction has been fetched from memory, the PC is incremented to refer to the next instruction in sequence | |
| This scheme should cause the computer to “run” through memory executing all instructions once and “fall off the end of memory” | |
| Computers have machine instructions to branch and jump, i.e. go to some instruction other than the next | |
| Jump and Branch change the PC after increment | |
| Programs generally repeat many instructions |
| The instruction add 884, 1004, 6618 | |
| Does not add 884 and 1004 together -- we can figure that out with a calculator … it adds whatever numbers are stored in those memory locations | |
| Different numbers in those locations produce different results |
| A computer memory location can store a byte of information (8 bits), enough for a keyboard character | ||
| A “normal” whole number (integer) uses 4 bytes | ||
| A machine instruction uses 4 bytes | ||
| Units of memory size are … | ||
| KB, kilobyte, 1024 bytes … just over a thousand bytes, a “K” | ||
| MB, megabyte, 1,048,576 bytes … just over a million bytes, a “meg” | ||
| GB, gigabyte, 1,073,741,824 bytes … just over a billion bytes, a “gig” | ||
| TB, terabyte, 1,099,511,627,776 bytes … just over a trillion bytes | ||
| Why do computers use such strange numbers??? | |||
| These numbers are powers of 2 | |||
| 210 = 1,024 call it a thousand | |||
| 220 = 1,048,576 call it a million | |||
| 230 = 1,073,741,824 call it a billion | |||
| 240 = 1,099,511,627,776 call it a trillion | |||
| When you buy a megabyte of memory you get 48,576 bytes for free! | |||
| Think of the clock rate of a computer as the rate it executes instructions, that is, how many Fetch/Execute cycles it can complete in a second (modern computers are very complex and can complete more than one instruction per cycle) | |
| hertz measures “cycles per second” | |
| 100MHz, specifies “100 million cycles per second” |
| Computers deterministically execute instructions to process information | |||
| Computers have five parts: ALU, Control, Memory, Input and Output | |||
| The control implements a process called the Fetch/Execute Cycle | |||
| The fetch/execute cycle is a fundamental method of deterministically performing operations, and the idea is used many places in computation … | |||
| The computer is an electronic fetch/execute cycle, ie, hardware | |||
| All other F/E cycles are implemented as programs, ie, software | |||