Exercise 1

1. How the processor uses the address bus, the data bus, and the control bus to communicate with the system memory?

2. Which of the following are unidirectional and which are bidirectional?

a. Address Bus

b. Data Bus

c. Control Bus

3. What are registers and what are the specific features of the accumulator, index registers, program counter, and program status word?
4. What is the size of the accumulator of a 64bit processor?

5. What is the difference between an instruction mnemonic and its opcode?
6. How are instructions classified into groups?

7. A combination of 8bits is called a byte. What is the name for 4bits and for 16bits?

8. What is the maximum memory 8088 can access?

9. List down the 14 registers of the 8088 architecture and briefly describe their uses.

10. What flags are defined in the 8088 FLAGS register? Describe the function of the zero flag, the carry flag, the sign flag, and the overflow flag.
11. Give the value of the zero flag, the carry flag, the sign flag, and the overflow flag after each of the following instructions if AX is initialized with 0x1254 and BX is initialized with 0x0FFF.
a. add ax, 0xEDAB

b. add ax, bx
c. add bx, 0xF001

12. What is the difference between little endian and big endian formats? Which format is used by the Intel 8088 microprocessor?

13. For each of the following words identify the byte that is stored at lower memory address and the byte that is stored at higher memory address in a little endi an computer.
a. 1234

b. ABFC
c. B100

d. B800

14. What are the contents of memory locations 200, 201, 202, and 203 if the word 1234 is stored at offset 200 and the word 5678 is stored at offset 202?

15. What is the offset at which the first exeutable instruction of a COM file must be placed?
16. Why was segmentation originally introduced in 8088 architecture?

17. Why a segment start cannot start from the physical address 55555.

18. Calculate the physical memory address generated by the following segment offset pairs.
a. 1DDD:0436

b. 1234:7920

c. 74F0:2123

d. 0000:6727
e. FFFF:4336

f. 1080:0100

g. AB01:FFFF

19. What are the first and the last physical memory addresses accessible using the following segment values?
a. 1000

b. 0FFF

c. 1002

d. 0001
e. E000

20. Write instructions that perform the following operations.

a. Copy BL into CL

b. Copy DX into AX

c. Store 0x12 into AL

d. Store 0x1234 into AX
e. Store 0xFFFF into AX

21. Write a program in assembly language that calculates the square of six by adding six to the accumulator six times.

SEGMENTED MEMORY MODEL

Rationale

In earlier processors like 8080 and 8085 the linear memory model was used to access memory. In linear memory model the whole memory appears like a single array of data. 8080 and 8085 could access a total memory of 64K using the 16 lines of their address bus. When designing iAPX88 the Intel designers wanted to remain compatible with 8080 and 8085 however 64K was too small to continue with, for their new processor. To get the best of both worlds they introduced the segmented memory model in 8088.

There is also a logical argument in favor of a segmented memory model in additional to the issue of compatibility discussed above. We have two logical parts of our program, the code and the data, and actually there is a third part called the program stack as well, but higher level languages make this invisible to us. These three logical parts of a program should appear as three distinct units in memory, but making this division is not possible in the linear memory model. The segmented memory model does allow this distinction.

Mechanism

The segmented memory model allows multiple functional windows into the main memory, a code window, a data window etc. The processor sees code from the code window and data from the data window. The size of one window is restricted to 64K. 8085 software fits in just one such window. It sees code, data, and stack from this one window, so downward compatibility is attained.

However the maximum memory iAPX88 can access is 1MB which can be accessed with 20 bits. Compare this with the 64K of 8085 that were accessed using 16 bits. The idea is that the 64K window just discussed can be moved anywhere in the whole 1MB. The four segment registers discussed in the Intel register architecture are used for this purpose. Therefore four windows can exist at one time. For example one window that is pointed to by the CS register contains the currently executing code.

To understand the concept, consider the windows of a building. We say that a particular window is 3 feet above the floor and another one is 20 feet above the floor. The reference point, the floor is the base of the segment called the datum point in a graph and all measurement is done from that datum point considering it to be zero. So CS holds the zero or the base of code. DS holds the zero of data. Or we can say CS tells how high code from the floor is, and DS tells how high data from the floor is, while SS tells how high the stack is. One extra segment ES can be used if we need to access two distant areas of memory at the same time that both cannot be seen through the same window. ES also has special role in string instructions. ES is used as an extra data segment and cannot be used as an extra code or stack segment.

Revisiting the conce pt again, like the datum point of a graph, the segment registers tell the start of our window which can be opened anywhere in the megabyte of memory available. The window is of a fixed size of 64KB. Base and offset are the two key variables in a segmented address. Segment tells the base while offset is added into it. The registers IP, SP, BP, SI, DI, and BX all can contain a 16bit offset in them and access memory relative to a segment base.

The IP register cannot work alone. It needs the CS register to ope n a 64K window in the 1MB memory and then IP works to select code from this window as offsets. IP works only inside this window and cannot go outside of this 64K in any case. If the window is moved i.e. the CS register is changed, IP will change its behavi or accordingly and start selecting from the new window. The IP register always works relatively, relative to the segment base stored in the CS register. IP is a 16bit register capable of accessing only 64K memory so how the whole megabyte can contain code anywhere. Again the same concept is there, it can access 64K at one instance of time. As the base is changed using the CS register, IP can be made to point anywhere is the whole megabyte. The process is illustrated with the following diagram.


Physical Address Calculation

Now for the whole megabyte we need 20 bits while CS and IP are both 16bit registers. We need a mechanism to make a 20bit number out of the two 16bit numbers. Consider that the segment value is stored as a 20 bi t number with the lower four bits zero and the offset value is stored as another 20 bit number with the upper four bits zeroed. The two are added to produce a 20bit absolute address. A carry if generated is dropped without being stored anywhere and the phenomenon is called address wraparound. The process is explained with the help of the following diagram.

18



15---------------------------- 0
Segment Address
16bit Segment Register 0000
----------------------------
15 0

0000 16bit Logical Address Offset Address

-----------------------------------

19 0

20bit Physical Address


Therefore memory is determined by a segment-offset pair and not alone by any one register which will be an ambiguous reference. Every offset register is assigned a default segment register to resolve such ambiguity. For example the program we wrote when loaded into memory had a value of 0100 in IP register and some value say 1DDD in the CS register. Making both 20 bit numbers, the segment base is 1DDD0 and the offset is 00100 and adding them we get the physical memory address of 1DED0 where the opcode B80500 is placed.

Paragraph Boundaries

As the segment value is a 16bit number and four zero bits are appended to the right to make it a 20bit number, segments can only be defined a 16byte boundaries called paragraph boundaries. The first possible segment value is 0000 meaning a physical base of 00000 and the next possible value of 0001 means a segment base of 00010 or 16 in decimal. Therefore segments can only be defined at 16 byte boundaries.

Overlapping Segments

We can also observe that in the case of our program CS, DS, SS, and ES all had the same value in them. This is called overlapping segments so that we can see the same memory from any window. This is the structure of a COM file.

Using partially overlapping segments we can produce a number of segment, offset pairs that all access the same memory. For example 1DDD:0100 and IDED:0000 both point to the same physical memory. To test this we can open a data window at 1DED:0000 in the debugger and change the first three bytes to “90” which is the opcode for NOP (no operation). The change is immediately visible in the code window which is pointed to by CS containing 1DDD. Similarly IDCD:0200 also points to the same memory location. Consider this like a portion of wall that three different people on three different floors are seeing through their own windows. One of them painted the wall red; it will be changed for all of them though their perspective is different. It is the same phenomenon occurring here.

The segment, offset pair is called a logical address, while the 20bit address is a physical address which is the real thing. Logical addressing is a mechanism to access the physical memory. As we have seen three different logical addresses accessed the same physical address.

Assembler, Linker, and Debugger

We need an assembler to assemble this program and convert this into executable binary code. The assembler that we will use during this course is “Netwide Assembler” or NASM. It is a free and open source assembler. And the tool that will be most used will be the debugger. We will use a free debugger called “A fullscreen debugger” or AFD. These are the whole set of weapons an assembly language programmer needs for any task whatsoever at hand.

To assemble we will give the following command to the processor assuming that our input file is named EX01.ASM.

nasm ex01.asm –o ex01.com –l ex01.lst

This will produce two files EX01.COM that is our executable file and EX01.LST that is a special listi ng file that we will explore now. The listing file produced for our example above is shown below with comments removed for neatness.


1
2 [org 0x0100]
3 00000000 B80500 mov ax, 5
4 00000003 BB0A00 mov bx, 10
5 00000006 01D8 add ax, bx
6 00000008 BB0F00 mov bx, 15
7 0000000B 01D8 add ax, bx
8
9 0000000D B8004C mov ax, 0x4c00
10 00000010 CD21 int 0x21


The first column in the above listing is offset of the listed instruction in the output file. Next column is the opcode into which our instruction was translated. In this case this opcode is B8. Whenever we move a constant into AX register the opcode B8 will be used. After it 0500 is appended which is the immediate operand to this instruction. An immediate operand is an operand which is placed directly inside the instruction. Now as the AX register is a word sized register, and one hexadecimal digit takes 4 bits so 4 hexadecimal digits make one word or two bytes. Which of the two bytes should be placed first in the instruction, the least significant or the most significant? Similarly for 32bit numbers either the orde r can be most significant, less significant, lesser significant, and least significant called the big-endian order used by Motorola and some other companies or it can be least significant, more significant, more significant, and most significant called the little -endian order and is used by Intel. The big-endian have the argument that it is more natural to read and comprehend while the little - endian have the argument that this scheme places the less significant value at a lesser address and more significant value at a higher address.

Because of this the constant 5 in our instruction was converted into 0500 with the least significant byte of 05 first and the most significant byte of 00 afterwards. When read as a word it is 0005 but when written in memory it will become 0500. As the first instruction is three bytes long, the listing file shows that the offset of the next instruction in the file is 3. The opcode BB is for moving a constant into the BX register, and the operand 0A00 is the number 10 in little -endian byte order. Similarly the offsets and opcodes of the remaining instructions are shown in order. The last instruction is placed at offset 0x10 or 16 in decimal. The size of the last instruction is two bytes, so the file of the complete COM file becomes 18 bytes. This can be verified from the directly listing that the COM file produced is exactly 18 bytes long.

Now the program is ready to be run inside the debugger. The debugger shows the values of registers, flags, stack, our code, and one or two areas of the system memory as data. Debugger allows us to step our program one instruction at a time and observe its effect on the registers and program data. The details of using the AFD debugger can be seen from the AFD manual.

After loading the program in the debugger observe that the first instruction is now at 0100 instead of absolute zero. This is the effect of the org directive at the start of our program. The first instruction of a COM file must be at offset 0100 (decimal 255) as a requirement. Also observe that the debugger isshowing your program even though it was provided only the COM file and neither of the listing file or the program source. This is because the translation from mnemonic to opcode is reversible and the debugger mapped back from the opcode to the instruction mnemonic. This will become apparent for instructions that have two mnemonics as the debugger might not show the one that was written in the source file.

As a result of program execution either registers or memory will change. Since our program yet doesn’t touch memory the only changes will be in the registers. Keenly observe the registers AX, BX, and IP change after every instruction. IP will change after every instruction to point to the next instruction while AX will accumulate the result of our addition.

Example

Example 1.1

001 ; a program to add three numbers using registers
002 [org 0x0100]
003 mov ax, 5 ; load first number in ax
004 mov bx, 10 ; load second number in bx
005 add ax, bx ; accumulate sum in ax
006 mov bx, 15 ; load third number in bx
007 add ax, bx ; accumulate sum in ax
008
009 mov ax, 0x4c00 ; terminate program
010 int 0x21

1 To start a comment a semicolon is used and the assembler ignores everything else on the same line. Comments must be extensively used in assembly language programs to make them readable.

002 Leave the org directive for now as it will be discussed later.

003 The constant 5 is loaded in one register AX.

The constant 10 is loaded in another register BX.
004


5 Register BX is again added to register AX now producing 15+15=30 in the AX register. So the program has computed 5+10+15=30

006 The constant 15 is loade d in the register BX.


7 Register BX is added to register AX and the result is stored in register AX. Register AX should contain 15 by now.

8 Vertical spacing must also be used extensively in assembly language programs to separate logical blocks of code.

009-010 The ending lines are related more to the operating system than to
assembly language programming. It is a way to inform DOS that our
program has terminated so it can display its command prompt
again. The computer may reboot or behave improperly if this
termination is not present.

Assembly Language Version

Assembly Language Version

Intel could have made their assembly language exactly identical to our program in plain English but they have abbreviated a lot of symbols to avoid unnecessarily lengthy program when the meaning could be conveyed with less effort. For example Intel has named their move instruction “mov” instead of “move.” Similarly the Intel order of placing source and destination is opposite to what we have used in our English program, just a change of interpretation. So the Intel way of writing things is:

operation destination, source operation destination operation source
operation

The later three variations are for instructions that have one or both of their operands implied or they work on a single or no operand. An implied operand means that it is always in a particular register say the accumulator, and it need not be mentioned in the instruction. Now we attempt to write our program in actual assembly language of the iapx88.

OUR FIRST PROGRAM

The first program that we will write will only add three numbers. This very simple program will clarify most of the basic concepts of assembly language. We will start with writing our algorithm in English and then moving on to convert it into assembly language.

English Language Version

“Program is an ordered set of instructions for the processor.” Our first program will be instructions manipulating AX and BX in plain English.

move 5 to ax move 10 to bx add bx to ax move 15 to bx add bx to ax

Even in this simple reflection of thoughts in English, there are some key things to observe. One is the concept of destination as every instruction has a “to destination” part and there is a source before it as well. For example the second line has a constant 10 as its source and the register BX as its destination. The key point in giving the first program in English is to convey that the concepts of assembly language are simple but fine. Try to understand them considering that all above is everyday English that you know very well and every concept will eventually be applicable to assembly language.

General Registers (AX, BX, CX, and DX)

The registers AX, BX, CX, and DX behave as general purpose registers in Intel architecture and do some specific functions in addition to it. X in their names stand for extended meaning 16bit registers. For example AX means we are referring to the extended 16bit “A” register. Its upper and lower byte are separately accessible as AH (A high byte) and AL (A low byte). All general purpose registers can be accessed as one 16bit register or as two 8bit registers. The two registers AH and AL are part of the big whole AX. Any change in AH or AL is reflected in AX as well. AX is a composite or extended register formed by gluing together the two parts AH and AL.

The A of AX stands for Accumulator. Even though all general purpose registers can act as accumulator in most instructions there are some specific variations which can only work on AX which is why it is named the accumulator. The B of BX stands for Base because of its role in memory addressing as discussed in the next chapter. The C of CX stands for Counter as there are certain instructions that work with an automatic count in the CX register. The D of DX stands for Destination as it acts as the destination in I/O operations. The A, B, C, and D are in letter sequence as well as depict some special functionality of the register.

Index Registers (SI and DI)

SI and DI stand for source index and destination index respectively. These are the index registers of the Intel architecture which hold address of data and used in memory access. Being an open and flexible architecture, Intel allows many mathematical and logical operations on these registers as well like the general registers. The source and destination are named because of their implied functionality as the source or the destination in a special class of instructions called the string instructions. However their use is not at all restricted to string instructions. SI and DI are 16bit and cannot be used as 8bit register pairs like AX, BX, CX, and DX.

Instruction Pointer (IP)

This is the special register containing the address of the next instruction to be executed. No mathematics or memory access can be done through this register. It is out of our direct control and is automatically used. Playing with it is dangerous and needs special care. Program control instructions change the IP register implicitly.

Stack Pointer (SP)

It is a memory pointer and is used indirectly by a set of instructions. This register will be explored in the discussion of the system stack.

Base Pointer (BP)

It is also a memory pointer containing the address in a special area of memory called the stack and will be explored alongside SP in the discussion of the stack.

Flags Register

The flags register as previously discussed is not meaningful as a unit rather it is bit wise significant and accordingly each bit is named separately. The bits not named are unused. The Intel FLAGS register has its bits organized as follows:


Segment Registers (CS, DS, SS, and ES)

The code segment register, data segment register, stack segment register, and the extra segment register are special registers related to the Intel segmented memory model and will be discussed later.

HISTORY

Intel did release some 4bit processors in the beginning but the first meaningful processor was 8080, an 8bit processor. The processor became popular due to its simplistic design and versatile architecture . Based on the experience gained from 8080, an advanced version was released as 8085. The processor became widely popular in the engineering community again due to its simple and logical nature.

Intel introduced the first 16bit processor named 8088 at a time when the concept of personal computer was evolving. With a maximum memory of 64K on the 8085, the 8088 allowed a whole mega byte. IBM embedded this processor in their personal computer. The first machines ran at 4.43 MHz; a blazing speed at that time. This was the right thing at the right moment. No one expected this to become the biggest success of computing history. IBM PC XT became so popular and successful due to its open architecture and easily available information.

The success was unexpected for the developers themselves. As when Intel introduced the processor it contained a timer tick count which was valid for five years only. They never anticipated the architecture to stay around for more than five years but the history took a turn and the architecture is there at every desk even after 25 years and the tick is to be specially handled every now and then.

INTEL IAPX88 ARCHITECTURE

Now we select a specific architecture to discuss these abstract ideas in concrete form. We will be using IBM PC based on Intel architecture because of its wide availability, because of free assemblers and debuggers available for it, and because of its wide use in a variety of domains. However the concepts discussed will be applicable on any other architecture as well; just the mnemonics of the particular language will be different.

Technically iAPX88 stands for “Intel Advanced Processor Extensions 88.” It was a very successful processor also called 8088 and was used in the very first IBM PC machines. Our discussion will revolve around 8088 in the first half of the course while in the second half we will use iAPX386 which is very advanced and powerful processor. 8088 is a 16bit processor with its accumulator and all registers of 16 bits. 386 on the other hand, is a 32bit processor. However it is downward compatible with iAPX88 meaning that all code written for 8088 is valid on the 386. The architecture of a processor means the organization and functionalities of the registers it contains and the instructions that are valid on the processor. We will discuss the register architecture of 8088 in detail below while its instructions are discussed in the rest of the book at appropriate places.

INSTRUCTION GROUPS

Usual opcodes in every processor exist for moving data, arithmetic and logical manipulations etc. However their mnemonics vary depending on the will of the manufacturer. Some manufacturers name the mnemonics for data movement instructions as “move,” some call it “load” and “store” and still other names are present. But the basic set of instructions is similar in every processor. A grouping of these instructions makes learning a new processor quick and easy. Just the group an instruction belongs tells a lot about the instruction.

Data Movement Instructions

These instructions are used to move data from one place to another. These places can be registers, memory, or even inside peripheral devices. Some examples are:

mov ax, bx lda 1234

Arithmetic and Logic Instructions

Arithmetic instructions like addition, subtraction, multiplication, division and Logical instructions like logical and, logical or, logical xor, or complement are part of this group. Some examples are:


and ax, 1234
add bx, 0534
add bx, [1200]

The bracketed form is a complex variation meaning to add the data placed at address 1200. Addressing data in memory is a detailed topic and is discussed in the next chapter.

Program Control Instructions

The instruction pointer points to the next instruction and instructions run one after the other with the help of this register. We can say that the instructions are tied with one another. In some situations we don’t want to follow this implied path and want to order the processor to break its flow if some condition becomes true instead of the spatially placed next instruction. In certain other cases we want the processor to first execute a separate block of code and then come back to resume processing where it left.

These are instructions that control the program execution and flow by playing with the instruction pointer and altering its normal behavior to point to the next instruction. Some examples are:

cmp ax, 0 jne 1234

We are changing the program flow to the instruction at 1234 address if the condition that we checked becomes true.

Special Instructions

Another group called special instructions works like the special service commandos. They allow changing specific processor behaviors and are used to play with it. They are used rarely but are certainly used in any meaningful program. Some examples are:

cli sti
Where cli clears the interrupt flag and sti sets it. Without delving deep into it, consider that the cli instruction instructs the processor to close its ears from the outside world and never listen to what is happening outside, possibly to do some very important task at hand, while sti restores normal behavior. Since these instructions change the processor behavior they are placed in the special instructions group.

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 ope rand 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. Naming is useful since the registers are few in number. This is called the nomenclature of the 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 registers do not hold data instead they are used to hold the address of data. There are other functions as well and the whole spectrum of register functionalities is quite large. However most of the details will become clear as the registers of the Intel architecture are discussed in detail.

Accumulator

There is a central register in every processor called the accumulator. Traditionally all mathematical and logical operations are performed on the accumulator. The word size of a processor is defined by the width of its accumulator. A 32bit processor has an accumulator of 32 bits.

Pointer, Index, or Base Register

The name varies from manufacturer to manufacturer, but the basic distinguishing property is that it does not hold data but holds the address of data. The rationale can be understood by examining a “for” loop in a higher level language, zeroing elements in an array of ten elements locate d in consecutive memory cells. The location to be zeroed changes with every iteration. That is the address where the operation is performed is changing. Index register is used in such a situation to hold the address of the current array location. Now the value in the index register cannot be treated as data, but it is the address of data. In general whenever we need access to a memory location whose address is not known until runtime we need an index register. Without this register we would have needed to explicitly code each iteration separately.

In newer architectures the distinction between accumulator and index registers has become vague. They have general registers which are more versatile and can do both functions. They do have some specialized behaviors but basic operations can be done on all general registers.

Flags Register or Program Status Word

This is a special register in every architecture called the flags register or the program status word. Like the accumulator it is an 8, 16, or 32 bits regi ster but unlike the accumulator it is meaningless as a unit, rather the individual bits carry different meanings. The bits of the accumulator work in parallel as a unit and each bit mean the same thing. The bits of the flags register work independently and individually, and combined its value is meaningless.

An example of a bit commonly present in the flags register is the carry flag. The carry can be contained in a single bit as in binary arithmetic the carry can only be zero or one. If a 16bit number is added to a 16bit accumulator, and the result is of 17 bits the 17th bit is placed in the carry bit of the flags register. Without this 17th bit the answer is incorrect. More examples of flags will be discussed when dealing with the Intel specific register set.

Program Counter or Instruction Pointer

Everything must translate into a binary number for our dumb processor to understand it, be it an operand or an operation itself. Therefore the instructions themselves must be translated into numbers. For example to add numbers we understand the word “add.” We translate this word into a number to make the processor understand it. This number is the actual instruction for the computer. All the objects, inheritance and encapsulation constructs in higher level languages translate down to just a number in assembly language in the end. Addition, multiplication, shifting; all big programs are made using these simple building blocks. A number is at the bottom line since this is the only thing a computer can understand.

A program is defined to be “an ordered set of instructions.” Order in this definition is a key part. Instructions run one after another, first, second, third and so on. Instructions have a positional relationship. The whole logic depends on this positioning. If the computer executes the fifth instructions after the first and not the second, all our logic is gone. The processor should ensure this ordering of instructions. A special register exists in every processor called the program counter or the instruction pointer that ensures this ordering. “The program counter holds the address of the next instruction to be executed.” A number is placed in the memory cell pointed to by this register and that number tells the processor which instruction to execute; for example 0xEA, 255, or 152. For the processor 152 might be the add instruction. Just this one number tells it that it has to add, where its operands are, and where to store the result. This number is called the opcode. The instruction pointer moves from one opcode to the next. This is how our program executes and progresses. One instruction is picked, its operands are read and the instruction is executed, then the next instruction is picked from the new address in instruction pointer and so on.

Remembering 152 for the add operation or 153 for the subtract operation is difficult. To make a simple way to remember difficult things we associate a symbol to every number. As when we write “add” everyone understands what we mean by it. Then we need a small program to convert this “add” of ours to 152 for the processor. Just a simple search and replace operation to translate all such symbols to their corresponding opcodes. We have mapped the numeric world of the processor to our symbolic world. “Add” conveys a meaning to us but the number 152 does not. We can say that add is closer to the programmer’s thinking. This is the basic motive of adding more and more translation layers up to higher level languages like C++ and Java and Visual Basic. These symbols are called instruction mnemonics. Therefore the mnemonic “add a to b” conveys more information to the reader. The dumb translator that will convert these mnemonics back to the original opcodes is a key program to be used throughout this course and is called the assembler.

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 readi ng 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.

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.

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.

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 leve l 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 grammer. He may need to translate a word into a sentence and destroy the beauty of the topic. And given that we do not know


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 90000 pixels per square inch. Now a processing on this picture requires 360000 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. 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.