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CAPABILITIES OF THE HARDWARE It ain t nuthin but aarn, one of my (Reynolds) instructors at Hewlett Packard used to say. He was big man from Georgia with a strong accent, and by aarn he meant iron. The computing machinery itself, without any operating system, is as useful to the average person as a chunk of iron. Maybe it is less useful, for a sufficiently large piece of iron could at least serve as a boat mooring. The precise list of things a particular computer can do directly is very short. The list is the instruction set of the computer. Modern computers have instruction sets of about 70 to 150 instructions. The instructions the machine understands allow the machine to move bits from one memory location to another, or to move bits to/from memory from/to a register, or to shift the bits in a computer word (most computers today regard 32 bits as a word ) some number of positions left or right, or to compare the values of two words, or to complement the bit values of a word (change the ones to zeros, and vice versa), or to add two values. The machine can also compare two values, and skip an instruction if the values are different (or, using a complementary instruction, if the values are the same). There s also a jump instruction to allow the machine to execute an instruction elsewhere in the program. Such primitive operations are a long way from doing anything really useful for people, and even at that, with nothing but the bare machine, one would have to know the bit code for each instruction, enter the sequence of instructions one bit at a time, and then press the start button! Early computers (even as recently as the early 1980s) usually had a front panel with rocker switches and lights to allow one to do just that. If one were to write a program to act as a simple four-function integer (no decimals) calculator, with nothing but the bare machine, one would write something like the following. 1 Enable a read from the keyboard by using one of I/O instructions in the machine instruction set to ready the keyboard interface (the electronics behind the connector port to which the keyboard is attached) to accept a character from the keyboard. 2 Wait by entering a loop, continuously testing the flag signal on the keyboard interface. If the flag is false, check it again, and again, and again, until it becomes true. When the flag becomes true, a character will have arrived at the keyboard interface from the keyboard. However, the character will arrive as the ASCII encoding of the character we know, not as a binary numeric value. You can see the encodings of various characters by looking at a table of ASCII encodings. One reference is this: http://www.lookuptables.com/. If you consult the table, you will see that the character 0 is encoded as the bit pattern equivalent to the decimal number 48. The character 1 is encoded as 49; the number 2 as 50; and so on, up to character 9, which is encoded as 57. Likewise, the equal sign is encoded as 61, the plus sign as 43, etc. If the user types a 3, it will arrive as the decimal value 51.
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3 When the user types a character, the signal flag on the keyboard interface becomes true, and our program exits its endless loop testing the signal flag. Now the program can use another I/O instruction to read the character on the keyboard interface into a register of the central processing unit (CPU). 4 Check to see if the character now in the register is one of the characters we re interested in, i.e., a number, operation sign or equal sign. If the character is not one of those, ignore the character, and enable another read from the keyboard. 5 If the character is a number, see if the previous character was a number. If so, then this is the next digit in a multidigit number. In that case, multiply the previous number by 10, add the new number to it, and store the result in a known memory location. Before you can add the new number, however, you must decode the ASCII and convert the character into a binary number. A commonly used trick is simply to subtract 48 from the coded value of the number (e.g., if the number is 3, subtracting 48 from 51, the encoding, returns the binary numeric value 3). On the other hand, if this is the first digit in a number, just decode the ASCII and store the binary value in a memory location set aside by the program for one of the operands. 6 If the character is one of the operation signs, store the sign in a memory location set aside for storing the operator. The program will refer to this later, after the program receives an equal sign. 7 If the character is an equal sign, load the operands saved in memory into CPU registers, retrieve the operator character, and, depending on the operator, jump to the instruction(s) to perform the arithmetic operation on the contents of the registers. Then store the result back in the memory location for operand 1, construct the character string to send as the result, and send the result back to the display. Here s how the program constructs the characters to send to the display: a If the result is greater than 9, then the result will require more than one character, so divide the result by 10 until further division results in a number less than 1. b Add 48 to the integer result to encode it as ASCII. c Load the interface register in the computer with the character, and issue the I/O instruction to have the first character sent to the display. d Wait (loop again) until the character is sent, and acknowledged by the display via the flag being set on the display interface. e Having sent the most significant digit, subtract the appropriate value from the result and repeat the formatting and output operations until the entire character string result is sent to the display. 8 Reenable the keyboard interface to read the next character. What a lot of work! And many programs need to read from the keyboard and write to the display! And many programs need to decode strings of numeric characters, and also to encode numbers into character strings! We don t want every programmer reinventing this code, and making similar mistakes over and over again. Obvious problems like this I/O programming and formatting prompted interest in common, efficient, debugged programs that different programmers could easily reuse. At first such code came into use as libraries of standard routines. Later, computer scientists created the first resident monitors or operating systems os which made it easier for programmers to use such shared code, and made use of the machine more efficient and secure. The operating system of a computer is a program, often called the kernel, which is always running on the computer, and which governs the execution of all the other programs that run on the computer. The operating system makes it much easier to write programs and execute them, because the operating system handles all the complex details, like performing I/O and formatting input and output. Two or three key motivations lay behind the development of the first operating systems. The first was to make the computer much more easily useful. The second was to use what were at the time very expensive computing resources efficiently. The third was to provide security and reliability in computing. In general, operating systems are programs written to run on the bare machine and provide services to user programs. The services operating systems provide include:
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Management of I/O Management of memory Scheduling of user processes (start, interrupt, and stop) A secure and reliable environment for programs and users
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