8086 assembler tutorial for beginners (part 2)

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]
[BX + DI]
[BP + SI]
[BP + DI] | [SI]
[DI]
d16 (variable offset only)
[BX] | [BX + SI + d8]
[BX + DI + d8]
[BP + SI + d8]
[BP + DI + d8] | [SI + d8]
[DI + d8]
[BP + d8]
[BX + d8] | [BX + SI + d16]
[BX + DI + d16]
[BP + SI + d16]
[BP + DI + d16] | [SI + d16]
[DI + d16]
[BP + 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, REGSREG: 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 the 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 the 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.

<<< previous part <<< >>> Next Part >>>

8086 assembler tutorial for beginners (part 3)

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 the source editor, and press F5 key to compile it and load 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 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 the 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[3]

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. 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.

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...

<<< previous part <<< >>> Next Part >>>

8086 assembler tutorial for beginners (part 4)

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 ; instruct compiler to make simple single segment .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 the source code editor, and press [Compile and Emulate] button. Run it!

See list of supported interrupts for more information about interrupts.

<<< previous part <<< >>> Next Part >>>