Decoded: Sopwith (1984) by David L. Clark
Source file: SWMAIN.C
Beginner friendly, line-by-line code walkthrough by MaiZure

SWMAIN.C is the main entry point to the program, which kicks off initialization and also includes the game loop at the end

Original code:
http://davidlclark.com/page/sopwith-source-code

Original code with line numbers
http://www.maizure.org/projects/decoded-sopwith/SWMAIN_linenum.txt

1     COMMENT
2     COMMENT
3     BLANK
4     COMMENT
5     COMMENT
6     COMMENT
7     COMMENT
8     COMMENT
9     COMMENT
10    COMMENT
11    COMMENT
12    COMMENT
13    COMMENT
14    BLANK
15    COMMENT
16    BLANK
17    COMMENT
18    COMMENT
19    COMMENT
20    COMMENT
21    COMMENT
22    COMMENT
23    COMMENT
24    COMMENT
25    COMMENT
26    COMMENT
27    Include main game header (sw.h)
28    BLANK
29    BLANK
30    BLANK

SOPWITH DECLARES MANY GLOBALS

31    Declares global variable for playmode (novice, single, multiple, async)
32    Declares global GAMES object for tracking game state
33    Declares global OBJECTS status array
34    Declares global target count array (used in collision loop)
35    Declares global for the screen mode. Not clear this is meaningfully used
36    Declares global for tick mode. Not clear that this is used
37    Declares global counters used in movement and networking
38    Declares global counters used for movement...mostly deprecated it seems
39    BLANK
40    Declares global game number counter. Generally each game per session gets
      more difficult
41    Declares global offsets for each game number
42    Declares global range variable
43    BLANK
44    Declares global structure for multiplayer communications
45    Begins a preprocessor case for IBM PCs  
46    Declares a global 8k display buffer for the IBM PC
47    Ends case for the IBM PC
48    Begins a preprocessor case for the Atari
49    Declares a global 16k display buffer for the Atari
50    Ends case for the Atari
51    BLANK
52    Declares a global integer to hold a keystroke
53    Declares a pointer for the multiplayer network buffer
54    BLANK
55    Begins a preprocessor case for IBM PCs
56    Declares a global address for the IBM PC disk controller
57    Ends case for the IBM PC
58    Begins a preprocessor case for the Atari
59    Declares a global address for the Atarti controller
60    Ends case for the Atari
61    BLANK
62    Declares a global used to track delay for multiplayer sync
63    Declares a global boolean for hi-resolution screen (40 vs 80 char width)
64    Declares a global boolean for display position -- not used
65    Declares a global boolean used to skip the title screen
66    Declares a global variable to hold debug infromation - not normally used
67    Declares a global boolean for sound on/off
68    Declares a global boolean to report game object usage (debug)
69    Declares a global boolean for joystick
70    Declares a global boolean for IBM type keyboards
71    Declares a global boolean for action gameplay state
72    Declares a global boolean for print screen keypress action
73    Declares a global for handling a keyboard press interrupt
74    Declares a global for missile usage
75    BLANK
76    Declares globals for left and right screen boundary positions
77    Declares a global for screen shift amount
78    Declares a global boolean for display initilization status
79    BLANK
80    Declares a global pointer to an object struct (not used)
81    Declares a global pointer to the game objects arrays
82    Declares a global pointer to player object arrays
83    Declares global pointers for object structure endpoints
84    Declares global pointers for an uninitialized object
85    Declares global pointerss for objects to be destroyed
86    Declares global pointers for objects in list by x position
87    BLANK
88    Declares a global array for potential AI targets
89    Declared a global array for left enemy territory boundaries (AI)
90    Sets array values based on positions
91    End of array
92    Declared a global array for right enemy territory boundaries (AI)
93    Sets array values based on positions
94    End of array
95    BLANK
96    Declares a global pointer to the highest allocated object in memory
97    Declares a global array for the end game stats per player
98    Declares a global counter variable for delaying for the ending sequence
99    Declares a global pointer to the player
100   Declares a global index for the current object
101   Declares a global boolean for current objects player status
102   Declares a global boolean for current objects computer status
103   Declares a display status array indexed by objects
104   Declares a global boolean for the end game sequence
105   Declares a global boolean for force redrawing of the ground
106   Declares global pointers to the history log i/o
107   Declares a global random seed used for generating exploding objects
108   BLANK
109   Declares a global delay threshold for keyboard input
110   Declares a global delay counter for keyboard input
111   Declares a global holding the end game status of the current object
112   Declares a global for the number of lives left
113   Declares a global for splatted birds
114   Declares a global counter for bird splats on screen (crash in to flock)
115   Declares a global for the splattered ox
116   Declares a global flag for the splattered ox (triggers splat display)
117   BLANK
118   BLOCK START - Precomputed sine table as integers [-256 - 256]
119   sin of 0, 22.5, 45, and 67.5 * 256
120   sin of 90, 112.5, 135, and 157.5 * 256
121   sin of 180, 202.5, 225, and 247.5 * 256
122   sin of 270, 292.5, 315, and 337.5 * 256
123   BLOCK END - Precomputed sine table as integers [-256 - 256]
124   BLANK
125   Declares a jump buffer to manipulate function calls (see setjmp.h)
126   COMMENT
127   BLANK
128   Imports a global procedure from SWUTIL.ASM for keyboard interrupts
129   BLANK
130   Begins a preprocessor case for IBM PCs
131   Imports a global system flag from BMBLIB.C
132   Ends a preprocessor case for IBM PCs
133   Begins a preprocessor case for the Atart
134   Declares a global system flag
135   Ends a preprocessor case for Atari
136   BLANK
137   BLANK
138   Main entry point with the standard argument count and element vector
139   Argument counter variable
140   Argument vector pointer

THE MAIN ENTRY POINT

141   BLOCK START - main
142   Prototype for malloc (behavior is compiler dependent...conflicts, etc)
143   BLANK
144   Allocates memory for a 100 object pool/slab (why 100 and not MAX_OBJS?)
145   Sets the system type to DOS
146   BLANK
147   Calls the big init function for the game
148   Saves the current environment (top-level main) in to the env buffer

THE GAME LOOP

149   BLOCK START - Game loop.  Infinite loop...FOREVER is 'for (;;)' in std.h
150   BLANK
151   COMMENT
152   COMMENT
153   COMMENT
154   COMMENT
155   Delays game for 'movemax' cycles (normally set to 15 in SWINIT)
156   Disables interrupts
157   Resets move tick to 0
158   Enables interrupts
159   BLANK
160   Moves all objects (SWMOVE.C)
161   Checks the status of the joystick (SWUTIL.ASM)
162   Updates the display (SWGRPH.ASM)
163   Checks the status of the joystick again
164   Processes collisions (SWCOLLSN.C)
165   Checks the status of the joystick yet again
166   Disables interrupts
167   BLOCK START - Print screen. Enters block if print screen was triggered
168   Resets the print flag
169   Resets the interrupt vector for koveride (for default printscreen)
170   Enables interrupts
171   Executes the print screen (SWUTIL.ASM)
172   Disable interrupts
173   Restores the application overrided keyboard interrupt
174   Segment arguments for the interrupt override
175   BLOCK END - Print screen
176   Enables interrupts
177   Performs sound operations
178   BLOCK END - Game Loop
179   BLOCK END - main
180   BLANK˙