am_map.c, am_map.h 	Automap
d_englsh.h 		Text strings used in the game (English versions)
d_event.h 		Event structure
d_french.h 		Text strings used in the game (French translations)
d_items.c, d_items.h 	A table which controls weapon animations
d_main.c, d_main.h 	System-independent main function, and setup code
d_net.c, d_net.h 	System-independent networking code
doomdata.h 		Basic data structures used to represent levels on disk
doomdef.c, doomdef.h 	Constants used throughout the source code
doomstat.c, doomstat.h 	Game mode variables
doomtype.h 		Common type definitions used throughout the source code
d_player.h 		Definition of the player_t structure, which represents players
dstrings.c, dstrings.h 	Quit messages
d_textur.h 		An unused data structure, probably obsolete
d_think.h 		Definitions of thinker structures used for AI
d_ticcmd.h 		Player movement command definition (ticcmd_t)
f_finale.c, f_finale.h 	Finale screen code
f_wipe.c, f_wipe.h 	Screen melt effect
g_game.c, g_game.h 	High-level game stuff: transitions between game states, level exits, main game event responder, setting up new games, demo recording
hu_lib.c, hu_lib.h 	Library functions for heads-up display
hu_stuff.c, hu_stuff.h 	Heads-up display code
i_main.c 		Main function
i_net.c, i_net.h 	System-dependent network code (UDP Unix version)
i_sound.c, i_sound.h 	System-dependent sound code
i_system.c, i_system.h 	Miscellaneous system-dependent code (error handling, timer, etc.)
i_video.c, i_video.h 	System-dependent video code (X11 Unix version)
info.c, info.h 	T	ables of data used by the game: sprite name table, thing data table, frame table
m_argv.c, m_argv.h 	Functions for searching the command line argument list
m_bbox.c, m_bbox.h 	Bounding boxes
m_cheat.c, m_cheat.h 	Cheat code parsing
m_fixed.c, m_fixed.h 	Fixed point number functions
m_menu.c, m_menu.h 	Menu
m_misc.c, m_misc.h 	Miscellaneous functions: text rendering, configuration file loading, writing screenshots
m_random.c, m_random.h 	Pseudo-random number generator
m_swap.c, m_swap.h 	Endianness conversion routines
p_ceilng.c 		Ceiling movement
p_doors.c 		Door movement
p_enemy.c 		Monster AI
p_floor.c 		Floor movement
p_inter.c, p_inter.h 	Object interaction: item pickup functions, object damage functions
p_lights.c 		Animated lights
p_local.h 		Global header for functions in the p_* section
p_map.c 		Object movement: collisions, teleportation, detecting for injury
p_maputl.c 		Library functions for p_map.c: blockmap code
p_mobj.c, p_mobj.h 	Basic object control functions: creating/destroying objects, movement, etc.
p_plats.c 		Elevators
p_pspr.c, p_pspr.h 	PSprite (weapon sprite) functions
p_saveg.c, p_saveg.h 	Savegames
p_setup.c, p_setup.h 	Loading of map data and setup
p_sight.c 		Line-of-sight/visibility checking for monsters
p_spec.c, p_spec.h 	Special effects: texture animations, line triggers, floor/ceiling/lighting changes
p_switch.c 		Wall switches
p_telept.c 		Handling of teleporter lines
p_tick.c, p_tick.h 	Base thinker code
p_user.c 		Player point-of-view code (weapon bobbing, player object thinker, etc.)
r_bsp.c, r_bsp.h 	BSP tree traversal
r_data.c, r_data.h 	Initialisation of data (textures, etc.) for use in the rendering code
r_defs.h 		Type definitions used in the rendering engine
r_draw.c, r_draw.h 	Fast drawing functions
r_local.h 		Includes all the r_* header files
r_main.c, r_main.h 	Entrypoint into rendering code
r_plane.c, r_plane.h 	Rendering of planes (floor/ceilings)
r_segs.c, r_segs.h 	Rendering of walls
r_sky.c, r_sky.h 	Sky rendering code (actually this is only some very basic initialisation code; the sky rendering code is really in r_plane.c)
r_state.h 		Definitions for global variables used by the rendering engine
r_things.c, r_things.h 	Rendering of sprites
sounds.c, sounds.h 	Sound/music tables
s_sound.c, s_sound.h 	System-independent sound code
st_lib.c, st_lib.h 	Library routines for status bar code
st_stuff.c, st_stuff.h 	Status bar main code
tables.c, tables.h 	Lookup tables for fast sine/cosine
v_video.c, v_video.h 	Generic image patch rendering (for menu, etc.)
wi_stuff.c, wi_stuff.h 	Intermission screens
w_wad.c, w_wad.h 	WAD loading code
z_zone.c, z_zone.h 	Zone memory allocator
[edit]
Sound server

These files are part of the sound server:
Files 	Description
linux.c 		Linux-specific sound output code
sounds.c,sounds.h 	List of sound effects
soundsrv.c,soundsrv.h 	Core sound server code
soundst.h 		Sound structures and prototypes
wadread.c,wadread.h 	WAD file reading code


Source code structure

The source code can be divided into sections. Files and functions within the source code have one- or two-letter prefixes to denote their subsystem.

AM_*
    Automap code 
D_*
    Initialisation/general code 
F_*
    "Finale" (end of game) and "screen melt" code. 
G_*
    Main game loop/control 
HU_*
    Heads-up display 
I_*
    System-specific code 
M_*
    Miscellaneous (includes the menu) 
P_*
    Game logic/behaviour 
R_*
    Rendering engine 
S_*
    Sound code 
ST_*
    Status bar 
V_*
    General graphic rendering 
WI_*
    End-of level "intermission" screen 
W_*
    WAD file loading 
Z_*
    Zone memory allocation system 

The following are common prefixes for functions, although they do not denote a particular subsystem, and there are no files with these prefixes:

A_*
    Action functions invoked in sprite movement frames (these are the functions used in Dehacked "code pointers") 
PIT_*
    Callback functions passed to P_BlockThingsIterator (see p_maputl.c) 
T_*
    "Thinker" functions set to be called each clock tic for some purpose (eg, moving platforms or flickering lights)