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1 CHAPTER 1 - SOME SIMPLE PROGRAMS It is now time to start writing assembler code. One of the problems with writing in assembler is that there is no way to get input into the program or output from the program until you are very far along with learning assembler language. This is a Catch-22 situation. You can't learn assembler easily without access to input and output, and you can't write i/o routines till you know assembler.{1} Help is at hand. Included on this disk is a file called asmhelp.obj. It is actually a series of programs that will allow you to get input from the keyboard and print output to the screen. It has some other features which will be explained later. The second problem at the start is that every assembler program has a lot of overhead. These are standard instructions and formats that you need to get the program to work AT ALL. This disk contains templates that contain all the overhead, so to write a program you just make a copy of the template and enter the code and data at the appropriate place. By the end of this sequence of lessons, you will know how to make templates yourself and know the meaning of each word in the template. For now, you have to have faith that what is written is necessary, and that you will learn the meaning of everything later. Let's start. At the end of this chapter is the template we will use for now - temp1.asm. These templates are in the subdirectory \template. Let's call the first program prog1.asm (very original). All programs in assembler must have the file extension .asm so make a copy by giving the command: >copy \template\temp1.asm prog1.asm You are now ready to enter code. Open up prog1.asm with your editor, and take a look at it. It should look the same as temp1.asm. Where it says "put name of program here" - that is for your personal use so you can see the program name while in the editor. The assembler ignores everything after a semicolon. All the lines that start with a semicolon are there for visual separation or for comments. The lines with asterisks separate segments. Yes, the assembler is going to make this program into three segments. You should put all code between the line labeled "START CODE BELOW THIS LINE" and the line labeled "END CODE ABOVE THIS LINE". ____________________ 1 Just to give you an idea of how contradictory the situation is, asmhelp.obj was written in assembler language and consists of about 3500 lines (that's about 50 pages), yet you need to be using it from day one. ______________________ The PC Assembler Tutor - Copyright (C) 1989 Chuck Nelson The PC Assembler Tutor 2 ______________________ Later you will get more flexibility. Also notice the lines starting with the word EXTRN. Those lines tell the assembler that the subroutines (such as print_num) are in a different file and must be found when this program is linked. The assembler enters each EXTRN name in a list and records each place that an EXTRN subroutine is requested. It is possible that one of these subroutines is never requested. That's fine. However, every one of these subroutines must be present at link time or you will get a link error. This is true even if the subroutine is on the list but never requested. Since the overhead is so long, and I am so lazy, the template will never be included in the description of the program. What you will get is the name of the template, then any data you need, then the code. It will look like this: TEMP1.ASM ; - - - - - START DATA BELOW THIS LINE the data is written here ; - - - - - END DATA ABOVE THIS LINE ; - - - - - START CODE BELOW THIS LINE the code is written here ; - - - - - END CODE ABOVE THIS LINE If there is no data, no data section will be included. If there is data, it should be written in the segment named DATASTUFF between the lines "START DATA..." and "END DATA ...". The code should be written between the lines that say "START CODE ..." and "END CODE ...". For our first program, the description will look like this: TEMP1.ASM ; - - - - - START CODE BELOW THIS LINE first_label: call get_num call print_num jmp first_label ; - - - - - END CODE ABOVE THIS LINE That's all we need. If we needed to write all the overhead (starting with the line "main proc far") we would have: main proc far start: push ds sub ax, ax push ax mov ax, DATASTUFF mov ds, ax ; + + + + + + + + + + + Chapter 1 - Some Simple Programs 3 ________________________________ first_label: call get_num call print_num jmp first_label ; + + + + + + + + + + + ret mainendp etc. You can see that a simple four line program has blossomed into a monster. I'm assuming some intelligence on your part. Until further notice, the code goes between the "START CODE' and the "END CODE" lines and the data goes in the DATASTUFF segment between the "START DATA" and the "END DATA" lines. It's time to type in the program listed above. Be careful when you type. When you are done I'll explain it. In assembler we need a way to label different spots in the code. We use labels. A LABEL is a name (at the beginning of a line) which is immediately followed by a colon. A label doesn't generate any code. The assembler merely keeps track of where the label is for future use. The label we are using is named first_label. The CALL instruction tells the assembler to call the subroutine listed after the call.{2} We are calling two subroutines; first get_num which gets a number, then print_num, which prints a number in a variety of styles. Finally, JMP tells the assembler that you want to jump to the label listed after it. It is the same as GOTO in BASIC. If you look at the program, you will notice that we have an infinite loop. It was designed that way. It takes a fair amount of code to exit gracefully, so we will always exit ungracefully. When you are tired of the program, simply press Control-C. That should get you out. That way you can try out something an indefinite number of times, and when you have finished you can press CTRL-C to quit the program. One warning about machine language before we start. There is no safety net, so before you start a machine language program, make sure all files are closed (i.e. that you have no other programs in memory). We will NEVER open a file in one of our programs. ____________________ 2 I am using the words subroutine, routine, program and procedure (the technical word) interchangebly throughout the book. A program is actually a group of one or more procedures, but I'm not going to be too strict about it. Context should tell whether we are talking about a single procedure or a whole program. The PC Assembler Tutor 4 ______________________ Your programs are almost certain to wind up in zombie space from time to time. If that happens, your choices are (1) hit CTRL-C. If that doesn't work, then (2) hit CTRL-ALT-DEL. As a last resort, you can (3) hit a reset button or shut the machine down. For that reason, memorize this mantra: BACKUP YOUR PROGRAMS AND BACKUP YOUR BACKUPS. Double check that you have typed in the assembler code correctly. Now it's time to assemble it. I am assuming that you have the Microsoft assembler.{3} Type: >masm prog1 ; The first thing you will see is the copyright notice: Microsoft (R) Macro Assembler Version 5.10 Copyright (C) Microsoft Corp 1981, 1988. All rights reserved. The file extension .asm is unnecessary; it's understood. The semicolon is to speed things up. If you don't use it, the assembler will ask you if you want to change any of the default choices. If you type just masm: >masm then you need to give the name of the assembler text file on the first line: Source filename [.ASM]: prog1 Object filename [prog1.OBJ]: Source listing [NUL.LST]: Cross-reference [NUL.CRF]: but press ENTER for the other options. If you type masm prog1: >masm prog1 You don't want to change any of the default settings: Object filename [prog1.OBJ]: Source listing [NUL.LST]: Cross-reference [NUL.CRF]: When the assembler asks you about options, hit the ENTER key. If you have made any errors, the assembler will tell you which line they are on and give you a description of the problem. Make a hard copy of them on the printer, then use your editor and find the line. Unfortunately, at this stage of the game, it will be ____________________ 3 If you are using A86, then consult A86.APP. If you are using Turbo Assembler, then consult TASM.APP. They are both located in the \APPENDIX subdirectory on disk 3. Chapter 1 - Some Simple Programs 5 ________________________________ very difficult for you to figure out what the problem is. You will have to struggle through the first 4 or 5 programs before things start getting easier. All the programs on these disks have been compiled on a Microsoft v5.1 assembler. They have assembled. They have been run, and they work. Don't tamper with the template and copy the code exactly and everything should work. If you haven't made any errors, the assembler will say: 0 Warning Errors 0 Severe Errors LINKING The assembler has given you back another program named prog1.obj - the same name with the extension .obj. I am assuming that you have all used the linker with compiled programs. If you haven't, you may be getting in over your head by using machine language. All the extra subroutines are in a program called asmhelp.obj. Its pathname is \asmhelp\asmhelp.obj. You want to put it in the root directory of your current drive. In the whole book, we will assume that its pathname is: \asmhelp.obj If you put it somewhere else, you will have to modify the pathname whenever it appears. Link the two modules by writing: >link prog1+\asmhelp ; The copyright notice will appear: Microsoft (R) Overlay Linker Version 3.61 Copyright (C) Microsoft Corp 1983-1987. All rights reserved. This time the file extensions are understood to be .obj. The semicolon is to avoid having to make default choices. If you type: >link then you need to put the module names after the first prompt: Object Modules [.OBJ]: prog1+\asmhelp Run File [PROG1.EXE]: List File [NUL.MAP]: Libraries [.LIB]: but press ENTER for the other choices. If you type: link prog1+\asmhelp You need to do nothing extra: Run File [PROG1.EXE]: The PC Assembler Tutor 6 ______________________ List File [NUL.MAP]: Libraries [.LIB]: When the linker asks for choices, simply press the ENTER key. The linker gives the executable file the name of the first object file on the line, so you should always put your program first and asmhelp.obj second. If there are no errors, you are ready to go. If there are errors, once again, they will be very difficult to trace. Go back and check everything from the beginning. You are now ready to run the program. Type: >prog1 The program will start. The first thing you will see is a copyright notice. The PC Assembler Helper Version 1.0 Copyright (C) 1989 Chuck Nelson All rights reserved. It appears the first time you call a subprogram in the module asmhelp.obj. The program will request a number. Give it any legal signed or unsigned number. It should be no longer than 5 digits. Press ENTER, and it will display the possible ways that that number can be thought of by the computer. Enter any decimal number 4410 HEX SIGNED UNSIGNED CHAR BINARY 113AH +04410 04410 11 : ** 0001000100111010 Enter any decimal number 30486 HEX SIGNED UNSIGNED CHAR BINARY 7716H +30486 30486 w 16 ** 0111011100010110 If the signed or unsigned number doesn't look the same as what you entered, then the number you entered is too big for a 16 bit computer. For signed numbers, the limits are +32767 to -32768 and for unsigned numbers, the limits are 0 to 65535. Enter any decimal number -64661 HEX SIGNED UNSIGNED CHAR BINARY 036BH +00875 00875 03 k ** 0000001101101011 Enter any decimal number 94547 HEX SIGNED UNSIGNED CHAR BINARY 7153H +29011 29011 q S * 0111000101010011 Lets look at the numbers. Each type of output is labeled. After a hex number, there is an 'H' and after the characters, there is a '*'. This is always true. Every time you print a hex number, there will be an 'H', and every time you print a character, there Chapter 1 - Some Simple Programs 7 ________________________________ will be a '*'. This is so you will always know what is being printed. Also notice that a signed integer ALWAYS has a sign and an unsigned integer NEVER has a sign. Not all characters are visible. Ascii 0 - 32 are invisible (32 is a blank). On the PC, ascii 33-255 are visible, but ascii 127 and ascii 255 are problematic. Therefore, if the ascii code is 0-32, 127 or 255, that character will be printed as a hex number, not a character, and print_num will signal the event by printing a double asterisk '**' instead of a single one. This has happened in the first two examples. ( 11 : ** ) and ( w 16 ** ). The first one is the hex number 11 followed by the character ':' and the second one is the character 'w' followed by the hex number 16. Both are signalled by the double asterisk '**' instead of the single asterisk '*'. Do a few examples. When you are done looking at the numbers, press CTRL-C and you will exit the program. Enter any decimal number ^C PROGRAM 2 The second program is almost the same as the first one. The program takes input from the keyboard and displays it in a variety of styles. This time, however, it is going to ask for different inputs: ascii, hex, binary and decimal. If you make an error in the input, the subroutine will prompt you again for the input. Here's the program: TEMP1.ASM ;+ + + + + + + + + + START CODE BELOW THIS LINE first_label: call get_num ; 1 to 5 digit signed or unsigned call print_num call get_ascii ; 1 or 2 characters call print_num call get_binary ; a 1 to 16 bit binary number call print_num call get_hex ; a 1 to 4 digit hex number call print_num jmp first_label ;+ + + + + + + + + + END CODE ABOVE THIS LINE The things to the right of the semicolons are comments. You do not need to type them in if you don't want to. Once again, assemble the program. (There should be no warning or severe errors. If something is wrong, it is most likely a typing error.) Then link it with asmhelp.obj. Remember - your program should be The PC Assembler Tutor 8 ______________________ the first one listed.{4} If all is well, run the program. It will ask you for a number (that is a signed or unsigned number), ascii characters, a binary number, and a 4 digit hex number (0-9,A-F). Enter any decimal number 27959 HEX SIGNED UNSIGNED CHAR BINARY 6D37H +27959 27959 m 7 * 0110110100110111 Enter one or two ascii characters $% HEX SIGNED UNSIGNED CHAR BINARY 2425H +09253 09253 $ % * 0010010000100101 Enter a two byte binary number 0101111001100010 HEX SIGNED UNSIGNED CHAR BINARY 5E62H +24162 24162 ^ b * 0101111001100010 Enter a two byte hex number 784d HEX SIGNED UNSIGNED CHAR BINARY 784DH +30797 30797 x M * 0111100001001101 Once again, this is an infinite loop, so in order to quit, you need to hit CTRL-C. The purpose of these first two programs is to remind you that the computer doesn't care whether you think you are storing binary numbers, characters, hex numbers, signed numbers or unsigned numbers. They all wind up in the computer as a series of 1s and 0s, and you can use these 1s and 0s any way you like. It's up to you to keep track of them. If you feel comfortable with the way we are writing, assembling and linking programs, you are ready to start looking at the 8086 itself. ____________________ 4 For your convenience, there is a batch file on the disk called asmlink.bat. Its pathname is \asmhelp\asmlink.bat. It is one line long and looks like this: link %1+\asmhelp ; If you use this batch file, you will never have an order problem. If your file is named myfile.asm, then type: >asmlink myfile Chapter 1 - Some Simple Programs 9 ________________________________ ; TEMP1.ASM The first assembler template ; put name of program here ; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - STACKSEG SEGMENT STACK 'STACK' dw 100 dup (?) STACKSEG ENDS ; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - DATASTUFF SEGMENT PUBLIC 'DATA' ; + + + + + + + + + + + + + + + START DATA BELOW THIS LINE ; + + + + + + + + + + + + + + + END DATA ABOVE THIS LINE DATASTUFF ENDS ; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - CODESTUFF SEGMENT PUBLIC 'CODE' EXTRN print_num:NEAR , get_num:NEAR EXTRN get_ascii:NEAR , get_hex:NEAR , get_binary:NEAR ASSUME cs:CODESTUFF, ds:DATASTUFF main proc far start: push ds ; set up for return sub ax,ax push ax mov ax, DATASTUFF mov ds,ax ; + + + + + + + + + + + + + + + START CODE BELOW THIS LINE ; + + + + + + + + + + + + + + + END CODE ABOVE THIS LINE ret main endp CODESTUFF ENDS ; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - END start The PC Assembler Tutor 10 ______________________ SUMMARY CALL CALL calls a subroutine. call get_num JMP JMP jumps to the indicated label. JMP label47 LABEL A label is a name at the beginning of a line which is followed by a colon. It is used to mark a spot in the program. label47: There are three different things which will be mentioned from time to time, so it's best to define them now. ASSEMBLER INSTRUCTIONS (CODE) is the text that you type in and give to the assembler. MACHINE CODE is the code that the assembler generates. After some adjustment by the linker, it is readable by the 8086. It is the actual code that controls the program. MICROCODE is the code that is imbedded in the 8086 itself. Each instruction has its own set of mini instructions within the 8086. This is the MICROCODE.