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

PROTECT.C provides copy protection services for the disk drive

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

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



1     COMMENT
2     COMMENT
3     COMMENT
4     BLANK
5     Includes the software interrupt header
6     BLANK
7     Import goodchk as defined in EXTERN.C
8     Declare a global integer to count steps
9     BLANK
10    Define macro for undefined values
11    Define macro for dont-care values
12    BLANK
13    Define a CRC value of 0x10
14    BLANK
15    Define a preconfigured register setup for reading rom from disk
16    AX = 0x206 (Read 6 sectors from disk...)
17    BX register 0 means no offset at destination
18    CX = 0x2701 (Read from sector 1, track 39)
19    DX is set before invoking 
20    SI don't care
21    DI don't care
22    DS don't care
23    ES is where we read to (ROM at 0xF800)
24    End of rom reading register setup
25    BLANK
26    Define a preconfigured register setup for reading the disk signature
27    AX = 0x201 (Read 1 sector from disk)
28    BX doesn't matter
29    CX = 0x2707 (Read sector 7 from track 39)
30    DX is set later
31    SI doesn't matter
32    DI doesn't matter
33    DS doesn't matter
34    ES doesn't matter
35    End signature read register setup
36    BLANK
37    Define a preconfigured register setup for checking signature
38    AX = 0x201 (Read 1 sector from disk)
39    BX doesn't matter
40    CX reads from sector 60 of the hard drive...Was this install specific?
41    DX...
42    SI...
43    DI...
44    DS...
45    ES don't matter
46    End set up for signature check
47    BLANK

CHECK DRIVE COPY PROTECTION

48    Define protect with one argument - the drive letter as integer
49    BLOCK START - protect, confirms copy protection
50    Declare an iterator and flags
51    Declare a register struct
52    Declare a sector-sized buffer
53    Declare a smaller buffer
54    BLANK
55    Increment the step counter
56    Pull the drive letter (casted to int) in to DX of all registers
57    Set the read target for the signatures to the data segment
58    Sig1 memory offset should be equal to the small buffer
59    Sig2 memory offset should be equal to the sector sized buffer
60    BLANK
61    Loop up to 7 times on error. (If flags&CF is true, it's an error)
62    BLOCK START - Read from ROM
63    Use the rom read register set
64    Reset the step counter
65    Invoke interrupt to read ROM and set flags
66    Increment step counter (it will reset if this failed)
67    BLOCK END - Read from ROM
68    If there was an error...
69    BLOCK START - ROM read error check
70    Reset the step counter
71    Return failure
72    BLOCK END - ROM read error check
73    ROM read successful, now loop up to 3 times to read signature
74    BLOCK START - Read signature 1
75    Set up registers for siganture 1
76    Reset step counter
77    Invoke read interrupt
78    Increment step counter
79    BLOCK END - Read signature 1
80    If there was an error...
81    BLOCK START - Signature read error
82    Reset step counter
83    Return failure
84    BLOCK END - Signature read error
85    Loop four times to compare signature integrity
86    BLOCK START - Verify signature
87    Set registers to read signature 2
88    Reset step counter
89    Invoke interrupt and get flags result
90    Increment step counter
91    Check if read was successful and the expected checksum is in AX
92    BLOCK START - success check
93    If the first 32 bytes of the buffers match...
94    We're good to go
95    Reset step counter
96    Return success
97    BLOCK END - success check
98    BLOCK END - Verify signature
99    Reset step counter
100   BLOCK END - protect
101   EOF˙