Analyzing the Assembly from Modern Warfare II campaign (2022)

#videogames #assembly #x86

This was originally posted to the Modern Warfare II subreddit, but it was deleted by the moderators for being off-topic :(

Some of you probably noticed that the missile control computers -- where you got asked to read the first letter of the second row and first column, etc -- had Assembly code shown.

I wanted to actually check if the Assembly made sense and was valid or if it was just garbage (shout-out to /r/ItsAUnixSystem).

Here's my annotated version* of the Assembly shown in the last mission

    # Put the base/frame pointer onto the top of the stack
    PUSHL %EBP

    # Make the base pointer point to the address of the stack pointer
    MOVL %ESP, %EBP

    # Lower the stack pointer by exactly 4 bytes. On x86 the stack "grows" down,
    # so it has to be decremented to make room for additional variables.
    SUBL $0x4, %ESP

    # "Zero out" the memory at address 0x00A9'FF1C
    MOVL $0X0, 0x00A9FF1C

    # Sets up a comparison between: 
    # * the contents of memory address 0x00A9'FF1C
    # * 0x636 (1590 in decimal).
    CMPL $0x636, 0x00A9FF1C

    # In the previous instruction, we compared the two values
    # (the result of that comparison is stored in separate registers)
    # This instruction is saying, if 0x636 is less than the value stored at address 0x00A9'FF1C, 
    # then "jump to" (i.e. set the program counter) address 0x001C'011AC
    JLE 0x001C011AC

    # Finally, jump to memory address 0x001C011B8
    JMP 0x001C011B8

This looks like a valid snippet so far.

And the first example, which is from the "Dark Water" mission:

    PUSHL %EBP
    MOVL %ESP, %EBP

    # It's kind of odd that this is here, since it's basically a useless instruction.
    # Unless it was done intentionally to show that there's no local variables going to be used.
    SUBL $0x0, %ESP
    MOVL $0x8, 0x00A9FF1C
    CMPL $0x65, 0x00A9FF1C
    JLE 0x001C011AC
    JMP 0x001C011B8

This is very similar to the other snippet -- using the same instructions, just some different values. Seems to mostly check out. Good job Infinity Ward.

DISCLAIMER: I'm not an expert in x86 assembly.

Top comments (2)

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