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Assembly Lib

A 16-bits x86 DOS Assembly library that provides many useful functions for developing programs. It has both VGA grapics functions as well as general purpose utilities. The main purpose of this library was to be able to implement simple DOS games (in Assembly) using VGA (320x200, 256 colors) display.

From oded8bit·Updated June 15, 2026·View on GitHub·

A 16-bits x86 DOS Assembly library that provides many useful functions for developing programs. It has both VGA grapics functions as well as general purpose utilities. The main purpose of this library was to be able to implement simple DOS games (in Assembly) using VGA (320x200, 256 colors) display. The project is written primarily in Assembly, first published in 2018. Key topics include: 16-bit, asm, assembler, assembly, game.

Assembly Library

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A 16-bits x86 DOS Assembly library that provides many useful functions for developing programs. It has both VGA grapics functions as well as general purpose utilities. The main purpose of this library was to be able to implement simple DOS games (in Assembly) using VGA (320x200, 256 colors) display.

Note The library was developed and tested using TASM 4.1 and DosBox

Graphics

The library provides the following graphics capabilities:

  1. General purpose graphics utilities
  2. Manipulate color palettes
  3. Draw lines
  4. Draw rectangles (outline and filled)
  5. Draw closed polygons (outline only)
  6. Draw circles (outline and filled)
  7. Draw Bitmaps
  8. Images & screen manipulations
  9. Draw sprites and animation
  10. Support for Double buffering

Utilities

The library contains an utility package with many useful macros and procedures such as:

  1. File management
  2. Keyboard management, including ISR support
  3. Mouse input
  4. Several Math and randomizer functions
  5. Memory management (allocation and deallocation on the heap)
  6. String operations
  7. Printing to the screen
  8. Support for playing sound
  9. Time functions for delays

Using the library

To use the libraries, include the files in your code as follows:

sh
CODESEG
    include "UtilLib.inc"
    include "GrLib.inc"

Note that UtilLib.inc MUST come before GrLib.inc

Here is a sample program that draws a line and waits for a key press, using double buffering:

sh
.486
IDEAL
MODEL small
STACK 256
DATASEG	

CODESEG
    include "UtilLib.inc"
    include "GrLib.inc"   
start:
    mov ax, @data
    mov ds,ax

    ; Init library with double buffering flag on
    mov ax, TRUE
    ut_init_lib ax
  
    ; Allocate double buffering memory space
    call AllocateDblBuffer

    ; set display to VGA mode
    gr_set_video_mode_vga

    ; Set initial pen color
    gr_set_color GR_COLOR_GREEN

    ; your code here - we will draw a line...
    grm_DrawLine  10, 10, 200, 200

exit:
    call WaitForKeypress    
    call ReleaseDblBuffer
    gr_set_video_mode_txt
    return 0
END start

Build the program using

sh
tasm /zi main.asm
tlink /v main

PROCs and MACROs

The library provides many Procedures and Macros that you can use directly in your code. To make life easier (and have the code look more like a high level language), a wrapper MACRO was provided to many of the PROCs. Both options yield the same outcome.

For example, you can draw a line using the PROC

sh
push x1
push y1
push x2
push y2
call GR_DrawLine

or using a wrapper MACRO

sh
grm_DrawLine x1, y1, x2, y2

Make sure you pass the arguments in the right order. All procedures preserve register values, unless stated otherwise.
Macros (not including wrapper macros), on the other hand, may alter register values. Check out their documentation for details.

Initializing the library

At the beginning of your code, you need to initialize the library by calling:

sh
CODESEG
    ; Init library
    mov ax, FALSE
    ut_init_lib ax

The flag shoule be set to TRUE if you intend to dynamically allocate memory in your program. See memory management

Note that TRUE and FALSE are defined by the library as 1 and 0.

Testing the library and code samples

The Tests folder includes a testing file that demonstrates the use of various parts of the library. the test program itself
can be found at the root and is called main.asm

TicTac Game

A simple TicTac game that uses many of the library's features, was added to the library. You can compile and run the game using:

sh
tasm /zi tictac.asm
tlink /v tictac

The game itself can be found under Tests/tic folder and the main file is tictac.asm

---------------------------------------------------------------

Graphics Library

Drawing a pixel

The most basic macros handle drawing a single pixel on the screen, using direct memory access (not interrupt).

sh
gr_set_pixel    x, y, color - draws a pixel in the given (x,y) coordinates with the given color
gr_set_pixel_xy x,y         - draws a pixel in the given (x,y) coordinates with the set color

Note that x, y and color cannot use the following registers: ax, bx, di, dx

This macro takes into account double buffering and will draw to the buffer (if set) instead of the video display. See
Double Buffering for details.

It is highly recommended to use this macro whenever you want to draw to the VGA screen.

Clearing the screen

There are 3 ways to clear the screen, or a portion of the screen

sh
clear_screen_vga    - macro that very efficiently clears the entire screen (VGA mode)
GR_ClearRect        - procedure that clears a rectangle on the screen (VGA mode)
clear_screen_txt    - macro that very efficiently clears the entire screen (TXT mode)

Drawing Shapes

Drawing Lines

Drawing a line using a PROC:

sh
push  50    ; x1 
push  60    ; y1 
push  120   ; x2 
push 180    ; y2
call GR_DrawLine

or using a MACRO:

sh
grm_DrawLine 50, 60, 120, 180

Drawing Rectangles

You can either draw a rectangle outline or a filled one.

Outline using a PROC:

sh
push  50    ; x (top)
push  60    ; y (top)
push  120   ; width
push 180    ; height
call GR_DrawRect

or using a MACRO:

sh
grm_DrawRect 50, 60, 120, 180

Filled using a PROC:

sh
push  50    ; x (top)
push  60    ; y (top)
push  120   ; width
push 180    ; height
call GR_FillRect

or using a MACRO:

sh
grm_FillRect 50, 60, 120, 180

Drawing Circles

You can either draw a circle outline or a filled one.

Outline using a PROC:

sh
push  150    ; X center
push  100    ; Y center
push  40   ; Radius
call GR_DrawCircle

or using a MACRO:

sh
grm_DrawCircle 150, 100, 40

Filled using a PROC:

sh
push  150    ; X center
push  100    ; Y center
push  40   ; Radius
call GR_FillCircle

or using a MACRO:

sh
grm_FillCircle 150, 100, 40

Drawing Polygons

To draw a polygon, you need to define an array and pass its pointer along with the number of points in the array.
The array should contain points (x and y) as follows:

sh
DATASEG	
    _poly    dw     5,30,100,50,200,100

This is an array with the following points:

  1. _poly[0] = (5,30)
  2. _poly[1] = (100,50)
  3. _poly[2] = (200,100)

Using a PROC:

sh
push 3              ; there are 3 points
push offset _poly   ; pointer to _poly array
call GR_DrawPolygon

or using a MACRO:

sh
mov ax, offset _poly
grm_DrawPolygon 3, ax

Using Bitmaps

The library support only 8-bit bitmaps in v3. To load a bitmap, you need to define the following variable:

sh
_bmp_file               db      "asset\\b.bmp",0
_bmp                    db      BMP_STRUCT_SIZE dup(0)

This will allocate the memory for the BMP headers (struct). Memory allocation for the pixels' data will be
allocated in RAM when loading the data.

You must use "call FreeProgramMem" before trying to load images or you will get an out of memory situation and the allocation will fail.
Note that you can implicitly call it by passing TRUE to ut_init_lib:

sh
mov ax, TRUE
ut_init_lib TRUE

To load a BMP image, call:

sh
mov dx, offset _bmp_file
mov cx, offset _bmp      

push dx                     ; path offset
push ds                     ; path segment
push cx                     ; bitmap struct offset
push ds                     ; bitmap struct segment
call LoadBMPImage

or using a MACRO

sh
mov dx, offset _bmp_file
mov cx, offset _bmp      

grm_LoadBMPImage dx, ds, cx, ds

After loading the bitmap, you can draw it on the screen

sh
mov cx, offset _bmp      

push cx                     ; bitmap struct offset
push ds                     ; bitmap struct segment
push 000Ah                  ; xTop = 10
push 000Bh                  ; yTop = 11
call DisplayBMP

; instead, you can tile the image on the screen using
push cx
push ds
call TileBmp

or using MACROS:

sh
mov cx, offset _bmp      

grm_DisplayBMP  cx, ds, 000Ah, 000Bh
; or
grm_TileBmp  cx, ds

You must free the bitmap memory (when it is not needed anymore) by calling

sh
mov cx, offset _bmp      

push cx                     ; bitmap struct offset
push ds                     ; bitmap struct segment
call FreeBmp

or using MACROS:

sh
grm_FreeBmp  cx, ds

Saving and Loading Palettes

The library provides methods for extracting color palettes of BMP files and storing them in a new file (binary) as well as load a palette file into
a buffer (possibly the palette of a BMP structure).

Here is an example of saving a palette to a file:

sh
mov dx, offset _bmp_file
mov ax, offset _bmp
grm_LoadBMPImage dx, [_dss], ax, [_dss]

mov ax, offset _bmp
mov bx, offset _paletteFile
grm_SavePalette ax, [_dss], bx

and loading it to a buffer:

sh
push offset _paletteFile
push ds
push offset _palette
push ds
call LoadPalette

Sprites

The library supports sprites, which is an image with multiple frames. Sprites are a great way to create animation or to hold multiple variants of an image.
See an example.

Since sprites are standard BMP files, you load them normally and then can display any frame you select.

sh
DATASEG
    _sprite_w       equ         30
    _sprite_frames  equ         6
    _sprite_file db      "asset\\sprite1.bmp",0
    _sprite      db      BMP_STRUCT_SIZE dup(0)

CODESEG    
    mov dx, offset _sprite_file         
    mov ax, offset _sprite          

    push dx                         ; path address
    push ds                         ; path segment
    push ax                         ; struct address
    push ds                         ; struct segment
    call LoadBMPImage

    push 0                  ; frame index
    push ax                 ; BMP struct address
    push ds                 ; BMP struct segment
    push 0064h              ; x coordinate
    push 0064h              ; y coordinate
    push _sprite_w          ; width of a single frame
    push _sprite_frames     ; number of frames in BMP
    call PlaySpriteInPlace

or using macros:

sh
CODESEG    
    mov dx, offset _sprite_file         
    mov ax, offset _sprite          

    grm_LoadBMPImage dx, ds, ax, ds
    grm_PlaySpriteInPlace 0, ax, ds, 0064h, 0064h, _sprite_w, _sprite_frames

Take a look at TestMySprite function as an example for playing animation using sprites.

Here is an example of a sprite image

Copy screen to buffer and visa versa

The library provide functions for copying a portion of the screen to a buffer and visa versa (from a buffer to the screen).

sh
grm_SaveScreen memAddress, memSeg, x, y, w, h
sh
grm_WriteScreen memAddress, memSeg, x, y, w, h

---------------------------------------------------------------

Utility Library

The library includes other utilities that help developing assembly programs.

sh
store_sp_bp         - macro that stores and sets BP value. Called at the beginning of PROC
restore_sp_bp       - macro that restores BP, SP values. Called at the end of PROC
return              - macro for returning control to DOS with a code
cmpv                - macro that compare two memory variables
movv                - moves a WORD from one memory to another via the stack

Data Segment

The library stores the original data segment (DS register) using a global variable _dss that you can access at any time if you need to restore its value.

sh
push [_dss]
pop ds

Time and Delays

The library provides 3 different ways to delay your program:

Sleep

If you want your program to halt for a given number of seconds call the Sleep procedure

sh
push 3              ; sleeps for 3 seconds
call Sleep

or using MACROS:

sh
utm_Sleep  3

Delay

Delay program execution by X number of milliseconds. Actually, the argument is the number of
1/18 of a second

sh
push 3              ; Delay for 3 * 1/18 of a sec
call Delay

Or:

sh
utm_Delay 3

DelayMS

This function delays execution for given number of microseconds which are specified in 2 variables, one for the high order and one for the low order.
For example, a 1 second delay (1 * 1,000,000 msec) is 000F 4240 and therfore the values will be

sh
push 000Fh
push 4240h
call DelayMS

Or

sh
utm_DelayMS 000Fh, 4240h

Another example, a 2 seconds delay is equal to (21,000,000) 001E 8480 and a 1 millisecond is (11000) 0000 03EB

Keyboard

Most keyboard functions are accessible via interrupts and the library focuses on some less trivial parts of handling the keyboard.
A list of keyboard scan codes can be found at keymap.asm

Repeat Rate

Setting keyboard yypematic rate to defalt (repeat delay and rate)

sh
call SetKeyboardRateDefault

Wait for keypress

Hold program execution until a key is pressed

sh
call WaitForKeypress

Keyboard Status

Getting keyboard status

sh
call GetKeyboardStatus

Returns:
ZF = 0 if a _Key pressed (even Ctrl-Break)
AX = 0 if no scan code is available
AH = scan code
AL = ASCII character or zero if special function _Key

Consume Key

Taking the pressed key out of the keyboard buffer

sh
call ConsumeKey

Get keyboard flags

sh
call GetKeyboardFlags

Returns AL:
|7|6|5|4|3|2|1|0| AL or BIOS Data Area 40:17
| | | | | | | ---- right shift key depressed | | | | | | ----- left shift key depressed
| | | | | ------ CTRL key depressed | | | | ------- ALT key depressed
| | | -------- scroll-lock is active | | --------- num-lock is active
| ---------- caps-lock is active ----------- insert is active

Install & Restore a Keyboard ISR

You can replace the default keyboard interrupt with your own by calling this procedure

sh
push isr_address 
call InstallKeyboardInterrupt

For example:

sh
lea dx,[my_interr]
push dx
call InstallKeyboardInterrupt

You must restore the interrupt at the end of the program by calling

sh
call RestoreKeyboardInterrupt

Sample Keyboard Interrupts

The library contains 2 sample implementations of a keyboard ISR.

  1. KeyboardSampleISR - implements a FIFO buffer and allows retrieving keys using getcISR. Note that this is not
    a complete implementation but merely an example. For instance, it does not convert the scancode to ASCII and does not
    treat SHIFT, CTRL and ALT combinations.
  2. KeyboardISREvents - a simple implementation that illustrates how to call the default keyboard handler

See TestKeyboardISR and TestSimpleISR

Mouse

Most mouse functions are accessible via interrupts and the library focuses on some less trivial parts of handling the mouse.

Show and Hide the mouse

Use these macros to show / hide the mouse

sh
ShowMouse

HideMouse

Get Mouse Status

Mouse position and button status can be retrieved by

sh
GetMouseStatus

On return:
CX = horizontal (X) position (0..639)
DX = vertical (Y) position (0..199)
BX = button status:

	|F-8|7|6|5|4|3|2|1|0|  Button Status
	  |  | | | | | | | `---- left button (1 = pressed)
	  |  | | | | | | `----- right button (1 = pressed)
	  `------------------- unused

Translate Mouse Coordinates

After getting mouse coordinates from GetMouseStatus, you can translate them to VGA coordinates by calling

sh
TranslateMouseCoords

Set Mouse Cursor Position

sh
mov cx, 3       ; horizontal
mov dx, 5       ; vertical
SetMousePosition

Installing and Restoring Mouse ISR

You can replace the default mouse interrupt with your own by calling this procedure

sh
push ISR address
push ISR segment
push mask
call InstallMouseInterrupt

You must restore the interrupt at the end of the program by calling

sh
call UninstallMouseInterrupt

File System

The library provide a set of functions to access and manipulate files. The library was designed to handle a single file at any time using the global variables

sh
_fHandle    - stores the handle of the openned file
_fErr       - stores the error value

Open a File

sh
push address_of_file_name
push segment_of_file_name
call fopen

or using MACROS:

sh
utm_fopen address_of_file_name, segment_of_file_name

Create a New File

sh
push address_of_file_name
push segment_of_file_name
call fnew

or using MACROS:

sh
utm_fnew address_of_file_name, segment_of_file_name

Close a File

sh
call fclose

or using MACROS:

sh
utm_fclose

Get File Size

sh
push address_of_file_name
push segment_of_file_name
call fsize

or using MACROS:

sh
utm_fsize address_of_file_name, segment_of_file_name

Read from a File

sh
push length
push address_of_buffer
push segment_of_buffer
call fread

or using MACROS:

sh
utm_fread address_of_buffer, segment_of_buffer

Write to a File

sh
push length
push address_of_buffer
push segment_of_buffer
call fwrite

or using MACROS:

sh
utm_fwrite address_of_buffer, segment_of_buffer

Delete a File

sh
push address_of_file_name
push segment_of_file_name
call fdelete

or using MACROS:

sh
utm_fdelete address_of_file_name, segment_of_file_name

Change File Attributes

sh
push attribute
push address_of_file_name
push segment_of_file_name
call fchangeAttr

or using MACROS:

sh
utm_fchangeAttr attribute, address_of_file_name, segment_of_file_name

File Seek

You can do a fseek in a file very similar to the C function

sh
grm_fseek SEEK_SET, 0, 40

Or:

sh
push SEEK_SET
push 0
push 40
call fseek

The first parameter can be SEEK_CUR, SEEK_SET or SEEK_END.
The second parameter is the high order of the offset and the third is the low order.

Directory Services

The library provides a few directory services, including:

sh
mkdir OR utm_mkdir   - create a directory
rmdir OR utm_rmdir   - delete a directory
chdir OR utm_chdir   - change a directory

Math

The library provide some basic math related functions and macros

Random Numbers

Before generating a random number, you need to initialize the number generator by calling

sh
call RandomSeed

and then you can use

sh
call RandomWord
or
call RandomByte

to get a random number in AX or AL

Abs(x)

You can get the absoilute value of a number

sh
gr_absolute number

Memory Management

The library allows managing (allocating, releasing) RAM memory. If you need to allocate dynamic memory, you must free unused memory at the
beginning of your program by calling:

sh
call FreeProgramMem

If you forget to call it, all memory allocations will fail on "out of memory".

Note that you can implicitly call it by passing TRUE to ut_init_lib:

sh
mov ax, TRUE
ut_init_lib TRUE

Allocating a block

Allocating a memory block is done by calling:

sh
push size
call malloc

Note that the size is measured in Paragraphs and therefore need to be divided by 16 (bytes)

Return value:
AX = segment address of allocated memory block (MCB + 1para)
0 on error
BX = size in paras of the largest block of memory available
0 on error
CF = 0 if successful
= 1 if error
Allocated memory is at AX:0000

Release an allocated memory block

Pass the segment address of the allocated memory block to release it

sh
push segment
call mfree

Releasing all allocated blocks

The library maintains an internal list of up to 50 allocated blocks that can be freed by calling

sh
call mfreeAll

Memory Copy

Copies memory from one address to another

sh
push from_address
push from_seg
push to_address
push to_seg
push length_in_bytes
call MemCpy

Initialize memory block

You can set a byte or word value to an entire memory block

sh
mov ax, offset _blcok

push 10             ; length_in_bytes
push ds             ; block_segment
push ax             ; block_offset
push 0              ; value
call SetMemByte

and

sh
mov ax, offset _blcok

push 5              ; length_in_words
push ds             ; block_segment
push ax             ; block_offset
push 0              ; value
call SetMemWord

Sound

You can make a beep with the following procedure. Note that the sound will continue until you stop it

sh
push frequency
call Beep

utm_Sleep  2        ; wait 2 seconds

call StopBeep

Or with macros:

sh
utm_Beep freq
utm_Sleep 2
utm_StopBeep

Print

This part of the library supports writting text to the screen

Printing to the screeb

sh
call PrintDecimal   - prints a value as a decimal number

call PrintChar      - prints DL as a char
PrintCharNewLine    - macro to print a char with new line

call PrintByte      - prints DL (number between 0 and 15)
PrintByteNewLine    - macro to print a byte with a new line

call PrintStr       - prints a string. DS:DX pointer to string ending in "$"
PrintStrNewLine     - macro to print a string with a new line

call PrintNewLine   - prints a new line
call PrintSpace     - prints a space char

call PrintHexByte   - prints the LSB BYTE in HEX
call PrintHexByte   - prints the WORD in HEX

Printing in VGA

sh
call PrintCharVGA   - prints a character on VGA display (DL: char, BL: color)
call PrintStrVGA    - prints a string to the VGA screen

Setting cursor position

sh
push x
push y
call SetCursorPosition

String Manipulations

The library contains several basic string manipulations functions that accept null terminating string

sh
Strlen       - Calculates length of string ending with NULL
StrlenDollar - Calculates length of string ending with '$'
Strcpy       - Copies string s2 into string s1
Strncpy      - Copies given number of chars from string s2 into string s1.
Strcat       - concatenate 2 strings
Strchr       - Searches for the first occurrence of the character in the given string
Strcmp       - Compares the string pointed to, by str1 to the string pointed to by str2.

License

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty
of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.

Credits

  1. Some ideas were taken from https://github.com/itay-grudev/assembly-dos-gx
  2. README file created using Dillinger

Contributors

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odedigo

odedigo

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This article is auto-generated from oded8bit/Assembly-Lib via the GitHub API.Last fetched: 6/28/2026