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2D GRAPHICS ENGINE

A freestanding 16-bit real-mode 2D graphics engine that builds to a DOS .COM executable (gui.com). It mixes 16-bit C (GCC .code16gcc) with NASM assembly and talks to hardware directly through BIOS interrupts — no standard library, no OS services, no malloc. Run it ships as a small interactive paint program: steer a cursor with the keyboard, cycle colors, and stamp rectangles.

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FEATURES

[+] Pure freestanding 16-bit code — boots straight into VGA mode 0x12 (640x480, 16 colors).

[+] Clean C-over-assembly layering: all BIOS interrupts isolated in one .asm file.

[+] Custom bump allocator with save/load checkpoints (no malloc, no heap fragmentation).

[+] Bit-packed shape primitives: points, lines, and rectangles.

[+] Interactive WASD paint demo with color cycling and anchor-based rectangles.

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REQUIREMENTS

[+] gcc with 16-bit support (-m16).

[+] nasm (assembler).

[+] ld (GNU linker).

[+] A real-mode environment to run the result, e.g. DOSBox or QEMU.

On Debian/Ubuntu:

sudo apt install gcc nasm binutils dosbox

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BUILD

make          # builds gui.com
make clean    # removes *.o and *.com

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RUN

gui.com is a DOS program and cannot run natively on Linux. Launch it in DOSBox:

dosbox gui.com

Or from the DOSBox prompt:

mount c .
c:
gui.com

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CONTROLS

[+] w / a / s / d — move the cursor (draws a trail in the current color).

[+] c — cycle the draw color (1-15).

[+] space — drop a rectangle anchor at the cursor.

[+] r — draw a rectangle from the anchor to the cursor.

[+] q — quit.

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PROJECT LAYOUT

[+] main.c — entry point, interactive paint loop, heap allocator, and basic I/O.

[+] shapes.c — shape constructors (mk*) and renderers (draw*).

[+] xgfx.asm — the only layer that issues BIOS interrupts; reserves the heap.

[+] include/ — core types, macros, packed shape structs, and prototypes.

[+] gui.ld — linker script producing the flat .COM binary at origin 0x100.

[+] Makefile — build rules and toolchain flags.

For architecture details and contribution conventions, see AGENTS.md.

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HOW IT WORKS

C logic in main.c and shapes.c never touches hardware directly — it calls the x* routines in xgfx.asm, which own every BIOS int call. Shapes compose from the bottom up: a line is a loop of points, a rectangle is four lines. Memory comes from a bump allocator; scratch allocations inside draw loops are reclaimed with save() / load() instead of being freed.

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