Chicken Invader 🐔🚀

A bare-metal x86 space-shooter game that runs directly on real hardware with no operating system. Written entirely in 16-bit x86 NASM assembly.

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Overview

Chicken Invader is a Space Invaders-style game running in BIOS real mode (16-bit). You pilot a ship at the bottom of the screen, shoot bullets upward, and destroy five chickens marching across the top. The chickens fire eggs downward; if one hits your ship, it's game over. The game runs as a raw bootable disk image — no OS, no runtime, just the CPU and BIOS.

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Files

File	Description
bootloader.asm	Stage 1 bootloader — loaded by BIOS at 0x7C00, loads game
spacegame.asm	Main game — loaded to 0x9000, runs the full game loop

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How It Works

Bootloader (bootloader.asm)

• Loaded by the BIOS at physical address 0x7C00
• Saves the boot drive ID from DL
• Tries LBA mode first (INT 13h/AH=0x41 to detect extensions, then AH=0x42 to read via the disk packet structure)
• Falls back to CHS mode (INT 13h/AH=0x02) if LBA extensions aren't supported (reads 5 sectors, cylinder 0, head 0, starting at sector 2)
• Both paths load the game to the same place: 0x0000:0x9000, then jump there
  • LBA path — the disk packet's buffer offset/segment fields point to 0x9000:0x0000
  • CHS path — ES:BX is set to 0x0000:0x9000 before the read
• On disk error, prints m@l (LBA read failure) or m@c (CHS read failure) to the screen and halts

Game (spacegame.asm)

• Executes from 0x9000 in 16-bit real mode
• Uses BIOS INT 10h (TTY video output, cursor positioning) and INT 16h (keyboard)
• All rendering is character-based (ASCII art on the text-mode terminal)
• Uses a software busy-wait double-counter timer (bx_time / dx_time) to pace animation

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Building

Requirements: NASM

nasm bootloader.asm -f bin -o bootloader.bin
nasm spacegame.asm  -f bin -o spacegame.bin
cat bootloader.bin spacegame.bin > chicken_invader.img

The image is padded with zeros inside spacegame.asm to reach 1,437,952 bytes (~1.37 MB), which is required by Ventoy to recognise the image as a valid bootable disk. This is handled by the line near the bottom of the file:

TIMES 1437952-($-$$) DB 0

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Running on Real Hardware (Ventoy / USB)

1. Build the image as above.
2. Copy chicken_invader.img to the Ventoy USB drive partition.
3. Boot the machine from the USB drive and select the image in the Ventoy menu.
4. The bootloader detects whether the drive supports LBA or falls back to CHS automatically.

Tested on Intel x86 hardware. The default timer values in spacegame.asm are tuned for real hardware clock speeds. Both the LBA and CHS load paths now land the game at the same address (0x9000), so the fallback works correctly on drives/BIOSes without INT 13h extensions support.

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Running with QEMU

QEMU's busy-wait loop runs slower than on real hardware, so with the default timer values (tuned for real hardware) the game runs extremely slow under QEMU. You must lower the timer values before building if you want the game to run at a normal pace under QEMU.

qemu-system-x86_64 -drive format=raw,file=chicken_invader.img

Timer Tuning for QEMU (2 GHz CPU, Linux)

The game uses two countdown variables, bx_time and dx_time, to pace every animated event. Each function checks whether the timer has reached a specific threshold before firing. For QEMU, replace the default values with the ones below.

dx_time (outer / slow counter)

Default value	QEMU value	Purpose
DW 0x001e	DW 0x000a	Global outer animation tick rate

bx_time thresholds (inner / fast counter)

Each function compares bx_time against a threshold constant. Change the CMP WORD [bx_time], ... value inside the relevant function:

Function	Default threshold	QEMU threshold	Notes
flames_anime (draw)	0x00de	0x010a	Draw * or + flame sprite
flames_anime (clear)	0x0008	0x000a	Clear flame sprite
chk_ship_destroy_by_egg	0x0001	0x0001	No change needed
check_weapons_clash	0x0001	0x0001	No change needed
init_egg_trojectory	0x0001	0x0001	No change needed
bullet_trogectry_drawer	0x031e	0x010a	Advance bullet position
cls_bullet_trogectry_drawer	0x001e	0x000a	Clear previous bullet position
displayhex	active by default	0x0080	Debug: shows timer values in top-left corner (see Known Limitations)

dx_time thresholds (outer counter comparisons inside functions)

Function	Default threshold	QEMU threshold	Notes
egg_trojectory_drawer	0x0015	0x0007	Move egg downward
cls_egg_trojectory_drawer	0x0009	0x0003	Clear previous egg position

Tip: The main game loop reload values at startsp copy bx_time and dx_time into bx_time_bak / dx_time_bak once at startup. The backup values are what the timer resets to on every cycle, so changing the DW declarations is all that is needed — no other code paths need touching for the baseline tick rate.

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Controls

Key	Scan code	Action
← Left	0x4B	Move ship left
→ Right	0x4D	Move ship right
↑ Up	0x48	Move ship up
↓ Down	0x50	Move ship down
Space	0x20	Fire bullet
Delete	0x53	Restart (triggers a keyboard-controller system reset via port 0x64, not just a soft game reset)

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Game Elements

Element	ASCII Art	Description
Player ship	**||*_/\_*|___/\___||__||__|	Your ship; moves freely on screen
Chicken	/-(o_o)-\V	5 enemies, march and descend over time
Bullet	....	Fired upward from the ship tip
Egg	0	Dropped downward by chickens
Flame (+)	+--+++--+++-++	Engine flame, alternates with * variant
Flame (*)	*--***--***-**	Engine flame alternate frame

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Collision Detection

• Bullet vs Chicken (cmp_and / check_collision) — pixel-accurate multi-point check across the bullet's three character positions against each chicken's 8-wide bounding box.
• Bullet vs Egg (check_weapons_clash) — direct coordinate comparison; both are cleared on impact.
• Egg vs Ship (checking_ship_crash) — checks the egg's position against the ship's full hitbox (two overlapping rectangular areas covering the body and fin). Triggers the DEAD game-over message on hit.

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Chicken Movement

Chickens move as a formation, driven by change_chicken_state. Their animation state machine cycles through:

1. Horizontal oscillation — left/right with wing-flap offsets
2. Diagonal upwards — the whole formation moves diagonally upwards
3. Reset — formation resets to starting positions once all are destroyed (chick_bak)
4. Descend downwards — every 16 animation cycles (chick_down_count), the entire formation moves down one row (chick_down).

Egg launch timing is randomised using a simple byte counter (randomkey) that increments by 2 on every ship movement, cycling 0 → 10 → 0.

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Memory Layout

Address Range	Contents
0x7C00–0x7DFF	Bootloader (bootloader.bin, 512 bytes)
0x7C00	Stack pointer (grows downward)
0x9000+	Game code and data (spacegame.bin), loaded here on both the LBA and CHS paths

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Known Limitations

• Single-frame character rendering (no double-buffering); fast CPUs may show flicker.
• Ship boundary checking is not enforced — the ship can be moved off screen.
• DEAD text is displayed but the game loop continues running; press Delete to restart (this triggers a full keyboard-controller reset, not a soft in-game reset).
• The debug displayhex routine (shows dx_time / bx_time in the top-left corner) is active by default, called unconditionally from the main loop (startspc). To disable it, comment out CALL displayhex in that loop.