How to write an assembly language program.

Transcription

1 Copyright 1999 by Barry B. Brey How to write an assembly language program. Someone asked about this and, at first, I thought this was a very difficult question until I thought about it for a few minutes. There is a general and simple way to get started. Step One: Know the form! To write a program you must place it into the correct form for the assembler..model SMALL END <<data goes here>> <<program goes here>> <<procedures go here>> The above template can be used to set up just about all assembly language programs. The only time that this will not work is if your program and procedures are larger than 64K bytes or if the amount of data is larger than 64K bytes. Once the form is know all we need to do is figure out what is data, a program, and procedure. Step Two: Know the data. Data comes in a few different forms. The most common are byte (8-bit) and word-sized (16-bit) data, but there is also doubleword (32-bit) and quadword (64-bit) data. For the most efficient operation of a program, it is important that the largest data I specified first in a data segment. That means if the program uses word and byte data, define all the word data first! To store data in the data section of a program we use DB (for bytes), DW (for words), DD (for doublewords), and DQ (for quadwords).

2 For example the DB directive can be used in the following three ways: DATA1 DB 34H DATA2 DB? DATA3 DB 4 DUP( Here the first line stores a 34 (hexadecimal) into a memory location called DATA1 The second line reserves a byte in memory at a location called DATA2 The last line sets aside 4 duplicate bytes at an area of memory called DATA3. This may still be puzzling, so I have drawn the contents of memory below: H? DATA1 DATA2 DATA Notice that the three assembly language lines have produced a section of memory that contains 6 bytes. The first byte (location ), contains a 34H and is called DATA1. The second byte (location 1), contains an unknown value (?) and is called DATA2. The final section contains 4 duplicates of. Memory location DATA3 points to the first byte of a block of 4 bytes. This is similar to a high-level language array. The array contains 4 bytes: DATA3, DATA3+1, DATA3+2, and DATA3+3. Think of these as array elements, 1, 2, and 3. A complete data section might appear as follows: TWO DW 2 LIST DW 2 DUP(?) ARRAY1 DW 1 DUP(?) MYNAME DB Barry SET1 DB 5 SET2 DB? Here we stored both bytes and words of data. Words (DW) are stored first and bytes (DB) last. The TWO is stored in 2 bytes of memory as a 2H. LIST requires 4 bytes of memory to reserve 2 words. ARRAY1 requires 2 bytes of memory to reserve 1 words. MYNAME holds an ASCII character string of 5 bytes in length. Finally SET1 and SET2 each set aside a byte of memory. In all this data section represents 249 bytes of memory.

3 Step Three: Know the program. This is the difficult part about programming. What do we put in as a program? Of course, there is no single answer for this, but there is a method. When a program is written there are always parts where you just get stumped. Where you just can t figure out what to do! How do you write a program when you are stuck? If you get stuck, CALL a procedure! Suppose that you want to write a program that produces the following list of information and accepts a number 1, 2, or 3 for a choice: My Breakfast Menu 1 Eggs 2 Bacon and Eggs 3 Ham and Eggs Enter Choice: This looks like it might be difficult. Is it? No. Identify the data needed for this display and setup your data block as I have below:.model SMALL DB 3 Ham and Eggs, 13,1,1 You might be thinking that this doesn t look the same, and you are correct, but it almost looks like the display. What is different are the 13, 1, and $. The 13 is an ASCII code called a carriage return. It returns the cursor to the left side of the video display. The 1 is an ASCII code called a line feed. The line feed moves the cursor down the screen one line. The $ is used to indicate that it is the end of the string of ASCII characters. This entire data block is one message. But what about the program? We are going to write the program now! What you want to do is display the menu and that allow someone to type a choice of 1, 2, or 3 to select one of the items. I don t have any idea how I am going to display the menu or accept the choice. I would write the program as listed on the next page.

4 .MODEL SMALL DB 3 Ham and Eggs, 13,1,1 SI,OFFSET M1 CALL MESSAGE CALL GETKEY Notice that the program is added to the data. I have a short program that contains 3 instructions. The first instruction is a move instruction that loads the address of the message into a register. I used SI here, but you can use any register. I just needed a place to store the address of the message so I could transfer it to a subroutine called MESSAGE. If you look at this program, I have completed the program, but I haven t filled in any details. I can clearly see that it displays a message and gets a key. I have no idea how I am going to display the message or get the key, but at least I have everything in the right places in the program. The next part is the difficult part, writing the procedures MESSAGE and GETKEY. The first place to start is by looking for a DOS function call that fits what you want to do. In this program we want to display a character string or message and we want to read a key. Next stop is the appendix in the textbook that lists all of the DOS INT 21H function calls. As you go down through this list you will find that function 1H seems to fit for reading a key and function 9H seems to fit for displaying a character string or message. Lets look at function 9H first in the appendix: Entry: AH = 9H and DS:DX = address of message. This tells us everything we need to know about displaying a message. is the DS or data segment and DX is where we need to load the offset address. INT AH,9 DX,OFFSET M1 21H The above will display our menu so we put it into our program as listed on the next page.

5 .MODEL SMALL DB 3 Ham and Eggs, 13,1,1 SI,OFFSET M1 CALL MESSAGE CALL GETKEY MESSAGE INT RET PROC NEAR AH,9H DX,SI 21H MESSAGE ENDP Only one more procedure and we have a working program, sort of. If you look at DOS INT 21H function 1, all you need to do to read a key is place 1 into AH. We need a filtered input of 1, 2, or 3 only. We use the repeat until construct to keep reading until a 1 or a 2 or a 3 are typed..repeat AH,1H INT 21H ;reads key.until AL==1 AL==2 AL==3 The completed program appears on the following page with everything in place.

6 .MODEL SMALL DB 3 Ham and Eggs, 13,1,1 SI,OFFSET M1 CALL MESSAGE CALL GETKEY MESSAGE INT RET PROC NEAR AH,9H DX,SI 21H MESSAGE ENDP GETKEY PROC NEAR.REPEAT AH,1H INT 21H ;reads key.until AL==1 AL==2 AL==3 RET GETKEY ENDP END