366: Systems Level Programming
================================

Low-level programming targeting DOS and x86 hardware, including device drivers and real-mode assembly.

Topics
------

- x86 real-mode assembly (NASM/MASM)
- DOS interrupt services (INT 21h, INT 10h)
- Device driver architecture (block and character drivers)
- Video memory-mapped I/O
- Keyboard and timer interrupt handling

Projects
--------

Missile Command Game (``366/src/rocks.asm``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A "Missile Command"-style arcade game written entirely in x86 assembly. Uses INT 10h for
video mode switching and direct VGA frame-buffer writes to animate missiles and explosions
in real time.

.. code-block:: nasm

   MAX_ROCKS       equ  5        ; maximum rocks on screen at a time
   NEW_ROCK_CHANCE equ  10
   TAU             equ  8        ; controls fall speed (4=fast, 12=slow)
   ROCK_MAX_Y      equ  20
   TIMER_INT       equ  1ch
   TIMER_INT_LOCATION equ 4*TIMER_INT

   ; A rock record (struct): x, y, active flag, momentum
   upper_left_x  equ  0   ; byte
   upper_left_y  equ  1   ; byte
   active        equ  2   ; byte
   momentum      equ  3   ; byte
   ROCK_SIZE     equ  4

   ; Hook the timer interrupt to drive game animation
   mov  ax, 3500h + TIMER_INT   ; DOS get interrupt vector
   int  21h
   mov  [old_timer_seg], es
   mov  [old_timer_off], bx

DOS Block Device Driver (``366/src/blkdrv.asm``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A dummy block device driver implemented as a DOS ``.SYS`` file. Demonstrates the DOS
device driver header structure, strategy/interrupt routines, and the request packet protocol
used by the DOS kernel to communicate with drivers.

.. code-block:: nasm

   ; DOS device driver header
   code    segment
           assume cs:code
           dw    -1          ; link to next driver (-1 = end of chain)
           dw    -1
           dw    2000h       ; attribute word (block device)
           dw    offset str  ; strategy routine offset
           dw    offset int  ; interrupt routine offset
           db    3           ; number of units (block devices)
           db    7 dup (?)   ; reserved for DOS

   int:
           cmp   byte ptr es:[bx+2], 0    ; only Init command supported
           je    init
           mov   word ptr es:[bx+3], 8102h ; return "device not ready"
           jmp   exit