INTRODUCTION TO PROGRAMMING THE 8086

Transcription

1 SAMPLE HELLO WORLD PROGRAM The following example shows a program that displays the traditional Hello, world! message on the screen. It contains the essential ingredients of an assembly language application. TITLE Program Hello World by Andrew H. Andersen, Jr. PAGE 60,132 ; Written Andrew H. Andersen, Jr. ; Brookdale Community College ; ELEC 242 ; Assignment x.model SMALL ; Setup a Stack.STACK 2000H ;Define values for predefined data.data prnt equ 9 done equ 4c00h msg db Hello world db 0Dh,0Ah,0Ah, $ ; We could also reserve storage for use ; under program control ; This is where we state what we want done ; We begin with a Main Procedure declaration.code.startup ; Now we should load the Data Segment register mov ax,@data mov ds,ax ; This program uses an MS/DOS BIOS function ; to send a message to the screen mov ah,prnt lea dx,msg int 21h ;or mov dx,offset msg ; The following 2 statements return control ; back to DOS since we are finished fini: mov ax,done int 21h.exit end Page 1

2 Here is a brief description of the important lines in the program: Line 1 contains the Title directive; all remaining characters on the line are treated as comments as are all characters on line 4. The source code for this program was written in assembly language and must be assembled into machine language before it can run. This program is compatible with both the Microsoft and Borland assemblers. Segments are the building blocks of programs: The code segment is where program instructions are stored; the data segment contains all variables, and the stack segment contains the program s runtime stack. The stack is a special area of memory that the program uses when calling and returning from subroutines. The.model small directive indicates that the program uses a maximum of 64K of memory for code, and 64K of memory for data. The.stack directive sets aside 2000h (8192) bytes of stack space for the program. The.data directive indicates the beginning of the data segment where variables are stored. In the declaration of message, the assembler allocates a block of memory to hold the string containing Hello, world!, along with two bytes containing a ASCII sequence in MS/DOS for the newline character sequence (0Dh for Carriage Return and 0Ah for LineFeed). The $ is a required string terminator character for the MS/DOS Function 9 console output routine being used. The.code directive indicates the beginning of the code segment. This is where the executable instructions are located..startup is an MASM directive that identifies the starting point of the program. The first two statements in the code segment are used to initialize the address of the data segment into the DS register. The next two statements in the code segment initialize two register for the MS/DOS Print Function. The entry requirements are We must load the address of a string into the DX register. DX is used as an address pointer and required by the routine. The MS/DOS BIOS function number is placed in the AH register. INT 21H causes DOS to execute the function The last two executable statements (beginning with the label fini) is the normal MS/DOS process end statement. This causes the program to halt and return control to the operating system..exit is the MASM directive, which is usually near the end directive. The last line of the program contains the end directive. This is the last line of the program to be assembled. Anything placed in an asm file after the end directive will be ignored by the assembler. After creating the Hello World source code program with a text editor, we would save it with the name HELLO.ASM. We would then invoke the assembler with the command: ml /Fl hello.asm The assembler would create an HELLO.OBJ, HELLO.LST, and HELLO.EXE. Entering the command HELLO at the MS/DOS prompt would cause the program to execute and display the message on the CRT. Here are the contents of HELLO.LST Page 2

3 Microsoft (R) Macro Assembler Version /29/00 08:36:36 Program Hello World by Andrew H. Andersen, Jr. Page 1-1 TITLE Program Hello World by Andrew H. Andersen, Jr. PAGE 60,132 ; Written Andrew H. Andersen, Jr. ; Brookdale Community College ; COMP ; Assignment x.model SMALL ; Setup a Stack.STACK 2000H ;Define values for predefined data 0000.DATA = 0009 prnt equ 9 = 4C00 done equ 4c00h C 6C 6F 20 msg db Hello world 77 6F 72 6C B 0D 0A 0A 24 db 0Dh,0Ah,0Ah, $ 0000.CODE.startup ; We could also reserve storage for use ; under program control ; This is where we state what we want done ; We begin with a Main Procedure declaration ; Now we should load the Data Segment register 0017 B R mov ax,@data 001A 8E D8 mov ds,ax ; This program uses an MS/DOS BIOS function ; to send a message to the screen 001C B4 09 mov ah,prnt 001E 8D R lea dx,msg ;or mov dx,offset msg 0022 CD 21 int 21h 0024 B8 4C00 fini: mov ax,done 0027 CD 21 int 21h ; The following 2 statements return control ; back to DOS since we are finished.exit end Page 3

4 Microsoft (R) Macro Assembler Version /16/00 11:49:32 Program Hello World by Andrew H. Andersen, Jr. Symbols 2-1 Segments and Groups: N a m e Size Length Align Combine Class DGROUP GROUP _DATA Bit 000F Word Public DATA STACK Bit 2000 Para Stack STACK _TEXT Bit 002D Word Public CODE Microsoft (R) Macro Assembler Version /16/00 11:49:32 Program Hello World by Andrew H. Andersen, Jr. Symbols 3-1 Symbols: N a m e Type Value Number Number Number Number L Near 0000 Text Text Text Text Text DGROUP done Number 4C00h fini L Near 0024 _TEXT msg Byte 0000 _DATA prnt Number 0009h 0 Warnings 0 Errors In C, the Hello World program source code is much smaller, and might be like the following: /* Hello World Program Written by Andrew H. Andersen, Jr. */ #include #include <stdio.h> <string.h> char msg[] = Hello world ; main() { printf( %s\n\n,msg); return 0 } This program would be called HELLO.C After compiling we will have an object file. Although the source program length is shorter in C, the executable program created by a C compiler may be over 12 times larger than the assembler version (over 8000 bytes vs. less than 600 bytes) Page 4

5 WRITING ASSEMBLY LANGUAGE PROGRAMS We are all aware that the English Language has specific rules that must be observed. Assembly language statements also have rules, which must be observed if we wish to have our program assembled. The source line in an assembly language statement is analogous to a sentence. The assembler recognizes three types of source lines: instructions, directives, and control. The assembly language instruction and assembler directives may consist of up to four fields as follows: NAME: OPCODE OPERAND ;COMMENT Any number of blanks may separate the fields, but they must be separated by at least one delimiter. Each statement must appear on a single line, and is terminated with a. The maximum length of an assembly language statement is 128 characters. The characters that may be used in assembly language source statements are the letters of the alphabet, and the digits 0 through 9. Most of the other printable characters may be used, although some of these characters are reserved. In addition, any ASCII character may appear in a data string enclosed in a single quote. Both upper and lower case alphabetical characters may be used, and most assemblers treat uppercase and lowercase characters as the same. For instance Val and val are treated as the same label or name. When a program displays data strings enclosed in single quotes, they are printed exactly as they appear. Certain characters have special meaning to the assembler, and these characters are called Delimiters. The delimiter defines the end of a source code statement, a field, or a component of a field. There are some delimiters that are reserved for use with Macros. DELIMITERS CHARACTER MEANING USE blank 1 or more blanks field separator or terminator field single quotes enclose a data string ( ) parentheses delimit an expression RETURN statement terminator HT TAB field separator or symbol terminator ; semicolon indicate start of a comment : colon delimiter for labels NAME FIELD The name is optional, and for some assemblers a colon must follow the label in the Code segment. A label is a symbolic name used to refer to a memory address or value. The label may use any alphanumeric character, and the first character of a label should be alphabetical. The student should use labels of six characters or less excluding the colon. This is primarily to do with spacing of lines and characters when printing the lst file. There are restrictions for label names. A label should not be the same as a mnemonic. We may not use the label SUB for a subroutine label since SUB is the assembly language mnemonic for subtract, but we could use the label SUB1. If we use a mnemonic or a single character that is the same as a register, or any reserved characters, the Assembler will flag the statement, and print a error message. If the assembler encounters a semicolon in the beginning of the label field, it treats the entire line as a comment. Page 5

6 OPCODE FIELD The opcode field must always contain a valid mnemonic or assembler directive. If it does not, the assembler will flag the statement and print an error message. All operands and assembler directives should begin at least one tab space from the left margin. OPERAND FIELD The operand field may contain a register name, data, or a label. If there is more than one operand required by the mnemonic, it must be separated by a comma. If the operand is data, it may be in binary, octal, decimal, hex, or ASCII. The numeric data type may be upper case or lower case. Binary data must be entered with no spaces, has a maximum of 8, 16, or 32 bits depending on the opcode, and a B as the last character, such as B. Octal data must be entered as a valid octal number up to 377Q for 8 bit data or Q for 16 bit data with no spaces depending on the opcode, and a Q as the character, such as 175Q. Decimal data is entered as a number up to 255 for 8 bit, 65,535 for 16 bit, or 4,294,967,296 for 32-bit unsigned data depending on the opcode. It may have no base designation or a D as the last digit of the number such a 37 or 37D. Hexadecimal data is entered as any valid hex number up to 0FFH for 8 bit, 0 FFFFH for 16 bit, or 0 FFFF FFFFH for 32-bit data depending on the opcode. It must have a number for the first digit and an H as the last digit of the number. When ASCII data is used in an operand, it must be enclosed in quotes. Whenever the $ is present in the operand field, it means the current address. In addition to specifying the number base in use, a valid operand could be a label with arithmetic or logical functions as part of the operand. OPERAND ARITHMETIC OPERATIONS OPERATION EXAMPLE EXPLANATION + LABEL+2 Address of LABEL + 2 bytes LABEL 1 Address 1 byte before LABEL * TOP*3 Equates to the value TOP times 3 / SIX/2 Equates to the value six divided by 2 or 3 NOT NOT TRUE the 1 s complement of TRUE AND WQ AND SE returns the value of label WQ AND SE OR WQ OR SE returns the value of label WQ OR SE XOR WQ XOR SE returns the value of label WQ XOR SE COMMENT FIELD The comment field must begin with a semicolon and may continue only until the end of the line. If it must continue on the next line, that line must begin with a semicolon. In the comments field the programmer should explain what events are taking place with that instruction or group of instructions. On line documentation is essential to enable the programmer or another individuals to determine what is happening. No programs will be accepted without proper documentation. Page 6

7 ASSEMBLER PSEUDO OPERATIONS A pseudo op is an assembler directive, and is located in the opcode field. Pseudo ops give instructions to the Assembler, and it may or may not generate machine code. Labels may be used to identify pseudo ops and most require operands. DATA ALLOCATION DIRECTIVES A variable is a symbolic name for a location in memory where some data are stored. A variable s offset is the distance from the beginning of the segment to the variable. A variable s name is automatically associated with its offset. An array is a group of sequential memory locations used for storage and whose address is represented by a symbolic name. The first element of the array is at the base address. Other elements of the array are accessed by adding an offset (the number of memory locations) to the base address. For example, if we declare an array containing four characters, the name alist identifies only the offset of the first character (A):.data mystring db This is a string mystring is a label or name. The purpose of the label is to associate a memory location in the data segment with user assigned name. This makes it easy for the programmer to access the data stored here. The DB directive instructs the assembler to reserve storage for data on a byte boundary. T is the first letter of the string, and is placed in the base address of mystring. The h is offset +1 or one byte after the T (the address of this memory location is mystring +1 byte). The character i is located at mystring+2 (the address of mystring plus two bytes), and so on for all characters in the string. The address of mystring is based on its location in the program. Address Contents mystring 54h mystring+1 68h mystring+2 69h mystring+3 73h mystring+4 20h And so on. In the table above, the numeric data is the hex value for the ASCII character. 20h is the ASCII character for a space. We use the following MASM data allocation directives to reserve storage on multiple byte boundaries. Different assemblers may us slightly different assembler directives. You should check the documentation for the proper declaration syntax. For MASM: Assember Directive Description Size DB Define byte 1 byte DW Define word 2 bytes DD Define doubleword 4 bytes DQ Define quadword 8 bytes DT Define tenbytes 10 bytes Page 7

8 DB The Define Byte directive is used to define memory storage of data on an 8 bit boundary. Labels are optional, but an operand is required. Multiple operand values may be entered in any acceptable format (numeric can be in binary, octal, decimal, or hex) and is separated by commas. Strings are enclosed in quotes. Unsigned integer values are in the range of 0 to 255 The following are examples of the DB directive: VALUE DB 100 ;DEFINE VALUE AS 100. VAL2 DB 10H ;DEFINE VAL2 AS 10 IN HEX. BINUM DB B ;DEFINE BINUM AS BINARY VALUE. MSG DB HELLO ;FIVE BYTE ASCII MESSAGE. NUM DB 1,2,3,4 ;DEFINE NUM AS THE FOUR BYTES ;1, 2, 3, AND 4 IN DECIMAL. INIT DB? ;INITIAL VALUE UNASSIGNED DUP The DUP operator, which is placed in the operand field, allows you to initialize storage to one or more multiple values. The DUP operator used with the DB directive is: NUM1 DB 80 DUP (20h) ;INITIALIZES 80 BYTES OF STORAGE TO ;20H, the ASCII value for a space VAL2 DB 5 DUP ( No ) ;DEFINE VAL2 AS NoNoNoNoNo DW The Define Word directive is similar to the Define Byte, except that it is used to define 16 bit data (A word) instead of 8 bit data. The syntax of the DW directive is similar to DB except that it stores the defined data on a word boundary. It is used to represent unsigned Integer values (0 to ) The DW format is: NUM1 DW 13H ;DEFINE NUM1 AS THE VALUE 0013H NUM2 DW 1000 ;DEFINE NUM2 AS THE VALUE 1000 CAL1 DW 256 * 2 ;ASSIGN THE VALUE 512 TO CAL1 BLANK DW 80 DUP (?) ;RESERVE 80 WORDS UNINITIALIZED BLANK DW 80 DUP (20H) ;RESERVE 80 WORDS OF SPACES (0020H) DD The Define DoubleWord directive is similar to DB and DW, except that it is used to define 32 bit data (A double word). The syntax of the DD directive is similar to DB except that it stores the defined data on a double word boundary. It is used to represent unsigned Integer values (0 to ) The DW format is: NUM1 DD 13H ;DEFINE NUM1 AS THE VALUE H DQ The Define QuadWord directive is similar to DB, except that it is used to define 64 bit data. Unsigned integer values range from 0 to (~1.8e19) The DW format is: NUM1 DQ 13H ;DEFINE NUM1 AS H Page 8

9 Equates Directive EQU The EQU directive is used to equate a symbolic name (or label) to another label or a value. You should make liberal use of labels and equate statements since they make the program easier to understand. Although the EQU statement may appear anywhere in a program, it is a good technique to place all the equate statements together in the DATA area. An example of the equate statement is: SIX EQU 6 COUNT EQU 25 NUM EQU 100H VAL2 EQU SIX VAL8 EQU SIX+2 Equal-Sign Directive = This is similar to the EQU, but is used as a redefinable equates. The value associated with the = may be reassigned during assembly. Like the EQU directive, it cannot change during program execution/ Examples are: count = maxlongint = 7FFF FFFFh ( ) minlongint = h ( ) maxunsignlongint = 0 FFFF FFFFh ( ) ORG The ORG directive is used to change the current address of a program to another address. It should be used in the beginning of the program, and is usually the last statement in the Equates Area. The addresses below 03FFH (4 groups of 256) are used to store the RESET and RESTART (vectored interrupt) and user defined vectored interrupt addresses. The programmer or operating system will usually load these addresses but they are not used to store source code. The ORG statement may be used at any point in the program, and may be used more than once. It could be used to define a hex lookup table for printing the ASCII value of single hex digits that begins at a specific address..data ORG 1000H HEXTBL DB DB DB ABCDE DB F END The END assembler directive identifies the last line of the source code program. Only one END statement may appear in a source code program as any statements after the end statement are ignored by the assembler. The END statement should be located in the OPCODE field, and labels and operands are optional when using simplified segment directives. Page 9

10 OFFSET Operator The OFFSET operator returns the offset of a memory location from the beginning of its segment. The destination operand must be a l6-bit register, as in the following example: mov si,offset addr ; SI points to addr SI points to addr because it contains its offset address. When the register contains an address that we wish to access,, we call it a pointer. This is a technique we will use later when we see how to access the contents of an array. In the following example, assume that bytesize is the first storage location in the data segment..data bytesize db 10h,25h,3h,4Fh wordsize dw 1000h,2000h,3000h.code mov di, offset bytesize ;DI points to address 0000 with respect to the ;base address of the data segment or DS:0000 mov bx, offset bytesize+1 ;BX points to the memory location bytesize+1 (0001) ;or DS:0001 mov si offset wordsize+2 ;SI points to the memory location DS:0006 PTR Operator The PTR operator forces an operand to a desired boundary. For instructions where the boundary of an operand might be ambiguous, the PTR operator can be used to define the operand s proper size in combination with the declared data size. For example, we might see: mov al,byte ptr Num1 mov si,word ptr Num2 mov eax,dword ptr listpointer When the size of an operand is not clear from the context of an instruction, you must make sure to use the PTR directive. Consider the following instruction, which would generate an operand must have size error message by the assembler: inc [si] In the previous instruction, the assembler cannot determine what size data SI is pointing to. By using the PTR operator, the assembler knows the data size. inc word ptr [si] The.MODEL is used to define the type of program being created. It automatically sets the default attribute for all procedures to either near or far. No. of Segments OS.model Data Code DOS Features tiny 1 1 Used for COM only, Data and Code combined small 1 1 Smallest EXE file medium 1 Multiple compact Multiple 1 large Multiple Multiple huge Multiple Multiple DOS/ Windows flat 1 1 Windows NT Uses normalized addressing Uses 32-bit addressing, Data and Code combined Page 10

11 There are two methods to do this with MASM. The Standard Segment Directives, which works with both older and newer versions of assemblers, and the Simplified Segment Directives, which only works with MASM 5 and higher. SEGMENT ENDS The 80x86 has different segments for the Stack, Data, and Code. Each of these three storage areas should be identified. Defines the end of the Segment. This must occur before the END directive STANDARD DIRECTIVE METHOD ML assembles and links in one command. If you install MASM on your home computer and you wish to edit and assemble files from your Floppy Disk, open AUTOEXEC.BAT with EDIT in C:\ and add the following to your PATH statement: C:\MASM611\BIN With MASM 6.11 and higher, the ML command assemble and links a.asm file ml hello.asm It produces a.obj file and a.exe file Option /Fl produces a source listing with an lst extension Option /Fm produces a map file with a map extension To produce both of these, type (with spaces) ml /Fl /Fm hello.asm PROCESSORS MASM supports a set of directives for selecting processors and coprocessors. Once you select a processor, you must use only the instruction set for that processor. The default is the 8086 processor. To enable a different processor modes and the additional instructions available on that processor, use the directives.186,.286,.386, and.486. The.286P,.386P, and.486p directives enable the instructions available only at higher privilege levels in addition to the normal instruction set for the given processor. Generally, you don t need privileged instructions unless you are writing operating-systems code or device drivers. STACK Segment The stack is the section of memory used for pushing or popping registers and storing the return address when a subroutine is called. The stack often holds temporary and local variables. If your main module is written in a high-level language, that language handles the details of creating a stack. Use the.stack directive only when you write a main module in assembly language. The.STACK directive creates a stack segment. By default, the assembler allocates 1K of memory for the stack. This size is sufficient for most small programs. To create a stack of a size other than the default size, give.stack a single numeric argument indicating stack size in bytes:.stack 2048 ; Setup 2K stack.stack 2000h ; Setup 2K stack exactly same as previous Page 11

12 DATA Segment Programs can contain both near and far data. In general, you should place important and frequently used data in the near data area, where data access is faster. This area can get crowded, however, because in 16-bit operating systems the total amount of all near data in all modules cannot exceed 64K. Therefore, you may want to place infrequently used or particularly large data items in a far data segment. The.DATA,.DATA?,.CONST,.FARDATA, and.fardata? directives create data segments. You can access the various segments within DGROUP without reloading segment registers (see Defining Segment Groups, later in this chapter). These five directives also prevent instructions from appearing in data segments by assuming CS to ERROR. Near Data Segments The.DATA directive creates a near data segment. This segment contains the frequently used data for your program. It can occupy up to 64K in MS-DOS or 512 megabytes under flat model in Windows NT. It is placed in a special group identified as DGROUP, which is also limited to 64K. When you use.model, the assembler automatically defines DGROUP for your near data segment. The segments in DGROUP form near data, which can normally be accessed directly through DS or SS. Far Data Segments The compact, large, and huge memory models use far data addresses by default. With these memory models, however, you can still construct data segments using.data,.data?, and.const. The effect of these directives does not change from one memory model to the next. They always contribute segments to the default data area, DGROUP, which has a total limit of 64K. When you use.fardata or.fardata? in the small and medium memory models, the assembler creates far data segments FAR_DATA and FAR_BSS, respectively. You can access variables with: mov ax, SEG farvar2 mov ds, ax Code Segment Whether you are writing a main module or a module to be called from another module, you can have both near and far code segments. This section explains how to use near and far code segments and how to use the directives and predefined equates that relate to code segments. Near Code Segments The small memory model is often the best choice for assembly programs that are not linked to modules in other languages, especially if you do not need more than 64K of code. This memory model defaults to near (two-byte) addresses for code and data, which makes the program run faster and use less memory. When you use.model and simplified segment directives, the.code directive in your program instructs the assembler to start a code segment. The next segment directive closes the previous segment; the END directive at the end of your program closes remaining segments. You can use the predefined to determine whether code pointers default to Page 12

13 Far Code Segments When you need more than 64K of code, use the medium, large, or huge memory model to create far segments. The medium, large, and huge memory models use far code addresses by default. In the larger memory models, the assembler creates a different code segment for each module. If you use multiple code segments in the small, compact, or tiny model, the linker combines the.code segments for all modules into one segment. For far code segments, the assembler names each code segment MODNAME_TEXT, in which MODNAME is the name of the module. With near code, the assembler names every code segment _TEXT, causing the linker to concatenate these segments into one. You can override the default name by providing an argument after.code. (For a complete list of segment names generated by MASM, see Appendix E, Default Segment Names. ) With far code, a single module can contain multiple code segments. The.CODE directive takes an optional text argument that names the segment. For instance, the following example creates two distinct code segments, FIRST_TEXT and SECOND_TEXT..CODE FIRST.. ; First set of instructions here..code SECOND.. ; Second set of instructions here The easiest way to begin and end an MS-DOS program is to use the.startup and.exit directives in the main module. The main module contains the starting point and usually the termination point. You do not need these directives in a module called by another module. These directives make MS-DOS programs easy to maintain. They automatically generate code appropriate to the stack distance specified with.model. However, they do not apply to flat-model programs written for 32-bit operating systems. Thus, you should not use.startup or.exit in programs written for Windows NT. To start a program, place the.startup directive where you want execution to begin. Usually, this location immediately follows the.code directive:.code.startup.. ; Place executable code here..exit END Note that.exit generates executable code, while END does not. The END directive informs the assembler that it has reached the end of the module. All modules must end with the END directive whether you use simplified or full segments. Page 13

14 If you do not use.startup, you must give the starting address as an argument to the END directive. For example, the following fragment shows how to identify a program s starting instruction with the label start:.code start:.. ; Place executable code here. END start Only the END directive for the module with the starting instruction should have an argument. When.STARTUP is present, the assembler ignores any argument to END. Sometimes MASM generates a warning stating the address of end was ignored. Do not worry about this warning as it is generated because the.exit and end directives are adjacent. For the default NEARSTACK attribute,.startup points DS to DGROUP and sets SS:SP relative to DGROUP, generating the following code: Page 14

15 SIMPLIFIED SEGMENT DIRECTIVE METHOD Title Segment Example TOTAL.ASM by Andrew H. Andersen, Jr. Page 60,132 ;.MODEL SMALL.DATA ORG 7000H POINTS DB 16 DUP(?) ;save room for 16 data bytes SUM DB? ;save room for result (1 byte).code.startup TOTAL: MOV AX,7000H ;load address of data area MOV DS,AX ;init data segment register MOV AL,0 ;clear result MOV BL,16 ;init loop counter LEA SI,POINTS ;init data pointer ADDUP: ADD AL,[SI] ;add data value to result INC SI ;increment data pointer DEC BL ;decrement loop counter JNZ ADDUP ;jump if counter not zero MOV SUM,AL ;save sum MOV AH,04CH ;Normal MS-DOS progam end INT 21H ;Using MS-DOS.EXIT END TOTAL Microsoft (R) Macro Assembler Version /20/00 08:37:41 Segment Example TOTAL.ASM by Andrew H. Andersen, Jr. Page 1-1 Title Segment Example TOTAL.ASM by Andrew H. Andersen, Jr. Page 60,132 ;.MODEL SMALL 0000.DATA ORG 7000H [ POINTS DB 16 DUP(?) ;save room for 16 data bytes 00 ] SUM DB? ;save room for result (1 byte) 0000.CODE.STARTUP 0017 B TOTAL: MOV AX,7000H ;load address of data area 001A 8E D8 MOV DS,AX ;init data segment register 001C B0 00 MOV AL,0 ;clear result 001E B3 10 MOV BL,16 ;init loop counter D R LEA SI,POINTS ;init data pointer ADDUP: ADD AL,[SI] ;add data value to result INC SI ;increment data pointer 0027 FE CB DEC BL ;decrement loop counter F9 JNZ ADDUP ;jump if counter not zero 002B A R MOV SUM,AL ;save sum 002E B4 4C MOV AH,04CH ;Normal MS-DOS progam end 0030 CD 21 INT 21H ;Using MS-DOS.EXIT END TOTAL Page 15

16 Microsoft (R) Macro Assembler Version /20/00 08:37:41 Segment Example TOTAL.ASM by Andrew H. Andersen, Jr. Symbols 2-1 Segments and Groups: N a m e Size Length Align Combine Class DGROUP GROUP _DATA Bit 7011 Word Public DATA _TEXT Bit 0036 Word Public CODE Microsoft (R) Macro Assembler Version /20/00 08:37:41 Segment Example TOTAL.ASM by Andrew H. Andersen, Jr. Symbols 3-1 Symbols: N a m e Type Value Number Number Number Number L Near 0000 Text Text Text Text Text DGROUP ADDUP L Near 0024 _TEXT POINTS Byte 7000 _DATA SUM Byte 7010 _DATA TOTAL L Near 0017 _TEXT 1 Warnings 0 Errors Both versions of the program accomplish the same task. The second version allows the Assembler to perform some housekeeping chores for us. Let us briefly look at a sample line of output in the total.lst file contents and what it represents: 8000 B TOTAL: MOV AX,7000H ;load address of data area 8000 is the Hex address in memory for this instruction B8 is the Intel HEX code for MOV AX 7000H is the hex value of the data TOTAL is the Label field MOV is the Opcode AX,7000H is the operand ;load is the comment. The assembler generated the first three, and the programmer typed the last 4 in the source code file. Page 16

17 TITLE Program STANDARD.ASM written by Andrew H. Andersen, Jr. Page 60,132 ;*************************************************** ; ; Program number ; Program STANDARD.ASM ; Written by Andrew H. Andersen, Jr. ; Course - Section ; ; This program, ; ; We use the procedures Clrscr, Gotoxy, Writestring ; Readint, and Writeint from LIBRARY.LIB ; ;***************************************************.model small.stack 1000h.data row1 row2 row3 row4 row5 equ 0516h equ 0616h equ 0916h equ 0a16h equ 0b20h prompt1 db This program adds two decimal numbers,0 prompt2 db that you enter from the keyboard.,0 get1st db Enter the 1st number -->,0 get2nd db Enter the 2nd number -->,0 showans db The sum is -->,0.code ; Define proc used from library.lib ; the \ is continue form previous line extern Writeint:proc, Writestring:proc, \ Readint:proc, Gotoxy:proc,.startup main: mov ax,@data mov ds,ax Page 17

18 ;************ ; ; Define what happens here ; ;************ init: call Clr call SetCur ; the rest of main is placed here fini: mov ah,04ch ;standard MS/DOS Process End int 21h ; ALL PROCEDURES ARE HERE ; Clear the screen using MS/DOS Video BIOS Clr proc ; Clear Screen with 25 line scroll. ; The screen is blue and the text is yellow Clr mov ah, 6 ;The Video BIOS function mov al, 0 ;Scroll # of lines 0 = all mov cx, 0 ;row/col at top mov dx, 1950h ;row/col at bottom 24 & 79 mov bh, 1eh ;foreground/background colors int 10h endp SetCur proc ; Now set Cursor to row 5 column 30 on page 0 mov ah, 2 ;The BIOS function mov dx, 051eh ;row/column position mov bh, 0 ;video page no. 0 int 10h ;using Video BIOS ret SetCur endp.exit end Page 18