ICS 232 Lab 1

Pentium Assembly Language Programming using DOS Debugger

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 concepts                

                                      

CPU Registers

The 8086 CPU is 16 bit processor, which 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 register

Debug program

To 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 that 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 disk

It 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:/>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:

0D62:0101 803ED3DF00    CMP     BYTE PTR [DFD3],00                 CS:DFD3=03

All 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.

Assembler structure

In 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 bh

Here "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, 25

In 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 program

The 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 this is done, the screen will produce the following lines:

C:\>debug

-a 100

0D62:0100 mov ax,0002

0D62:0103 mov bx,0004

0D62:0106 add ax,bx

0D62:0108 nop

0D62:0109

Type the command "t" (trace), to execute each instruction of this program, example:

-t

AX=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,0004

You see that the value 2 moves to AX register. Type the command "t" (trace), again, and you see the second instruction is executed.

-t

AX=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,BX

Type the command "t" (trace) to see the instruction add is executed, you will see the follow lines:

-t

AX=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            NOP

The 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.

Storing and loading the programs

It 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 and 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:010A

To 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 program's final address (10A), and the program's 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 000a

The "n" command allows us to name the program.

-n test.com

The "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

:000a

Lastly, the "w" command writes our program on the disk, indicating how many bytes it wrote.

-w

Writing 000A bytes

To 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 20

The 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.

Activity 1.1: Enter the following program instructions in assembly code at the offset memory location 100h by typing A 100 at the DEBUG program prompt then press Enter-key.

MOV AX, 2864

ADD AX, 3749

MOV BX, AX

SUB BX, 2805

NOP

Activity 1.2: Use DEBUG command U 100 to unassembled the instructions in Activity 1.1. What is the machine code corresponding to each assembly code instruction?

Assembly code	Machine Code
MOV AX, 2864
ADD AX, 3749
MOV BX, AX
SUB BX, 2805
NOP

Activity 1.3: How many bytes does it need to represent each instruction in binary?

Assembly code	Number of bytes
MOV AX, 2864
ADD AX, 3749
MOV BX, AX
SUB BX, 2805
NOP

Activity 1.4: What are the contents of CS, IP, AX, and BX? Use DEBUG command R to display these information?

Register	Content
CS
IP
AX
BX

Activity 1.5: Predict the contents of the following registers after execution of each instruction: CS, IP, AX, and BX.

Register	MOV AX, 2864	ADD AX, 3749	MOV BX, AX	SUB BX, 2805
CS
IP
AX
BX

Debug Flag symbols

Status Flag	Set (1) Symbol	Clear (0) Symbol
CF	CY(Carry)	NC(No carry)
PF	PE (Even parity)	PO (odd parity)
AF	AC(Auxiliary carry)	NA (No Auxiliary carry)
ZF	ZR(zero)	NZ(No zero)
SF	NG(negative)	PL (plus)
OF	OV(Overflow)	NV(no overflow)
Control Flags
DF	DN(Down)	UP(up)
IF	EI(Enable Interrupt)	DI(disable interrupt)

Activity 1.6

a) Run the program given in by trace command given in Activity 1.1 and note the status of each flag for each instruction.

b) For each of the following instructions, give new destination contents and new setting of CF, SF, ZF, PE, and OF. Suppose that the flags are initially 0 in each part of this question.

i)                    ADD AX, BX                  AX=7FFFH and BX=0001H

ii)                  SUB AL, BL                    AL=01H, and BL=FFH

iii)                DEC AL                           AL=00H

iv)                ADD AL, BL                   AL=80H and BL=FFH

Working with Memory locations

Debugger’s “E” command can be used to change the content of a memory location.  The “D n” command is used to display the content of memory location “n.”  In assembly language, a memory location’s content can be accessed by specifying its address within square brackets.  For example, following sequence of 80x86 assembly transfer the content of memory location 200 to 201:

Mov Al, [200]

MOV [201],AL

INT 20

 The result of execution is shown below:

Note that locations 200 and 2001 now contain same value.  

Activity 2.1

Write an assembly language program to add the 10 values saved in locations starting from address 200.  Save the result in location 300.