8086 Assembler Tutorial PDF

8086 assembler tutorial for beginners (part 1) This tutorial is intended for those who are not familiar with assembler at all, or have a very distant idea about it. of course if you have knowledge of some other programming language (basic, c/c++, pascal...) that may help you a lot. but even if you...

8086 assembler tutorial for beginners (part 1) This tutorial is intended for those who are not familiar with assembler at all, or have a very distant idea about it. of course if you have knowledge of some other programming language (basic, c/c++, pascal...) that may help you a lot. but even if you are familiar with assembler, it is still a good idea to look through this document in order to study emu8086 syntax. It is assumed that you have some knowledge about number representation (hex/bin), if not it is highly recommended to study numbering systems tutorial before you proceed. what is assembly language? assembly language is a low level programming language. you need to get some knowledge about computer structure in order to understand anything. the simple computer model as i see it: the system bus (shown in yellow) connects the various components of a computer. the CPU is the heart of the computer, most of computations occur inside the CPU. RAM is a place to where the programs are loaded in order to be executed. inside the cpu general purpose registers 8086 CPU has 8 general purpose registers, each register has its own name:  AX - the accumulator register (divided into AH / AL).  BX - the base address register (divided into BH / BL).  CX - the count register (divided into CH / CL).  DX - the data register (divided into DH / DL).  SI - source index register.  DI - destination index register.  BP - base pointer.  SP - stack pointer. despite the name of a register, it's the programmer who determines the usage for each general purpose register. the main purpose of a register is to keep a number (variable). the size of the above registers is 16 bit, it's something like: 0011000000111001b (in binary form), or 12345 in decimal (human) form. 4 general purpose registers (AX, BX, CX, DX) are made of two separate 8 bit registers, for example if AX= 0011000000111001b, then AH=00110000b and AL=00111001b. therefore, when you modify any of the 8 bit registers 16 bit register is also updated, and vice-versa. the same is for other 3 registers, "H" is for high and "L" is for low part. because registers are located inside the CPU, they are much faster than memory. Accessing a memory location requires the use of a system bus, so it takes much longer. Accessing data in a register usually takes no time. therefore, you should try to keep variables in the registers. register sets are very small and most registers have special purposes which limit their use as variables, but they are still an excellent place to store temporary data of calculations. segment registers  CS - points at the segment containing the current program.  DS - generally points at segment where variables are defined.  ES - extra segment register, it's up to a coder to define its usage.  SS - points at the segment containing the stack. although it is possible to store any data in the segment registers, this is never a good idea. the segment registers have a very special purpose - pointing at accessible blocks of memory. segment registers work together with general purpose register to access any memory value. For example if we would like to access memory at the physical address 12345h (hexadecimal), we should set the DS = 1230h and SI = 0045h. This is good, since this way we can access much more memory than with a single register that is limited to 16 bit values. CPU makes a calculation of physical address by multiplying the segment register by 10h and adding general purpose register to it (1230h * 10h + 45h = 12345h): the address formed with 2 registers is called an effective address. by default BX, SI and DI registers work with DS segment register; BP and SP work with SS segment register. other general purpose registers cannot form an effective address! also, although BX can form an effective address, BH and BL cannot. special purpose registers  IP - the instruction pointer.  flags register - determines the current state of the microprocessor. IP register always works together with CS segment register and it points to currently executing instruction. flags register is modified automatically by CPU after mathematical operations, this allows to determine the type of the result, and to determine conditions to transfer control to other parts of the program. generally you cannot access these registers directly, the way you can access AX and other general registers, but it is possible to change values of system registers using some tricks that you will learn a little bit later. Memory Access to access memory we can use these four registers: BX, SI, DI, BP. combining these registers inside [ ] symbols, we can get different memory locations. these combinations are supported (addressing modes): [BX + SI] [SI] [BX + SI + d8] [BX + DI] [DI] [BX + DI + d8] [BP + SI] d16 (variable offset only) [BP + SI + d8] [BP + DI] [BX] [BP + DI + d8] [SI + d8] [BX + SI + d16] [SI + d16] [DI + d8] [BX + DI + d16] [DI + d16] [BP + d8] [BP + SI + d16] [BP + d16] [BX + d8] [BP + DI + d16] [BX + d16] d8 - stays for 8 bit signed immediate displacement (for example: 22, 55h, -1, etc...) d16 - stays for 16 bit signed immediate displacement (for example: 300, 5517h, -259, etc...). displacement can be a immediate value or offset of a variable, or even both. if there are several values, assembler evaluates all values and calculates a single immediate value.. displacement can be inside or outside of the [ ] symbols, assembler generates the same machine code for both ways. displacement is a signed value, so it can be both positive or negative. generally the compiler takes care about difference between d8 and d16, and generates the required machine code. for example, let's assume that DS = 100, BX = 30, SI = 70. The following addressing mode: [BX + SI] + 25 is calculated by processor to this physical address: 100 * 16 + 30 + 70 + 25 = 1725. by default DS segment register is used for all modes except those with BP register, for these SS segment register is used. there is an easy way to remember all those possible combinations using this chart: you can form all valid combinations by taking only one item from each column or skipping the column by not taking anything from it. as you see BX and BP never go together. SI and DI also don't go together. here are an examples of a valid addressing modes: [BX+5] , [BX+SI] , [DI+BX-4] the value in segment register (CS, DS, SS, ES) is called a segment, and the value in purpose register (BX, SI, DI, BP) is called an offset. When DS contains value 1234h and SI contains the value 7890h it can be also recorded as 1234:7890. The physical address will be 1234h * 10h + 7890h = 19BD0h. if zero is added to a decimal number it is multiplied by 10, however 10h = 16, so if zero is added to a hexadecimal value, it is multiplied by 16, for example: 7h = 7 70h = 112 in order to say the compiler about data type, these prefixes should be used: byte ptr - for byte. word ptr - for word (two bytes). for example: byte ptr [BX] ; byte access. or word ptr [BX] ; word access. assembler supports shorter prefixes as well: b. - for byte ptr w. - for word ptr in certain cases the assembler can calculate the data type automatically. MOV instruction  copies the second operand (source) to the first operand (destination).  the source operand can be an immediate value, general-purpose register or memory location.  the destination register can be a general-purpose register, or memory location.  both operands must be the same size, which can be a byte or a word. these types of operands are supported: MOV REG, memory MOV memory, REG MOV REG, REG MOV memory, immediate MOV REG, immediate REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP. memory: [BX], [BX+SI+7], variable, etc... immediate: 5, -24, 3Fh, 10001101b, etc... for segment registers only these types of MOV are supported: MOV SREG, memory MOV memory, SREG MOV REG, SREG MOV SREG, REG SREG: DS, ES, SS, and only as second operand: CS. REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP. memory: [BX], [BX+SI+7], variable, etc... The MOV instruction cannot be used to set the value of the CS and IP registers. here is a short program that demonstrates the use of MOV instruction: ORG 100h ; this directive required for a simple 1 segment.com program. MOV AX, 0B800h ; set AX to hexadecimal value of B800h. MOV DS, AX ; copy value of AX to DS. MOV CL, 'A' ; set CL to ASCII code of 'A', it is 41h. MOV CH, 1101_1111b ; set CH to binary value. MOV BX, 15Eh ; set BX to 15Eh. MOV [BX], CX ; copy contents of CX to memory at B800:015E RET ; returns to operating system. you can copy & paste the above program to emu8086 code editor, and press [Compile and Emulate] button (or press F5 key on your keyboard). the emulator window should open with this program loaded, click [Single Step] button and watch the register values. how to do copy & paste: 1. select the above text using mouse, click before the text and drag it down until everything is selected. 2. press Ctrl + C combination to copy. 3. go to emu8086 source editor and press Ctrl + V combination to paste. as you may guess, ";" is used for comments, anything after ";" symbol is ignored by compiler. you should see something like that when program finishes: actually the above program writes directly to video memory, so you may see that MOV is a very powerful instruction Variables Variable is a memory location. For a programmer it is much easier to have some value be kept in a variable named "var1" then at the address 5A73:235B, especially when you have 10 or more variables. Our compiler supports two types of variables: BYTE and WORD. Syntax for a variable declaration: name DB value name DW value DB - stays for Define Byte. DW - stays for Define Word. name - can be any letter or digit combination, though it should start with a letter. It's possible to declare unnamed variables by not specifying the name (this variable will have an address but no name). value - can be any numeric value in any supported numbering system (hexadecimal, binary, or decimal), or "?" symbol for variables that are not initialized. As you probably know from part 2 of this tutorial, MOV instruction is used to copy values from source to destination. Let's see another example with MOV instruction: ORG 100h MOV AL, var1 MOV BX, var2 RET ; stops the program. VAR1 DB 7 var2 DW 1234h Copy the above code to emu8086 source editor, and press F5 key to compile and load it in the emulator. You should get something like: As you see this looks a lot like our example, except that variables are replaced with actual memory locations. When compiler makes machine code, it automatically replaces all variable names with their offsets. By default segment is loaded in DS register (when COM files is loaded the value of DS register is set to the same value as CS register - code segment). In memory list first row is an offset, second row is a hexadecimal value, third row is decimal value, and last row is an ASCII character value. Compiler is not case sensitive, so "VAR1" and "var1" refer to the same variable. The offset of VAR1 is 0108h, and full address is 0B56:0108. The offset of var2 is 0109h, and full address is 0B56:0109, this variable is a WORD so it occupies 2 BYTES. It is assumed that low byte is stored at lower address, so 34h is located before 12h. You can see that there are some other instructions after the RET instruction, this happens because disassembler has no idea about where the data starts, it just processes the values in memory and it understands them as valid 8086 instructions (we will learn them later). You can even write the same program using DB directive only: ORG 100h ; just a directive to make a simple.com file (expands into no code). DB 0A0h DB 08h DB 01h DB 8Bh DB 1Eh DB 09h DB 01h DB 0C3h DB 7 DB 34h DB 12h Copy the above code to emu8086 source editor, and press F5 key to compile and load it in the emulator. You should get the same disassembled code, and the same functionality! As you may guess, the compiler just converts the program source to the set of bytes, this set is called machine code, processor understands the machine code and executes it. ORG 100h is a compiler directive (it tells compiler how to handle the source code). This directive is very important when you work with variables. It tells compiler that the executable file will be loaded at the offset of 100h (256 bytes), so compiler should calculate the correct address for all variables when it replaces the variable names with their offsets. Directives are never converted to any real machine code. Why executable file is loaded at offset of 100h? Operating system keeps some data about the program in the first 256 bytes of the CS (code segment), such as command line parameters and etc. Though this is true for COM files only, EXE files are loaded at offset of 0000, and generally use special segment for variables. Maybe we'll talk more about EXE files later. Arrays Arrays can be seen as chains of variables. A text string is an example of a byte array, each character is presented as an ASCII code value (0..255). Here are some array definition examples: a DB 48h, 65h, 6Ch, 6Ch, 6Fh, 00h b DB 'Hello', 0 b is an exact copy of the a array, when compiler sees a string inside quotes it automatically converts it to set of bytes. This chart shows a part of the memory where these arrays are declared: You can access the value of any element in array using square brackets, for example: MOV AL, a You can also use any of the memory index registers BX, SI, DI, BP, for example: MOV SI, 3 MOV AL, a[SI] If you need to declare a large array you can use DUP operator. The syntax for DUP: number DUP ( value(s) ) number - number of duplicate to make (any constant value). value - expression that DUP will duplicate. for example: c DB 5 DUP(9) is an alternative way of declaring: c DB 9, 9, 9, 9, 9 one more example: d DB 5 DUP(1, 2) is an alternative way of declaring: d DB 1, 2, 1, 2, 1, 2, 1, 2, 1, 2 Of course, you can use DW instead of DB if it's required to keep values larger then 255, or smaller then -128. DW cannot be used to declare strings. Getting the Address of a Variable There is LEA (Load Effective Address) instruction and alternative OFFSET operator. Both OFFSET and LEA can be used to get the offset address of the variable. LEA is more powerful because it also allows you to get the address of an indexed variables. Getting the address of the variable can be very useful in some situations, for example when you need to pass parameters to a procedure. Reminder: In order to tell the compiler about data type, these prefixes should be used: BYTE PTR - for byte. WORD PTR - for word (two bytes). For example: BYTE PTR [BX] ; byte access. or WORD PTR [BX] ; word access. emu8086 supports shorter prefixes as well: b. - for BYTE PTR w. - for WORD PTR in certain cases the assembler can calculate the data type automatically. Here is first example: ORG 100h MOV AL, VAR1 ; check value of VAR1 by moving it to AL. LEA BX, VAR1 ; get address of VAR1 in BX. MOV BYTE PTR [BX], 44h ; modify the contents of VAR1. MOV AL, VAR1 ; check value of VAR1 by moving it to AL. RET VAR1 DB 22h END Here is another example, that uses OFFSET instead of LEA: ORG 100h MOV AL, VAR1 ; check value of VAR1 by moving it to AL. MOV BX, OFFSET VAR1 ; get address of VAR1 in BX. MOV BYTE PTR [BX], 44h ; modify the contents of VAR1. MOV AL, VAR1 ; check value of VAR1 by moving it to AL. RET VAR1 DB 22h END Both examples have the same functionality. These lines: LEA BX, VAR1 MOV BX, OFFSET VAR1 are even compiled into the same machine code: MOV BX, num num is a 16 bit value of the variable offset. Please note that only these registers can be used inside square brackets (as memory pointers): BX, SI, DI, BP! (see previous part of the tutorial). Constants Constants are just like variables, but they exist only until your program is compiled (assembled). After definition of a constant its value cannot be changed. To define constants EQU directive is used: name EQU < any expression > For example: k EQU 5 MOV AX, k The above example is functionally identical to code: MOV AX, 5 You can view variables while your program executes by selecting "Variables" from the "View" menu of emulator. To view arrays you should click on a variable and set Elements property to array size. In assembly language there are not strict data types, so any variable can be presented as an array. Variable can be viewed in any numbering system:  HEX - hexadecimal (base 16).  BIN - binary (base 2).  OCT - octal (base 8).  SIGNED - signed decimal (base 10).  UNSIGNED - unsigned decimal (base 10).  CHAR - ASCII char code (there are 256 symbols, some symbols are invisible). You can edit a variable's value when your program is running, simply double click it, or select it and click Edit button. It is possible to enter numbers in any system, hexadecimal numbers should have "h" suffix, binary "b" suffix, octal "o" suffix, decimal numbers require no suffix. String can be entered this way: 'hello world', 0 (this string is zero terminated). Arrays may be entered this way: 1, 2, 3, 4, 5 (the array can be array of bytes or words, it depends whether BYTE or WORD is selected for edited variable). Expressions are automatically converted, for example: when this expression is entered: 5+2 it will be converted to 7 etc... Interrupts Interrupts can be seen as a number of functions. These functions make the programming much easier, instead of writing a code to print a character you can simply call the interrupt and it will do everything for you. There are also interrupt functions that work with disk drive and other hardware. We call such functions software interrupts. Interrupts are also triggered by different hardware, these are called hardware interrupts. Currently we are interested in software interrupts only. To make a software interrupt there is an INT instruction, it has very simple syntax: INT value Where value can be a number between 0 to 255 (or 0 to 0FFh), generally we will use hexadecimal numbers. You may think that there are only 256 functions, but that is not correct. Each interrupt may have sub-functions. To specify a sub-function AH register should be set before calling interrupt. Each interrupt may have up to 256 sub-functions (so we get 256 * 256 = 65536 functions). In general AH register is used, but sometimes other registers maybe in use. Generally other registers are used to pass parameters and data to sub-function. The following example uses INT 10h sub-function 0Eh to type a "Hello!" message. This functions displays a character on the screen, advancing the cursor and scrolling the screen as necessary. ORG 100h ; directive to make a simple.com file. ; The sub-function that we are using ; does not modify the AH register on ; return, so we may set it only once. MOV AH, 0Eh ; select sub-function. ; INT 10h / 0Eh sub-function ; receives an ASCII code of the ; character that will be printed ; in AL register. MOV AL, 'H' ; ASCII code: 72 INT 10h ; print it! MOV AL, 'e' ; ASCII code: 101 INT 10h ; print it! MOV AL, 'l' ; ASCII code: 108 INT 10h ; print it! MOV AL, 'l' ; ASCII code: 108 INT 10h ; print it! MOV AL, 'o' ; ASCII code: 111 INT 10h ; print it! MOV AL, '!' ; ASCII code: 33 INT 10h ; print it! RET ; returns to operating system. Copy & paste the above program to emu8086 source code editor, and press [Compile and Emulate] button. Run it! See list of supported interrupts for more information about interrupts. Library of common functions - emu8086.inc To make programming easier there are some common functions that can be included in your program. To make your program use functions defined in other file you should use the INCLUDE directive followed by a file name. Compiler automatically searches for the file in the same folder where the source file is located, and if it cannot find the file there - it searches in Inc folder. Currently you may not be able to fully understand the contents of the emu8086.inc (located in Inc folder), but it's OK, since you only need to understand what it can do. To use any of the functions in emu8086.inc you should have the following line in the beginning of your source file: include 'emu8086.inc' emu8086.inc defines the following macros:   PUTC char - macro with 1 parameter, prints out an ASCII char at current cursor position.  GOTOXY col, row - macro with 2 parameters, sets cursor position.  PRINT string - macro with 1 parameter, prints out a string.  PRINTN string - macro with 1 parameter, prints out a string. The same as PRINT but automatically adds "carriage return" at the end of the string.  CURSOROFF - turns off the text cursor.  CURSORON - turns on the text cursor. To use any of the above macros simply type its name somewhere in your code, and if required parameters, for example: include emu8086.inc ORG 100h PRINT 'Hello World!' GOTOXY 10, 5 PUTC 65 ; 65 - is an ASCII code for 'A' PUTC 'B' RET ; return to operating system. END ; directive to stop the compiler. When compiler process your source code it searches the emu8086.inc file for declarations of the macros and replaces the macro names with real code. Generally macros are relatively small parts of code, frequent use of a macro may make your executable too big (procedures are better for size optimization). emu8086.inc also defines the following procedures:  PRINT_STRING - procedure to print a null terminated string at current cursor position, receives address of string in DS:SI register. To use it declare: DEFINE_PRINT_STRING before END directive.  PTHIS - procedure to print a null terminated string at current cursor position (just as PRINT_STRING), but receives address of string from Stack. The ZERO TERMINATED string should be defined just after the CALL instruction. For example:  CALL PTHIS db 'Hello World!', 0 To use it declare: DEFINE_PTHIS before END directive.  GET_STRING - procedure to get a null terminated string from a user, the received string is written to buffer at DS:DI, buffer size should be in DX. Procedure stops the input when 'Enter' is pressed. To use it declare: DEFINE_GET_STRING before END directive.  CLEAR_SCREEN - procedure to clear the screen, (done by scrolling entire screen window), and set cursor position to top of it. To use it declare: DEFINE_CLEAR_SCREEN before END directive.  SCAN_NUM - procedure that gets the multi-digit SIGNED number from the keyboard, and stores the result in CX register. To use it declare: DEFINE_SCAN_NUM before END directive.  PRINT_NUM - procedure that prints a signed number in AX register. To use it declare: DEFINE_PRINT_NUM and DEFINE_PRINT_NUM_UNS before END directive.  PRINT_NUM_UNS - procedure that prints out an unsigned number in AX register. To use it declare: DEFINE_PRINT_NUM_UNS before END directive. To use any of the above procedures you should first declare the function in the bottom of your file (but before the END directive), and then use CALL instruction followed by a procedure name. For example: include 'emu8086.inc' ORG 100h LEA SI, msg1 ; ask for the number CALL print_string ; CALL scan_num ; get number in CX. MOV AX, CX ; copy the number to AX. ; print the following string: CALL pthis DB 13, 10, 'You have entered: ', 0 CALL print_num ; print number in AX. RET ; return to operating system. msg1 DB 'Enter the number: ', 0 DEFINE_SCAN_NUM DEFINE_PRINT_STRING DEFINE_PRINT_NUM DEFINE_PRINT_NUM_UNS ; required for print_num. DEFINE_PTHIS END ; directive to stop the compiler. First compiler processes the declarations (these are just regular the macros that are expanded to procedures). When compiler gets to CALL instruction it replaces the procedure name with the address of the code where the procedure is declared. When CALL instruction is executed control is transferred to procedure. This is quite useful, since even if you call the same procedure 100 times in your code you will still have relatively small executable size. Seems complicated, isn't it? That's ok, with the time you will learn more, currently it's required that you understand the basic principle. Arithmetic and logic instructions Most Arithmetic and Logic Instructions affect the processor status register (or Flags) As you may see there are 16 bits in this register, each bit is called a flag and can take a value of 1 or 0.  Carry Flag (CF) - this flag is set to 1 when there is an unsigned overflow. For example when you add bytes 255 + 1 (result is not in range 0...255). When there is no overflow this flag is set to 0.  Zero Flag (ZF) - set to 1 when result is zero. For none zero result this flag is set to 0.  Sign Flag (SF) - set to 1 when result is negative. When result is positive it is set to 0. Actually this flag take the value of the most significant bit.  Overflow Flag (OF) - set to 1 when there is a signed overflow. For example, when you add bytes 100 + 50 (result is not in range - 128...127).  Parity Flag (PF) - this flag is set to 1 when there is even number of one bits in result, and to 0 when there is odd number of one bits. Even if result is a word only 8 low bits are analyzed!  Auxiliary Flag (AF) - set to 1 when there is an unsigned overflow for low nibble (4 bits).  Interrupt enable Flag (IF) - when this flag is set to 1 CPU reacts to interrupts from external devices.  Direction Flag (DF) - this flag is used by some instructions to process data chains, when this flag is set to 0 - the processing is done forward, when this flag is set to 1 the processing is done backward. There are 3 groups of instructions. First group: ADD, SUB,CMP, AND, TEST, OR, XOR These types of operands are supported: REG, memory memory, REG REG, REG memory, immediate REG, immediate REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP. memory: [BX], [BX+SI+7], variable, etc... immediate: 5, -24, 3Fh, 10001101b, etc... After operation between operands, result is always stored in first operand. CMP and TEST instructions affect flags only and do not store a result (these instruction are used to make decisions during program execution). These instructions affect these flags only: CF, ZF, SF, OF, PF, AF.  ADD - add second operand to first.  SUB - Subtract second operand to first.  CMP - Subtract second operand from first for flags only.  AND - Logical AND between all bits of two operands. These rules apply: 1 AND 1 = 1 1 AND 0 = 0 0 AND 1 = 0 0 AND 0 = 0 As you see we get 1 only when both bits are 1.  TEST - The same as AND but for flags only.  OR - Logical OR between all bits of two operands. These rules apply: 1 OR 1 = 1 1 OR 0 = 1 0 OR 1 = 1 0 OR 0 = 0  As you see we get 1 every time when at least one of the bits is 1.  XOR - Logical XOR (exclusive OR) between all bits of two operands. These rules apply: 1 XOR 1 = 0 1 XOR 0 = 1 0 XOR 1 = 1 0 XOR 0 = 0 As you see we get 1 every time when bits are different from each other. Second group: MUL, IMUL, DIV, IDIV These types of operands are supported: REG memory REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP. memory: [BX], [BX+SI+7], variable, etc... MUL and IMUL instructions affect these flags only: CF, OF When result is over operand size these flags are set to 1, when result fits in operand size these flags are set to 0. For DIV and IDIV flags are undefined.  MUL - Unsigned multiply: when operand is a byte: AX = AL * operand. when operand is a word: (DX AX) = AX * operand.  IMUL - Signed multiply: when operand is a byte: AX = AL * operand. when operand is a word: (DX AX) = AX * operand.  DIV - Unsigned divide:  when operand is a byte: AL = AX / operand AH = remainder (modulus).. when operand is a word: AX = (DX AX) / operand DX = remainder (modulus)..  IDIV - Signed divide: when operand is a byte: AL = AX / operand AH = remainder (modulus).. when operand is a word: AX = (DX AX) / operand DX = remainder (modulus).. Third group: INC, DEC, NOT, NEG These types of operands are supported: REG memory REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP. memory: [BX], [BX+SI+7], variable, etc... INC, DEC instructions affect these flags only: ZF, SF, OF, PF, AF. NOT instruction does not affect any flags! NEG instruction affects these flags only: CF, ZF, SF, OF, PF, AF.  NOT - Reverse each bit of operand.  NEG - Make operand negative (two's complement). Actually it reverses each bit of operand and then adds 1 to it. For example 5 will become -5, and -2 will become 2. program flow control Controlling the program flow is a very important thing, this is where your program can make decisions according to certain conditions.  unconditional jumps The basic instruction that transfers control to another point in the program is JMP. The basic syntax of JMP instruction: JMP label To declare a label in your program, just type its name and add ":" to the end, label can be any character combination but it cannot start with a number, for example here are 3 legal label definitions: label1: label2: a: Label can be declared on a separate line or before any other instruction, for example: x1: MOV AX, 1 x2: MOV AX, 2 here's an example of JMP instruction: org 100h mov ax, 5 ; set ax to 5. mov bx, 2 ; set bx to 2. jmp calc ; go to 'calc'. back: jmp stop ; go to 'stop'. calc: add ax, bx ; add bx to ax. jmp back ; go 'back'. stop: ret ; return to operating system.  Of course there is an easier way to calculate the some of two numbers, but it's still a good example of JMP instruction. As you can see from this example JMP is able to transfer control both forward and backward. It can jump anywhere in current code segment (65,535 bytes).  Short Conditional Jumps Unlike JMP instruction that does an unconditional jump, there are instructions that do a conditional jumps (jump only when some conditions are in act). These instructions are divided in three groups, first group just test single flag, second compares numbers as signed, and third compares numbers as unsigned. Jump instructions that test single flag Opposite Instruction Description Condition Instruction JZ , JE Jump if Zero (Equal). ZF = 1 JNZ, JNE Jump if Carry (Below, Not Above JC , JB, JNAE CF = 1 JNC, JNB, JAE Equal). JS Jump if Sign. SF = 1 JNS JO Jump if Overflow. OF = 1 JNO JPE, JP Jump if Parity Even. PF = 1 JPO JNZ , JNE Jump if Not Zero (Not Equal). ZF = 0 JZ, JE JNC , JNB, Jump if Not Carry (Not Below, Above CF = 0 JC, JB, JNAE JAE Equal). JNS Jump if Not Sign. SF = 0 JS JNO Jump if Not Overflow. OF = 0 JO JPO, JNP Jump if Parity Odd (No Parity). PF = 0 JPE, JP  as you may already notice there are some instructions that do that same thing, that's correct, they even are assembled into the same machine code, so it's good to remember that when you compile JE instruction - you will get it disassembled as: JZ, JC is assembled the same as JB etc...  different names are used to make programs easier to understand, to code and most importantly to remember. very offset dissembler has no clue what the original instruction was look like that's why it uses the most common name. if you emulate this code you will see that all instructions are assembled into JNB, the operational code (opcode) for this instruction is 73h this instruction has fixed length of two bytes, the second byte is number of bytes to add to the IP register if the condition is true. because the instruction has only 1 byte to keep the offset it is limited to pass control to -128 bytes back or 127 bytes forward, this value is always signed.   jnc a  jnb a  jae a   mov ax, 4  a: mov ax, 5  ret   Jump instructions for signed numbers Instruction Description Condition Opposite Instruction Jump if Equal (=). JE , JZ ZF = 1 JNE, JNZ Jump if Zero. Jump if Not Equal (). JNE , JNZ ZF = 0 JE, JZ Jump if Not Zero. ZF = 0 Jump if Greater (>). JG , JNLE and JNG, JLE Jump if Not Less or Equal (not =). JGE , JNL SF = OF JNGE, JL Jump if Not Less (not ). CF = 0 JA , JNBE Jump if Not Below or Equal (not and JNA, JBE =). JAE , JNB, Jump if Not Below (not check for an update from the menu. Another, yet rarely used method is providing an immediate value instead of label. When immediate value starts with $ relative jump is performed, otherwise compiler calculates instruction that jumps directly to given offset. For example: org 100h ; unconditional jump forward: ; skip over next 3 bytes + itself ; the machine code of short jmp instruction takes 2 bytes. jmp $3+2 a db 3 ; 1 byte. b db 4 ; 1 byte. c db 4 ; 1 byte. ; conditional jump back 5 bytes: mov bl,9 dec bl ; 2 bytes. cmp bl, 0 ; 3 bytes. jne $-5 ; jump 5 bytes back ret Procedures Procedure is a part of code that can be called from your program in order to make some specific task. Procedures make program more structural and easier to understand. Generally procedure returns to the same point from where it was called. The syntax for procedure declaration: name PROC ; here goes the code ; of the procedure... RET name ENDP name - is the procedure name, the same name should be in the top and the bottom, this is used to check correct closing of procedures. Probably, you already know that RET instruction is used to return to operating system. The same instruction is used to return from procedure (actually operating system sees your program as a special procedure). PROC and ENDP are compiler directives, so they are not assembled into any real machine code. Compiler just remembers the address of procedure. CALL instruction is used to call a procedure. Here is an example: ORG 100h CALL m1 MOV AX, 2 RET ; return to operating system. m1 PROC MOV BX, 5 RET ; return to caller. m1 ENDP END The above example calls procedure m1, does MOV BX, 5, and returns to the next instruction after CALL: MOV AX, 2. There are several ways to pass parameters to procedure, the easiest way to pass parameters is by using registers, here is another example of a procedure that receives two parameters in AL and BL registers, multiplies these parameters and returns the result in AX register: ORG 100h MOV AL, 1 MOV BL, 2 CALL m2 CALL m2 CALL m2 CALL m2 RET ; return to operating system. m2 PROC MUL BL ; AX = AL * BL. RET ; return to caller. m2 ENDP END In the above example value of AL register is update every time the procedure is called, BL register stays unchanged, so this algorithm calculates 2 in power of 4, so final result in AX register is 16 (or 10h). Here goes another example, that uses a procedure to print a Hello World! message: ORG 100h LEA SI, msg ; load address of msg to SI. CALL print_me RET ; return to operating system. ; ========================================================== ; this procedure prints a string, the string should be null ; terminated (have zero in the end), ; the string address should be in SI register: print_me PROC next_char: CMP b.[SI], 0 ; check for zero to stop JE stop ;  MOV AL, [SI] ; next get ASCII char. MOV AH, 0Eh ; teletype function number. INT 10h ; using interrupt to print a char in AL. ADD SI, 1 ; advance index of string array. JMP next_char ; go back, and type another char. stop: RET ; return to caller. print_me ENDP ; ========================================================== msg DB 'Hello World!', 0 ; null terminated string. END "b." - prefix before [SI] means that we need to compare bytes, not words. When you need to compare words add "w." prefix instead. When one of the compared operands is a register it's not required because compiler knows the size of each register The Stack Stack is an area of memory for keeping temporary data. Stack is used by CALL instruction to keep return address for procedure, RET instruction gets this value from the stack and returns to that offset. Quite the same thing happens when INT instruction calls an interrupt, it stores in stack flag register, code segment and offset. IRET instruction is used to return from interrupt call. We can also use the stack to keep any other data, there are two instructions that work with the stack: PUSH - stores 16 bit value in the stack. POP - gets 16 bit value from the stack. Syntax for PUSH instruction: PUSH REG PUSH SREG PUSH memory PUSH immediate REG: AX, BX, CX, DX, DI, SI, BP, SP. SREG: DS, ES, SS, CS. memory: [BX], [BX+SI+7], 16 bit variable, etc... immediate: 5, -24, 3Fh, 10001101b, etc... Syntax for POP instruction: POP REG POP SREG POP memory REG: AX, BX, CX, DX, DI, SI, BP, SP. SREG: DS, ES, SS, (except CS). memory: [BX], [BX+SI+7], 16 bit variable, etc... Notes:  PUSH and POP work with 16 bit values only!  Note: PUSH immediate works only on 80186 CPU and later! The stack uses LIFO (Last In First Out) algorithm, this means that if we push these values one by one into the stack: 1, 2, 3, 4, 5 the first value that we will get on pop will be 5, then 4, 3, 2, and only then 1. It is very important to do equal number of PUSHs and POPs, otherwise the stack maybe corrupted and it will be impossible to return to operating system. As you already know we use RET instruction to return to operating system, so when program starts there is a return address in stack (generally it's 0000h). PUSH and POP instruction are especially useful because we don't have too much registers to operate with, so here is a trick:  Store original value of the register The Stack Stack is an area of memory for keeping temporary data. Stack is used by CALL instruction to keep return address for procedure, RET instruction gets this value from the stack and returns to that offset. Quite the same thing happens when INT instruction calls an interrupt, it stores in stack flag register, code segment and offset. IRET instruction is used to return from interrupt call. We can also use the stack to keep any other data, there are two instructions that work with the stack: PUSH - stores 16 bit value in the stack. POP - gets 16 bit value from the stack. Syntax for PUSH instruction: PUSH REG PUSH SREG PUSH memory PUSH immediate REG: AX, BX, CX, DX, DI, SI, BP, SP. SREG: DS, ES, SS, CS. memory: [BX], [BX+SI+7], 16 bit variable, etc... immediate: 5, -24, 3Fh, 10001101b, etc... Syntax for POP instruction: POP REG POP SREG POP memory REG: AX, BX, CX, DX, DI, SI, BP, SP. SREG: DS, ES, SS, (except CS). memory: [BX], [BX+SI+7], 16 bit variable, etc... Notes:  PUSH and POP work with 16 bit values only!  Note: PUSH immediate works only on 80186 CPU and later! The stack uses LIFO (Last In First Out) algorithm, this means that if we push these values one by one into the stack: 1, 2, 3, 4, 5 the first value that we will get on pop will be 5, then 4, 3, 2, and only then 1. It is very important to do equal number of PUSHs and POPs, otherwise the stack maybe corrupted and it will be impossible to return to operating system. As you already know we use RET instruction to return to operating system, so when program starts there is a return address in stack (generally it's 0000h). PUSH and POP instruction are especially useful because we don't have too much registers to operate with, so here is a trick:  Store original value of the register in stack (using PUSH).  Use the register for any purpose.  Restore the original value of the register from stack (using POP). Here is an example: ORG 100h MOV AX, 1234h PUSH AX ; store value of AX in stack. MOV AX, 5678h ; modify the AX value. POP AX ; restore the original value of AX. RET END Another use of the stack is for exchanging the values, here is an example: ORG 100h MOV AX, 1212h ; store 1212h in AX. MOV BX, 3434h ; store 3434h in BX PUSH AX ; store value of AX in stack. PUSH BX ; store value of BX in stack. POP AX ; set AX to original value of BX. POP BX ; set BX to original value of AX. RET END The exchange happens because stack uses LIFO (Last In First Out) algorithm, so when we push 1212h and then 3434h, on pop we will first get 3434h and only after it 1212h. The stack memory area is set by SS (Stack Segment) register, and SP (Stack Pointer) register. Generally operating system sets values of these registers on program start. "PUSH source" instruction does the following:  Subtract 2 from SP register.  Write the value of source to the address SS:SP. "POP destination" instruction does the following:  Write the value at the address SS:SP to destination.  Add 2 to SP register. The current address pointed by SS:SP is called the top of the stack. For COM files stack segment is generally the code segment, and stack pointer is set to value of 0FFFEh. At the address SS:0FFFEh stored a return address for RET instruction that is executed in the end of the program. You can visually see the stack operation by clicking on [Stack] button on emulator window. The top of the stack is marked with "