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

LOAD.C loads title screen graphics by directly manipulating video memory

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

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



1     COMMENT
2     COMMENT
3     COMMENT
4     COMMENT
5     COMMENT
6     BLANK
7     Include the game header file
8     BLANK
9     Defines macro for the hardware mode address in BIOS memory BIOS:0x65
10    Defines macro for the address of the 6845 Mode control register 
11    Defines macro mask for the B&W enable bit in the mode register
12    Defines macro for the address of the color selection register
13    Defines macro for the background intensity bit of the color register
14    Defines macro...not used
15    BLANK
16    Declare a pointer to a byte
17    Initialize variables for the video block size and a file descriptor
18    BLANK

SHOW THE TITLE PICTURE

19    Defines epyx_yuck with no arguments
20    BLOCK START - epyx_yuck, show the epyx title logo
21    Import the tick counter from DOS.ASM
22    Initialize an integer for the current video mode (text)
23    Import sbrk to allocate memory later
24    BLANK
25    If we're in Monochrome mode or there is no graphics file...
26    Return to caller - nothing to do
27    Allocate memory for graphics but if we fail..
28    Keep trying to allocate with progressively smaller blocks until success
29    Set video mode to 320x200, 4 color CGA
30    Load and display the graphics
31    Reset the tick counter
32    If we're logging (we're not)
33    Loop forever (dead code)
34    Repeat loop
35    Otherwise, if no logfile
36    Loop while there's no use input
37    Repeat loop
38    If there is input...
39    Read the input
40    End check for logfile
41    Reset video mode back to the original text mode
42    Undo memory allocation for graphics
43    Reset tick counter to 0
44    BLOCK END - epyx_yuck
45    BLANK

INITIALIZES SCREEN FOR GRAPHICS

46    Defines src_load with no arguments
47    BLOCK START - scr_load, initializes the screen for graphics display
48    Declare integers for the palette and background settings
49    Declare integers for the video mode and the color bit
50    BLANK
51    Load the graphics file in to video memory
52    BLANK
53    Read the palette settings from byte 8012 of the video now in video mem
54    Read the background settings from byte 8013 (these are custom formats)
55    BLANK
56    If the palette setting is above three...
57    Then we can enable high intensity
58    Initialize burst (bw mode) to 0
59    SWITCH on the palette settings
60    BLOCK START - Switch on palette
61    CASE palette setting is 2 (fallthrough)
62    CASE palette setting is 5
63    Set the burst flag to 1 (bw mode)
64    CASE palette setting is 0 (fallthrough)
65    CASE palette setting is 3
66    Palette setting is 1
67    End of check for palette setting
68    CASE palette is 1 (fallthrough)
69    CASE palette is 4
70    Set palette to 0
71    End check for palette setting
72    BLOCK END - Switch on palette
73    BLANK
74    Output the background to 0x3d9 -- first 5 bits matter
75    Read the current mode without the BW flag (using file burst setting)
76    If we read in the burst bit...
77    Assert the black and white mode
78    Then send the updated mode back to the BIOS parameter block
79    Output the new mode to the video hardware at 0x3d8
80    BLOCK END - scr_load
81    BLANK

LOADS GRAPHICS IN TO VIDEO MEMORY

82    Define bload with one argument
83    Argument one is the segment number we'll offset from for memory access
84    BLOCK START - bload, loads the binary data in to video memory
85    Initializes the offset to 0 and declares a read counter
86    BLANK
87    Read 7 bytes from the file...assume good
88    Seek past this amount
89    Loop while we continue to read..
90    Output the bytes read in to the video memory
91    Increment the offset and check it's bounds (should never get this far)
92    Break if we've read more than video memory allows
93    Repeat loop while we have more to read
94    BLOCK END - bload
95    BLANK

GETS THE CURRENT DRIVE LETTER

96    Define find_drive with no arguments
97    BLOCK START - find_drive, finds the drive using a DOS software interrupt
98    Initializes an integer holding the current drive number from offset 0x19
      of the BIOS parameter block
99    Gets the save game drive letter
100   Allocates space for a file name
101   BLANK
102   If the drive letter is a letter (is valid)...
103   BLOCK START - Set drive number
104   If the environment variable drive letter is capitalized...
105   The drive number is offset from capital A
106   Otherwise the drive letter is lower case...
107   Offset from lower case a
108   BLOCK END - Set drive number
109   Set the file name
110   Overwrite the file name with the real drive letter
111   Check file accessibility?? (This function isn't included in the source)
112   Return the drive letter
113   BLOCK END - find_drive
114   EOF˙