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Assembly Language Programming Lecture Notes Delivered by Belal Hashmi Compiled by Junaid Haroon Preface Assembly language programming develops a very basic and low level understanding of the computer.
Assembly Language Programming Lecture Notes Delivered by Belal Hashmi Compiled by Junaid Haroon Preface Assembly language programming develops a very basic and low level understanding of the computer. In higher level languages there is a distance between the computer and the programmer. This is because higher level languages are designed to be closer and friendlier to the programmer, thereby creating distance with the machine. This distance is covered by translators called compilers and interpreters. The aim of programming in assembly language is to bypass these intermediates and talk directly with the computer. There is a general impression that assembly language programming is a difficult chore and not everyone is capable enough to understand it. The reality is in contrast, as assembly language is a very simple subject. The wrong impression is created because it is very difficult to realize that the real computer can be so simple. Assembly language programming gives a freehand exposure to the computer and lets the programmer talk with it in its language. The only translator that remains between the programmer and the computer is there to symbolize the computer s numeric world for the ease of remembering. To cover the practical aspects of assembly language programming, IBM PC based on Intel architecture will be used as an example. However this course will not be tied to a particular architecture as it is often done. In our view such an approach does not create versatile assembly language programmers. The concepts of assembly language that are common across all platforms will be developed in such a manner as to emphasize the basic low level understanding of the computer instead of the peculiarities of one particular architecture. Emphasis will be more on assembly language and less on the IBM PC. Before attempting this course you should know basic digital logic operations of AND, OR, NOT etc. You should know binary numbers and their arithmetic. Apart from these basic concepts there is nothing much you need to know before this course. In fact if you are not an expert, you will learn assembly language quickly, as non-experts see things with simplicity and the basic beauty of assembly language is that it is exceptionally simple. Do not ever try to find a complication, as one will not be there. In assembly language what is written in the program is all that is there, no less and no more. After successful completion of this course, you will be able to explain all the basic operations of the computer and in essence understand the psychology of the computer. Having seen the computer from so close, you will understand its limitations and its capabilities. Your logic will become fine grained and this is one of the basic objectives of teaching assembly language programming. Then there is the question that why should we learn assembly language when there are higher level languages one better than the other; C, C++, Java, to name just a few, with a neat programming environment and a simple way to write programs. Then why do we need such a freehand with the computer that may be dangerous at times? The answer to this lies in a very simple example. Consider a translator translating from English to Japanese. The problem faced by the translator is that every language has its own vocabulary and grammar. He may need to translate a word into a sentence and destroy the beauty of the topic. And given that we do not know ii Japanese, so we cannot verify that our intent was correctly conveyed or not. Compiler is such a translator, just a lot dumber, and having a scarce number of words in its target language, it is bound to produce a lot of garbage and unnecessary stuff as a result of its ignorance of our program logic. In normal programs such garbage is acceptable and the ease of programming overrides the loss in efficiency but there are a few situations where this loss is unbearable. Think about a four color picture scanned at 300 dots per inch making pixels per square inch. Now a processing on this picture requires operations per square inch, one operation for each color of each pixel. A few extra instructions placed by the translator can cost hours of extra time. The only way to optimize this is to do it directly in assembly language. But this doesn t mean that the whole application has to be written in assembly language, which is almost never the case. It s only the performance critical part that is coded in assembly language to gain the few extra cycles that matter at that point. Consider an arch just like the ones in mosques. It cannot be made of big stones alone as that would make the arch wildly jagged, not like the fine arch we are used to see. The fine grains of cement are used to smooth it to the desired level of perfection. This operation of smoothing is optimization. The core structure is built in a higher level language with the big blocks it provides and the corners that need optimization are smoothed with the fine grain of assembly language which allows extreme control. Another use of assembly language is in a class of time critical systems called real time systems. Real time systems have time bound responses, with an upper limit of time on certain operations. For such precise timing requirement, we must keep the instructions in our total control. In higher level languages we cannot even tell how many computer instructions were actually used, but in assembly language we can have precise control over them. Any reasonable sized application or a serious development effort has nooks and corners where assembly language is needed. And at these corners if there is no assembly language, there can be no optimization and when there is no optimization, there is no beauty. Sometimes a useful application becomes useless just because of the carelessness of not working on these jagged corners. The third major reason for learning assembly language and a major objective for teaching it is to produce fine grained logic in programmers. Just like big blocks cannot produce an arch, the big thick grained logic learnt in a higher level language cannot produce the beauty and fineness assembly language can deliver. Each and every grain of assembly language has a meaning; nothing is presumed (e.g. div and mul for input and out put of decimal number). You have to put together these grains, the minimum number of them to produce the desired outcome. Just like a for loop in a higher level language is a block construct and has a hundred things hidden in it, but using the grains of assembly language we do a similar operation with a number of grains but in the process understand the minute logic hidden beside that simple for construct. Assembly language cannot be learnt by reading a book or by attending a course. It is a language that must be tasted and enjoyed. There is no other way to learn it. You will need to try every example, observe and verify the things you are told about it, and experiment a lot with the computer. Only then you will know and become able to appreciate how powerful, versatile, and simple this language is; the three properties that are hardly ever present together. Whether you program in C/C++ or Java, or in any programming paradigm be it object oriented or declarative, everything has to boil down to the bits and bytes of assembly language before the computer can even understand it. Virtual University of Pakistan ii Table of Contents Preface Table of Contents 1 Introduction to Assembly Language Basic Computer Architecture Registers Instruction Groups Intel iapx88 Architecture History Register Architecture Our First Program Segmented Memory Model 12 2 Addressing Modes Data Declaration Direct Addressing Size Mismatch Errors Register Indirect Addressing Register + Offset Addressing Segment Association Address Wraparound Addressing Modes Summary 27 3 Branching Comparison and Conditions Conditional Jumps Unconditional Jump Relative Addressing Types of Jump Sorting Example 38 4 Bit Manipulations Multiplication Algorithm Shifting and Rotations Multiplication in Assembly Language Extended Operations Bitwise Logical Operations Masking Operations 51 5 Subroutines Program Flow Our First Subroutine Stack Saving and Restoring Registers Parameter Passing Through Stack Local Variables 67 6 Display Memory 71 i iii iv 6.1. ASCII Codes Display Memory Formation Hello World in Assembly Language Number Printing in Assembly Screen Location Calculation 79 7 String Instructions String Processing STOS Example Clearing the Screen LODS Example String Printing SCAS Example String Length LES and LDS Example MOVS Example Screen Scrolling CMPS Example String Comparison 92 8 Software Interrupts Interrupts Hooking an Interrupt BIOS and DOS Interrupts 99 9 Real Time Interrupts and Hardware Interfacing Hardware Interrupts I/O Ports Terminate and Stay Resident Programmable Interval Timer Parallel Port Debug Interrupts Debugger using single step interrupt Debugger using breakpoint interrupt Multitasking Concepts of Multitasking Elaborate Multitasking Multitasking Kernel as TSR Video Services BIOS Video Services DOS Video Services Secondary Storage Physical Formation Storage Access Using BIOS Storage Access using DOS Device Drivers Serial Port Programming Introduction Serial Communication Protected Mode Programming Introduction bit Programming VESA Linear Frame Buffer Interrupt Handling Interfacing with High Level Languages 179 Virtual University of Pakistan iv TABLE OF CONTENTS v Calling Conventions Calling C from Assembly Calling Assembly from C Comparison with Other Processors Motorolla 68K Processors Sun SPARC Processor 184 Virtual University of Pakistan v 1 Introduction to Assembly Language 1.1. BASIC COMPUTER ARCHITECTURE Address, Data, and Control Buses A computer system comprises of a processor, memory, and I/O devices. I/O is used for interfacing with the external world, while memory is the processor s internal world. Processor is the core in this picture and is responsible for performing operations. The operation of a computer can be fairly described with processor and memory only. I/O will be discussed in a later part of the course. Now the whole working of the computer is performing an operation by the processor on data, which resides in memory. The scenario that the processor executes operations and the memory contains data elements requires a mechanism for the processor to read that data from the memory. That data in the previous sentence much be rigorously explained to the memory which is a dumb device. Just like a postman, who must be told the precise address on the letter, to inform him where the destination is located. Another significant point is that if we only want to read the data and not write it, then there must be a mechanism to inform the memory that we are interested in reading data and not writing it. Key points in the above discussion are: There must be a mechanism to inform memory that we want to do the read operation There must be a mechanism to inform memory that we want to read precisely which element There must be a mechanism to transfer that data element from memory to processor The group of bits that the processor uses to inform the memory about which element to read or write is collectively known as the address bus. Another important bus called the data bus is used to move the data from the memory to the processor in a read operation and from the processor to the memory in a write operation. The third group consists of miscellaneous independent lines used for control purposes. For example, one line of the bus is used to inform the memory about whether to do the read operation or the write operation. These lines are collectively known as the control bus. These three buses are the eyes, nose, and ears of the processor. It uses them in a synchronized manner to perform a meaningful operation. Although the programmer specifies the meaningful operation, but to fulfill it the processor needs the collaboration of other units and peripherals. And that collaboration is made available using the three buses. This is the very basic description of a computer and it can be extended on the same lines to I/O but we are leaving it out just for simplicity for the moment. The address bus is unidirectional and address always travels from processor to memory. This is because memory is a dumb device and cannot predict which element the processor at a particular instant of time needs. Data moves from both, processor to memory and memory to processor, so the data bus is bidirectional. Control bus is special and relatively complex, because different lines comprising it behave differently. Some take information from the processor to a peripheral and some take information from the peripheral to the processor. There can be certain events outside the processor that are of its interest. To bring information about these events the data bus cannot be used as it is owned by the processor and will only be used when the processor grants permission to use it. Therefore certain processors provide control lines to bring such information to processor s notice in the control bus. Knowing these signals in detail is unnecessary but the general idea of the control bus must be conceived in full. PROCESSOR MEMORY PERIPHERALS We take an example to explain the collaboration of the processor and memory using the address, control, and data buses. Consider that you want your uneducated servant to bring a book from the shelf. You order him to bring the fifth book from top of the shelf. All the data movement operations are hidden in this one sentence. Such a simple everyday phenomenon seen from this perspective explains the seemingly complex working of the three buses. We told the servant to bring a book and the one which is fifth from top, precise location even for the servant who is much more intelligent then our dumb memory. The dumb servant follows the steps one by one and the book is in your hand as a result. If however you just asked him for a book or you named the book, your uneducated servant will stand there gazing at you and the book will never come in your hand. Even in this simplest of all examples, mathematics is there, fifth from top. Without a number the servant would not be able to locate the book. He is unable to understand your will. Then you tell him to put it with the seventh book on the right shelf. Precision is involved and only numbers are precise in this world. One will always be one and two will always be two. So we tell in the form of a number on the address bus which cell is needed out of say the 2000 cells in the whole memory. A binary number is generated on the address bus, fifth, seventh, eighth, tenth; the cell which is needed. So the cell number is placed on the address bus. A memory cell is an n-bit location to store data, normally 8-bit also called a byte. The number of bits in a cell is called the cell width. The two dimensions, cell width and number of cells, define the memory completely just like the width and depth of a well defines it completely. 200 feet deep by 15 feet wide and the well is completely described. Similarly for memory we define two dimensions. The first dimension defines how many parallel bits are there in a single memory cell. The memory is called 8-bit or 16-bit for this reason and this is also the word size of the memory. This need not match the size of a processor word which has other parameters to define it. In general the memory cell cannot be wider than the width of the data bus. Best and simplest operation requires the same size of data bus and memory cell width. Virtual University of Pakistan 2 As we previously discussed that the control bus carries the intent of the processor that it wants to read or to write. Memory changes its behavior in response to this signal from the processor. It defines the direction of data flow. If processor wants to read but memory wants to write, there will be no communication or useful flow of information. Both must be synchronized, like a speaker speaks and the listener listens. If both speak simultaneously or both listen there will be no communication. This precise synchronization between the processor and the memory is the responsibility of the control bus. Control bus is only the mechanism. The responsibility of sending the appropriate signals on the control bus to the memory is of the processor. Since the memory never wants to listen or to speak of itself. Then why is the control bus bidirectional. Again we take the same example of the servant and the book further to elaborate this situation. Consider that the servant went to fetch the book just to find that the drawing room door is locked. Now the servant can wait there indefinitely keeping us in surprise or come back and inform us about the situation so that we can act accordingly. The servant even though he was obedient was unable to fulfill our orders so in all his obedience, he came back to inform us about the problem. Synchronization is still important, as a result of our orders either we got the desired cell or we came to know that the memory is locked for the moment. Such information cannot be transferred via the address or the data bus. For such situations when peripherals want to talk to the processor when the processor wasn t expecting them to speak, special lines in the control bus are used. The information in such signals is usually to indicate the incapability of the peripheral to do something for the moment. For these reasons the control bus is a bidirectional bus and can carry information from processor to memory as well as from memory to processor REGISTERS The basic purpose of a computer is to perform operations, and operations need operands. Operands are the data on which we want to perform a certain operation. Consider the addition operation; it involves adding two numbers to get their sum. We can have precisely one address on the address bus and consequently precisely one element on the data bus. At the very same instant the second operand cannot be brought inside the processor. As soon as the second is selected, the first operand is no longer there. For this reason there are temporary storage places inside the processor called registers. Now one operand can be read in a register and added into the other which is read directly from the memory. Both are made accessible at one instance of time, one from inside the processor and one from outside on the data bus. The result can be written to at a distinct location as the operation has completed and we can access a different memory cell. Sometimes we hold both operands in registers for the sake of efficiency as what we can do inside the processor is undoubtedly faster than if we have to go outside and bring the second operand. Registers are like a scratch pad ram inside the processor and their operation is very much like normal memory cells. They have precise locations and remember what is placed inside them. They are used when we need more than one data element inside the processor at one time. The concept of registers will be further elaborated as we progress into writing our first program. Memory is a limited resource but the number of memory cells is large. Registers are relatively very small in number, and are therefore a very scarce and precious resource. Registers are more than one in number, so we have to precisely identify or name them. Some manufacturers number their registers like r0, r1, r2, others name them like A, B, C, D etc. Naming is useful since the registers are few in number. This is called the nomenclature of the Virtual University of Pakistan 3 particular architecture. Still other manufacturers name their registers according to their function like X stands for an index register. This also informs us that there are special functions of registers as well, some of which are closely associated to the particular architecture. For example index register
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