asm_shapes

Introduction

This is a project I wrote when I was teaching myself assembly. It is written in 8086
processor 16-bit DOS assembler language and as the title suggests uses BIOS interrupts
to draw 2D shapes.

Motivation

The project started when found out the interrupt routine that draws a single pixel.
With a nested loop I was able to draw a square, a rectangle, and more and more
shapes.
I then included these algorithms in my own functions, so I was able to draw a number
of shapes of a variety of size and colours.
Added some user interaction features. The user was then able to select what shape
to draw and where exactly on the screen.
Its memory requirements are minimal - the emulator allocates it only 1 MB of RAM.

Installation

Clone the project.
The source code is compiled and run on the emu8086 emulator, which is
available here.
Open the emulator and open main.asm. Click "emulate" on the menu and when it's
finished compiling it will be ready to run.

User Guide

Once the project starts running, it's time to draw a shape. This is accomplished
in 4 steps including keystrokes and clicks. Brief instructions are displayed when
the program starts to run.

#####1.##### [key] Press a key to select shape. There are four available, selected by entering
a number between 1 and 4. Any other key will cause the program to exit.
#####2.##### [key] Press a key to select colour. Colours corresponding from 0 to 9 are documented,
however more can be drawn with a little bit knowledge of binary arithmetic and palettes.
[0] Black
[1] Blue
[2] Green
[3] Aqua
[4] Red
[5] Fuchsia
[6] Dark orange/ light brown
[7] Grey (light)
[8] Grey (dark)
[9] Steel Blue

(skip this part if you're not interested in technical details)
Drawing a colour not on this list is a bit trickier. Some explanation in the procedure to
capture a key from the user is needed.
Having pressed a key, an interrupt is called and it is saved as char in the al register.
'0' (or 048 in decimal) is subtracted from al to covert it to integer.
This integer is stored in a variable which stores the colour. Therefore to select a
desired colour:

key_pressed = colour + 048

How do we select the colour, e.g. if we wish to draw in yellow?
The range of colours that can be used is from 0 to 0x0f, or 16 in decimal, or 1111 in binary. Therefore
the colour is 4 bits wide. The foremost bit contains information about grey, and the
next three about RGB.

MSB       LSB
 v         v       
+--+--+--+--+
|Gr|R |G |B |
+--+--+--+--+

Yellow consists of Grey, Red, and Green = 1110b = 14 Therefore the key we'd have to press is
14 + 48 = 62 = > in ASCII.
(end of technical part)
#####3.##### [key] The third key is a flag that takes effect only in case triangle has previously
been selected.
If it is 0, the triangle is drawn pointing upwards otherwise downwards.
#####4.##### [click] The program waits for two clicks. These will define the position and size of
the shape. For the rectangle, these are the top left and bottom right vertices but for
the other shapes this is not the shapes due to the difficulty of dealing with floats in
this particular language or adding excessive complexity to the program. The definition
points are illustrated below.

Testing

An example output as it being synthesised is shown below.

Known bugs

When choosing to draw a circle, if the second click is left of the first one, the program
crashes.

Todo

Fix bug 1.
More colours (all 16).
Add a background randomiser option, which draws pixels or whole shapes at random positions.

leonmavr / asm_shapes