Decoded: Rogue (1980) by Toy, Arnold, Wichman
DOS version (1983) by Mel Sibony and Jon Lane
Source file: FAKEDOS.C
Beginner friendly, line-by-line code walkthrough by MaiZure

FAKEDOS.C gives you bad workers a way to have fun and avoid the man. 
Fake DOS shell!

Original code:
https://britzl.github.io/roguearchive/

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




1     COMMENT
2     COMMENT
3     COMMENT
4     BLANK
5     Include the game header
6     Include the console management header
7     BLANK
8     Declare a function pointer to the strip blank function (in STRINGS.C)
9     BLANK

FAKE DOS PROMPT

10    Define fakedos with no arguments
11    BLOCK START - fakedos, imitates the DOS prompt (launch with ! key)
12    Declare a buffer to hold the command line input
13    Declare string pointers for the directory and the stripped input
14    BLANK
15    Save the game's current console output
16    Clear the screen
17    Move the cursor to the top left
18    Display the blinking cursor
19    Get the current drive letter and offset by 'A' (ex. C drive returns 0x02
      but to display C, we have to add 0x40, 'A')
20    Loop while input doesn't match 'rogue' or bit 7 isn't asserted
21    Clear the command line
22    Print a prompt using the current drive letter
23    Get input from the user
24    Strip all spaces
25    Remove the end of line characters from input
26    Redo loop until the boss is gone (type 'rogue')!
27    Input the current drive letter to ensure proper drive is saved on exit
28    Disable the blinking cursor
29    Restore the game's console output
30    BLOCK END - fakedos
31    BLANK
32    COMMENT
33    COMMENT
34    COMMENT

FAKE DOS INTERPRETER

35    Define dodos with one argument
36    Argument 1 is a string representing the user command line input
37    BLOCK START - dodos, process command line input
38    If the first byte is unprintable or the user typed 'rogue'...
39    Return false - this ends the fake DOS prompt and returns to the game
40    If the 2nd char is : and 3rd is NULL, assume we're changing drives so...
41    Subtract away the drive 'letter' to get the base number
42    BLANK
43    Print out a blank line
44    If the input isn't a letter or the drive doesn't exist...
45    Print out an error message
46    Otherwise if there was some user input...
47    Print ou good old BCOFN
48    Return true to keep the fake DOS alive
49    BLOCK END - dodos
50    EOF˙