In this blog post, we’ll delve into the world of 8086 assembly language to explore a fundamental arithmetic operation: adding two 16-bit numbers.

; Data Segment

data segment
    a dw 0202h      ; 16-bit variable 'a' initialized with 0202h
    b dw 0408h      ; 16-bit variable 'b' initialized with 0408h
    c dw ?          ; 16-bit variable 'c' to store the result
data ends

; Code Segment

code segment
assume cs:code, ds:data
start:
    mov ax, data    ; Load the address of the data segment into AX
    mov ds, ax      ; Initialize the data segment register

    mov ax, a       ; Load value of 'a' into AX
    mov bx, b       ; Load value of 'b' into BX
    add ax, bx      ; Perform addition (AX = AX + BX)

    mov c, ax       ; Store the result in 'c'

    int 3           ; Halt program execution for debugging

code ends
end start           ; Mark the end of the program

16-bit addition in 8086: It’s like learning a new language, except the language is binary and the grammar is brutal 😉

Understanding the code:

Data Segment

• a dw 0202h: This line declares a 16-bit word variable named a and initializes it with the hexadecimal value 0202.
• b dw 0408h: Similarly, this declares a 16-bit word variable b and initializes it with the hexadecimal value 0408.
• c dw ?: This declares a 16-bit word variable c but does not initialize it. It will store the result of the addition operation.

Code Segment

• mov ax,data: This instruction moves the address of the data segment into the AX register.
• mov ds,ax: This sets the Data Segment (DS) register to the value in AX, effectively making the data segment accessible for data operations.
• mov ax,a: This moves the value of a (0202h) into the AX register.
• mov bx,b: This moves the value of b (0408h) into the BX register.
• add ax,bx: This adds the contents of BX to the contents of AX. The result (060Ah) is stored in AX.
• mov c,ax: This moves the result from AX to the variable c.
• int 3: This is a breakpoint instruction that causes the program to halt, allowing for debugging and inspection of registers and memory.

Flowchart

Overall Functionality

The code performs the following steps:

1. Initialization: Sets up the data segment and initializes variables a and b.
2. Addition: Loads the values of a and b into registers AX and BX, respectively. Adds the two values and stores the result in AX.
3. Result Storage: Stores the result from AX into the variable c.
4. Program Termination: Halts the program execution using the int 3 instruction.

In essence, this code adds two 16-bit hexadecimal numbers (0202h and 0408h) and stores the result (060Ah) in the variable c.

Output

C:\TASM>masm an16add.asm
Microsoft (R) Macro Assembler Version 5.00
Copyright (C) Microsoft Corp 1981-1985, 1987.  All rights reserved.
 
Object filename [an16add.OBJ]:
Source listing  [NUL.LST]:
Cross-reference [NUL.CRF]:
 
  50402 + 450254 Bytes symbol space free
 
      0 Warning Errors
      0 Severe  Errors
 
C:\TASM>link an16add.obj
 
Microsoft (R) Overlay Linker  Version 3.60
Copyright (C) Microsoft Corp 1983-1987.  All rights reserved.
 
Run File [AN16ADD.EXE]:
List File [NUL.MAP]:
Libraries [.LIB]:
LINK : warning L4021: no stack segment
 
C:\TASM>debug an16add.exe
-g
 
AX=060A  BX=0408  CX=0022  DX=0000  SP=0000  BP=0000  SI=0000  DI=0000
DS=0B97  ES=0B87  SS=0B97  CS=0B98  IP=0011   NV UP EI PL NZ NA PE NC
0B98:0011 CC            INT     3
-d 0B97:0000
0B97:0000  02 02 08 04 0A 06 00 00-00 00 00 00 00 00 00 00   ................
0B97:0010  B8 97 0B 8E D8 A1 00 00-8B 1E 02 00 03 C3 A3 04   ................
0B97:0020  00 CC 86 72 FF 77 15 8A-86 70 FF 2A E4 50 B8 FD   ...r.w...p.*.P..
0B97:0030  05 50 FF 36 24 21 E8 77-63 83 C4 06 FF 36 24 21   .P.6$!.wc....6$!
0B97:0040  B8 0A 00 50 E8 47 5E 83-C4 04 5E 8B E5 5D C3 90   ...P.G^...^..]..
0B97:0050  55 8B EC 81 EC 84 00 C4-5E 04 26 80 7F 0A 00 74   U.......^.&....t
0B97:0060  3E 8B 46 08 8B 56 0A 89-46 FC 89 56 FE C4 5E FC   >.F..V..F..V..^.
0B97:0070  26 8A 47 0C 2A E4 40 50-8B C3 05 0C 00 52 50 E8   &.G.*[email protected].
-q
 
C:\TASM>

Understanding the Memory Dump

The memory dump provided by the -d 0B97:0000 command displays the contents of memory starting from the DS (data segment) base address, which is 0B97:0000 in this case.

Here is the relevant portion of the memory:

0B97:0000  02 02 08 04 0A 06 00 00-00 00 00 00 00 00 00 00

Here, 1A 06 is the result of a + b = 0202h + 0408h = 060Ah, stored in c. In the memory dump, values are stored in little-endian format, meaning the least significant byte appears first. Hence the result 060A is stored as 0A 06 in memory.