Assembly Language free essay sample

In other words, assembly language programs are specific to a particular hardware. Assembly language programs for a Mac will not work on a PC. But this can be an advantage for programmers who are targeting a specific platform and need full control over the hardware. Table of Contents:| 1. Introduction 2. Basic Concepts Assembler language Basic concepts Using debug program| | 3. Assembler programming Assembly process More assembler programs Types of instructions 4. Assembler language instructions Transfer instructions Loading instructions Stack instructions Logic instructions Arithmetic instructions Jump instructions Instructions for cycles: loop Counting Instructions Comparison Instructions Flag Instructions 5. Interruptions and file managing Internal hardware interruptions External hardware interruptions Software interruptions Most Common interruptions 6. Macros and procedures| INTRODUCTION In the introductory section some of the elemental concepts regarding computer systems are mentioned, along with the concepts of the assembly language itself, and continues with the tutorial itself. Why learn assembler language The first reason to work with assembler is that it provides the opportunity of knowing more the operation of your PC, which allows the development of software in a more consistent manner. The second reason is the total control of the PC which you can have with the use of the assembler. Another reason is that the assembly programs are quicker, smaller, and have larger capacities than ones created with other languages. Lastly, the assembler allows an ideal optimization in programs, be it on their size or on their execution. Assembler language Basic conceptsInformation UnitsIn order for the PC to process information, it is necessary that this information be in special cells called registers. The registers are groups of 8 or 16 flip-flops. A flip-flop is a device capable of storing two levels of voltage, a low one, regularly 0. 5 volts, and another one, commonly of 5 volts. The low level of energy in the flip-flop is interpreted as off or 0, and the high level as on or 1. These states are usually known as bits, which are the smallest information unit in a computer. A group of 16 bits is known as word; a word can be divided in groups of 8 bits called bytes, and the groups of 4 bits are called nibbles. Numeric systemsThe numeric system we use daily is the decimal system, but this system is not convenient for machines since the information is handled codified in the shape of on or off bits; this way of codifying takes us to the necessity of knowing the positional calculation which will allow us to express a number in any base where we need it. It is possible to represent a determined number in any base through the following formula:Where n is the position of the digit beginning from right to left and numbering from zero. D is the digit on which we operate and B is the used numeric base. TOPConverting binary numbers to decimalsWhen working with assembly language we come on the necessity of converting numbers from the binary system, which is used by computers, to the decimal system used by people. The binary system is based on only two conditions or states, be it on(1) or off(0), thus its base is two. For the conversion we can use the positional value formula:For example, if we have the binary number of 10011, we take each digit from right to left and multiply it by the base, elevated to the new position they are:Binary: 1 1 0 0 1Decimal: 1*2^0 + 1*2^1 + 0*2^2 + 0*2^3 + 1*2^4= 1 + 2 + 0 + 0 + 16 = 19 decimal. The ^ character is used in computation as an exponent symbol and the * character is used to represent multiplication. Converting decimal numbers to binaryThere are several methods to convert decimal numbers to binary; only one will be analyzed here. Naturally a conversion with a scientific calculator is much easier, but one cannot always count with one, so it is convenient to at least know one formula to do it. The method that will be explained uses the successive division of two, keeping the residue as a binary digit and the result as the next number to divide. Let us take for example the decimal number of 43. 43/2=21 and its residue is 121/2=10 and its residue is 110/2=5 and its residue is 05/2=2 and its residue is 12/2=1 and its residue is 01/2=0 and its residue is 1Building the number from the bottom , we get that the binary result is 101011Hexadecimal systemOn the hexadecimal base we have 16 digits which go from 0 to 9 and from the letter A to the F, these letters represent the numbers from 10 to 15. Thus we count 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E, and F. The conversion between binary and hexadecimal numbers is easy. The first thing done to do a conversion of a binary number to a hexadecimal is to divide it in groups of 4 bits, beginning from the right to the left. In case the last group, the one most to the left, is under or less than 4 bits, the missing places are filled with zeros. Taking as an example the binary number of 101011, we divide it in 4 bits groups and we are left with:10;1011Filling the last group with zeros (the one from the left):0010;1011Afterwards we take each group as an independent number and we consider its decimal value:0010=2;1011=11But since we cannot represent this hexadecimal number as 211 because it would be an error, we have to substitute all the values greater than 9 by their respective representation in hexadecimal, with which we obtain:2BH, where the H represents the hexadecimal base. In order to convert a hexadecimal number to binary it is only necessary to invert the steps: the first hexadecimal digit is taken and converted to binary, and then the second, and so on. Data representation methods in a computer. ASCII codeASCII is an acronym of American Standard Code for Information Interchange. This code assigns the letters of the alphabet, decimal digits from 0 to 9 and some additional symbols a binary number of 7 bits, putting the 8th bit in its off state or 0. This way each letter, digit or special character occupies one byte in the computer memory. We can observe that this method of data representation is very inefficient on the numeric aspect, since in binary format one byte is not enough to represent numbers from 0 to 255, but on the other hand with the ASCII code one byte may represent only one digit. Due to this inefficiency, the ASCII code is mainly used in the memory to represent text. BCD MethodBCD is an acronym of Binary Coded Decimal. In this notation groups of 4 bits are used to represent each decimal digit from 0 to 9. With this method we can represent two digits per byte of information. Even when this method is much more practical for number representation in the memory compared to the ASCII code, it still less practical than the binary since with the BCD method we can only represent digits from 0 to 99. On the other hand in binary format we can represent all digits from 0 to 255. This format is mainly used to represent very large numbers in mercantile applications since it facilitates operations avoiding mistakes. Floating point representationThis representation is based on scientific notation; this is, to represent a number in two parts: its base and its exponent. As an example, the number 1234000, can be represented as 1. 23*10^6, in this last notation the exponent indicates to us the number of spaces that the decimal point must be moved to the right to obtain the original result. In case the exponent was negative, it would be indicating to us the number of spaces that the decimal point must be moved to the left to obtain the original result. Using Debug programP rogram creation processFor the creation of a program it is necessary to follow five steps: * Design of the algorithm, stage the problem to be solved is established and the best solution is proposed, creating squematic diagrams used for the better solution proposal. Coding the algorithm, consists in writing the program in some programming language; assembly language in this specific case, taking as a base the proposed solution on the prior step. * Translation to machine language is the creation of the object program, in other words, the written program as a sequence of zeros and ones that can be interpreted by the processor. * Test the program, after the translation the program into machine language, execute the program in the computer machine. * The last stage is the elimination of detected faults on the program on the test stage. The correction of a fault normally requires the repetition of all the steps from the first or second. CPU RegistersThe CPU has 4 internal registers, each one of 16 bits. The first four, AX, BX, CX, and DX are general use registers and can also be used as 8 bit registers, if used in such a way it is necessary to refer to them for example as: AH and AL, which are the high and low bytes of the AX register. This nomenclature is also applicable to the BX, CX, and DX registers. The registers known by their specific names:AX Accumulator BX Base register CX Counting register DX Data register DS Data segment register ES Extra segment register SS Battery segment register CS Code segment register BP Base pointers register SI Source index register DI Destiny index register SP Battery pointer register IP Next instruction pointer register F Flag registerDebug programTo create a program in assembler two options exist, the first one is to use the TASM or Turbo Assembler, of Borland, and the second one is to use the debugger on this first section we will use this last one since it is found in any PC with the MS-DOS, which makes it available to any user who has access to a machine with these characteristics. Debug can only create files with a . COM extension, and because of the characteristics of these kinds of programs they cannot be larger than 64 kb, and they also must start with displacement, offset, or 0100H memory direction inside the specific segment. Debug provides a set of commands that lets you perform a number of useful operations:A Assemble symbolic instructions into machine code D Display the contents of an area of memory E Enter data into memory, beginning at a specific location G Run the executable program in memory N Name a program P Proceed, or execute a set of related instructions Q Quit the debug program R Display the contents of one or more registers T Trace the contents of one instruction U Unassembled machine code into symbolic code W Write a program onto diskIt is possible to visualize the values of the internal registers of the CPU using the Debug program. To begin working with Debug, type the following prompt in your computer:C:/gt;Debug [Enter]On the next line a dash will appear, this is the indicator of Debug, at this moment the instructions of Debug can be introduced using the following command:-r[Enter]AX=0000 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0100 NV EI PL NZ NA PO NC 0D62:0100 2E CS: D62:0101 803ED3DF00 CMP BYTE PTR [DFD3],00 CS:DFD3=03All the contents of the internal registers of the CPU are displayed; an alternative of viewing them is to use the r command using as a parameter the name of the register whose value wants to be seen. For example:-rbx BX 0000 :This instruction will only display the content of the BX register and the Debug indicator changes from - to :When the prompt is like this, it is possible to change the value of the register which was seen by typing the new value and [Enter], or the old value can be left by pressing [Enter] without typing any other value. TOPAssembler structureIn assembly language code lines have two parts, the first one is the name of the instruction which is to be executed, and the second one are the parameters of the command. For example: add ah bhHere add is the command to be executed, in this case an addition, and ah as well as bh are the parameters. For example:mov al, 25In the above example, we are using the instruction mov, it means move the value 25 to al register. The name of the instructions in this language is made of two, three or four letters. These instructions are also called mnemonic names or operation codes, since they represent a function the processor will perform. Sometimes instructions are used as follows:add al,[170]The brackets in the second parameter indicate to us that we are going to work with the content of the memory cell number 170 and not with the 170 value, this is known as direct addressing. Creating basic assembler programThe first step is to initiate the Debug, this step only consists of typing debug[Enter] on the operative system prompt. To assemble a program on the Debug, the a (assemble) command is used; when this command is used, the address where you want the assembling to begin can be given as a parameter, if the parameter is omitted the assembling will be initiated at the locality specified by CS:IP, usually 0100h, which is the locality where programs with . COM extension must be initiated. And it will be the place we will use since only Debug can create this specific type of programs. Even though at this moment it is not necessary to give the a command a parameter, it is recommendable to do so to avoid problems once the CS:IP registers are used, therefore we type:a 100[enter] mov ax,0002[enter] mov bx,0004[enter] add ax,bx[enter] nop[enter][enter]What does the program do? , move the value 0002 to the ax register, move the value 0004 to the bx register, add the contents of the ax and bx registers, the instruction, no operation, to finish the program. In the debug program. After to do this, appear on the screen some like the follow lines:C:\gt;debug -a 100 0D62:0100 mov ax,0002 0D62:0103 mov bx,0004 0D62:0106 add ax,bx 0D62:0108 nop 0D62:0109Type the command t (trace), to execute each instruction of this program, example:-tAX=0002 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0103 NV EI PL NZ NA PO NC 0D62:0103 BB0400 MOV BX,0004You see that the value 2 move to AX register. Type the command t (trace), again, and you see the second instruction is executed. tAX=0002 BX=0004 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0106 NV EI PL NZ NA PO NC 0D62:0106 01D8 ADD AX,BXType the command t (trace) to see the instruction add is executed, you will see the follow lines:-tAX=0006 BX=0004 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=0D62 ES=0D62 SS=0D62 CS=0D62 IP=0108 NV EI PL NZ NA PE NC 0D62:0108 90 NOPThe possibility that the registers contain different values exists, but AX and BX must be the same, since they are the ones we just modified. To exit Debug use the q (quit) command. TOPStoring and loading the programsIt would not seem practical to type an entire program each time it is needed, and to avoid this it is possible to store a program on the disk, with the enormous advantage that by being already assembled it will not be necessary to run Debug again to execute it. The steps to save a program that it is already stored on memory are:Obtain the length of the program subtracting the final address from the initial address, naturally in hexadecimal system. Give the program a name and extension. Put the length of the program on the CX register. Order Debug to write the program on the disk. By using as an example the following program, we will have a clearer idea of how to take these steps:When the program is finally assembled it would look like this:0C1B:0100 mov ax,0002 0C1B:0103 mov bx,0004 0C1B:0106 add ax,bx 0C1B:0108 int 20 0C1B:010ATo obtain the length of a program the h command is used, since it will show us the addition and subtraction of two numbers in hexadecimal. To obtain the length of ours, we give it as parameters the value of our programs final address (10A), and the programs initial address (100). The first result the command shows us is the addition of the parameters and the second is the subtraction. h 10a 100 020a 000aThe n command allows us to name the program. -n test. comThe rcx command allows us to change the content of the CX register to the value we obtained from the size of the file with h, in this case 000a, since the result of the subtraction of the final address from the initial address. -rcx CX 0000 :000aLastly, the w command writes our pr ogram on the disk, indicating how many bytes it wrote. -w Writing 000A bytesTo save an already loaded file two steps are necessary:Give the name of the file to be loaded. Load it using the l (load) command. To obtain the correct result of the following steps, it is necessary that the above program be already created. Inside Debug we write the following:-n test. com -l -u 100 109 0C3D:0100 B80200 MOV AX,0002 0C3D:0103 BB0400 MOV BX,0004 0C3D:0106 01D8 ADD AX,BX 0C3D:0108 CD20 INT 20The last u command is used to verify that the program was loaded on memory. What it does is that it disassembles the code and shows it disassembled. The parameters indicate to Debug from where and to where to disassemble. Debug always loads the programs on memory on the address 100H, otherwise indicated. | | Assembler programming| Table of ContentsBuilding Assembler programs Assembly process More assembler programs Types of instructionsBuilding Assembler programsIn order to be able to create a program, several tools are needed:First an editor to create the source program. Second a compiler, which is nothing more than a program that translates the source program into an object program. And third, a linker that generates the executable program from the object program. The editor can be any text editor at hand, and as a compiler we will use the TASM macro assembler from Borland, and as a linker we will use the Tlink program. The extension used so that TASM recognizes the source programs in assembler is . ASM; once translated the source program, the TASM creates a file with the . OBJ extension, this file contains an intermediate format of the program, called like this because it is not executable yet but it is not a program in source language either anymore. The linker generates, from a . OBJ or a combination of several of these files, an executable program, whose extension usually is . EXE though it can also be .COM, depending of the form it was assembled. Assembler ProgrammingTo build assembler programs using TASM programs is a different program structure than from using debug program. Its important to include the following assembler directives:. MODEL SMALL Assembler directive that defines the memory model to use in the program. CODE Assembler directive that defines the program instructions. STACK Assembler directive that reserves a memory space for program instructions in the stackEND Assembler directive that finishes the assembler programLets programFirst stepuse any editor program to create the source file. Type the following lines:TOPfirst example; use ; to put comments in the assembler program . MODEL SMALL; memory model .STACK; memory space for program instructions in the stack .CODE; the following lines are program instructions mov ah,1h; moves the value 1h to register ah mov cx,07h;moves the value 07h to register cx int 10h;10h interruption mov ah,4ch;moves the value 4 ch to register ah int 21h;21h interruption END; finishes the program codeThis assembler program changes the size of the computer cursor. Second stepSave the file with the following name: examp1. sm Dont forget to save this in ASCII format. Third stepUse the TASM program to build the object program. Example:C:\gt;tasm exam1. asm Turbo Assembler Version 2. 0 Copyright (c) 1988, 1990 Borland InternationalAssembling file: exam1. asm Error messages: None Warning messages: None Passes: 1 Remaining memory: 471kThe TASM can only create programs in . OBJ format, which are not executable by themselves, but rather it is necessa ry to have a linker which generates the executable code. Fourth stepUse the TLINK program to build the executable program example:C:\gt;tlink exam1. bj Turbo Link Version 3. 0 Copyright (c) 1987, 1990 Borland InternationalC:\gt;Where exam1. obj is the name of the intermediate program, . OBJ. This generates a file directly with the name of the intermediate program and the . EXE extension. Fifth stepExecute the executable programC:\gt;exam1[enter]Remember, this assembler program changes the size of the cursor. Assembly process. TOPSEGMENTSThe architecture of the x86 processors forces to the use of memory segments to manage the information, the size of these segments is of 64kb. The reason of being of these segments is that, considering that the maximum size of a number that the processor can manage is given by a word of 16 bits or register, it would not be possible to access more than 65536 localities of memory using only one of these registers, but now, if the PCs memory is divided into groups or segments, each one of 65536 localities, and we use an address on an exclusive register to find each segment, and then we make each address of a specific slot with two registers, it is possible for us to access a quantity of 4294967296 bytes of memory, which is, in the present day, more memory than what we will see installed in a PC. In order for the assembler to be able to manage the data, it is necessary that each piece of information or instruction be found in the area that corresponds to its respective segments. The assembler accesses this information taking into account the localization of the segment, given by the DS, ES, SS and CS registers and inside the register the address of the specified piece of information. It is because of this that when we create a program using the Debug on each line that we assemble, something like this appears:1CB0:0102 MOV AX,BXWhere the first number, 1CB0, corresponds to the memory segment being used, the second one refers to the address inside this segment, and the instructions which will be stored from that address follow. The way to indicate to the assembler with which of the segments we will work with is with the . CODE, . DATA and . STACK directives. The assembler adjusts the size of the segments taking as a base the number of bytes each assembled instruction needs, since it would be a waste of memory to use the whole segments. For example, if a program only needs 10kb to store data, the data segment will only be of 10kb and not the 64kb it can handle. SYMBOLS CHARTEach one of the parts on code line in assembler is known as token, for example on the code line:MOV AX,Varwe have three tokens, the MOV instruction, the AX operator, and the VAR operator. What the assembler does to generate the OBJ code is to read each one of the tokens and look for it on an internal equivalence chart known as the reserved words chart, which is where all the mnemonic meanings we use as instructions are found. Following this process, the assembler reads MOV, looks for it on its chart and identifies it as a processor instruction. Likewise it reads AX and recognizes it as a register of the processor, but when it looks for the Var token on the reserved words chart, it does not find it, so then it looks for it on the symbols chart which is a table where the names of the variables, constants and labels used in the program where their addresses on memory are included and the sort of data it contains, are found. Sometimes the assembler comes on a token which is not defined on the program, therefore what it does in these cased is to pass a second time by the source program to verify all references to that symbol and place it on the symbols chart. There are symbols which the assembler will not find since they do not belong to that segment and the program does not know in what part of the memory it will find that segment, and at this time the linker comes into action, which will create the structure necessary for the loader so that the segment and the token be defined when the program is loaded and before it is executed. TOPMore assembler programsAnother examplefirst stepuse any editor program to create the source file. Type the following lines:;example11 . model small .stack .code mov ah,2h ;moves the value 2h to register ah mov dl,2ah ;moves de value 2ah to register dl ;(Its the asterisk value in ASCII format) int 21h ;21h interruption mov ah,4ch ;4ch function, goes to operating system int 21h ;21h interruption nd ;finishes the program codesecond stepSave the file with the following name: exam2. asm Dont forget to save this in ASCII format. third stepUse the TASM program to build the object program. C:\gt;tasm exam2. asm Turb o Assembler Version 2. 0 Copyright (c) 1988, 1990 Borland International Assembling file: exam2. asm Error messages: None Warning messages: None Passes: 1 Remaining memory: 471kfourth stepUse the TLINK program to build the executable programC:\gt;tlink exam2. obj Turbo Link Version 3. 0 Copyright (c) 1987, 1990 Borland InternationalC:\gt;fifth stepExecute the executable programC:\gt;ejem11[enter] * C:\gt;This assembler program shows the asterisk character on the computer screenTOPTypes of instructions. Data movementIn any program it is necessary to move the data in the memory and in the CPU registers; there are several ways to do this: it can copy data in the memory to some register, from register to register, from a register to a stack, from a stack to a register, to transmit data to external devices as well as vice versa. This movement of data is subject to rules and restrictions. The following are some of them:*It is not possible to move data from a memory locality to another directly; it is necessary to first move the data of the origin locality to a register and then from the register to the destiny locality. *It is not possible to move a constant directly to a segment register; it first must be moved to a register in the CPU. It is possible to move data blocks by means of the movs instructions, which copies a chain of bytes or words; movsb which copies n bytes from a locality to another; and movsw copies n words from a locality to another. The last two instructions take the values from the defined addresses by DS:SI as a group of data to move and ES:DI as the new localization of the data. To move data there are also structures called batteries, where the data is introduced with the push instruction and are extracted with the pop instruction. In a stack the first data to be introduced is the last one we can take, this is, if in our program we use these instructions:PUSH AX PUSH BX PUSH CXTo return the correct values to each register at the moment of taking them from the stack it is necessary to do it in the following order:POP CX POP BX POP AXFor the communication with external devices the out command is used to send information to a port and the in command to read the information received from a port. The syntax of the out command is:OUT DX,AXWhere DX contains the value of the port which will be used for the communication and AX contains the information which will be sent. The syntax of the in command is:IN AX,DXWhere AX is the register where the incoming information will be kept and DX contains the address of the port by which the information will arrive. Logic and arithmetic operationsThe instructions of the logic operations are: and, not, or and xor. These work on the bits of their operators. To verify the result of the operations we turn to the cmp and test instructions. The instructions used for the algebraic operations are: to add, to subtract sub, to multiply mul and to divide div. Almost all the comparison instructions are based on the information contained in the flag register. Normally the flags of this register which can be directly handled by the programmer are the data direction flag DF, used to define the operations about chains. Another one which can also be handled is the IF flag by means of the sti and cli instructions, to activate and deactivate the interruptions. Jumps, loops and proceduresThe unconditional jumps in a written program in assembler language are given by the jmp instruction; a jump is to moves the flow of the execution of a program by sending the control to the indicated address. A loop, known also as iteration, is the repetition of a process a certain number of times until a condition is fulfilled. | |