		
        .text:004027D8  call  ds:GetVersion
        .text:004027DE  mov   [esp+5A0h+pcbBuffer], eax
        .text:004027E2  and   eax, 0FFFFh
        .text:004027E7  cmp   eax, 6
        .text:004027EA  jg    short loc_402817
        .text:004027EC  jz    short loc_40280C
        .text:004027EE  sub   eax, 4
        .text:004027F1  jz    short loc_402801
        .text:004027F3  dec   eax
        .text:004027F4  jnz   short loc_402825
        .text:004027F6  lea   edx, [esp+5A0h+Buffer]
        .text:004027FA  mov   edi, offset aWin2000 ; "Win2000"
        .text:004027FF  jmp   short loc_402844

This code snippet demonstrates how to potentially identify the Windows operating system version using the GetVersion() function from the Win32 API.
Click on the function name anywhere to open it's official API page.

It would be helpful if you have some basic familiarity with - x86, Endianness, Registers, Instructions, Local variables, Datatypes, Handles, Stack, Heap & Offset.
Let's Start!

Here's a breakdown of each instruction:

.text:004027D8 call ds:GetVersion

This instruction calls a function named GetVersion. The `ds:` prefix might indicate the function is located in the data segment, but it's more likely that `GetVersion` is an imported function from the operating system's API (Application Programming Interface). The purpose of this call is to retrieve information about the operating system version. The `call` instruction executes the `GetVersion` function, and by convention in many architectures, function calls typically return values in the eax register. So, after the function finishes executing, the return value containing the version information is placed in eax for further processing within this code block.

.text:004027DE mov [esp+5A0h+pcbBuffer], eax

This instruction moves the value from the eax register (which now holds the return value from `GetVersion`) to a memory location on the stack.
- esp: This refers to the stack pointer, a register that always points to the top of the stack. The stack is a Last-In-First-Out (LIFO) data structure used to store temporary data during function calls.
- `5A0h`: This is a hexadecimal value (1440 in decimal). It represents an offset relative to the esp value.
- `pcbBuffer`: This is likely a local variable name that defines an offset within the current stack frame. A stack frame is the memory space allocated for a function's execution. It typically holds the function's arguments, local variables, and saved registers.
- `[esp+5A0h+pcbBuffer]`: This combines the base address (esp), the offset (`5A0h`), and the variable offset (`pcbBuffer`) to calculate the final memory location where the value from eax will be stored.
The retrieved version information (potentially some integer value) is stored on the stack. This is likely done because the current function might need to use this value later in its execution. The stack provides temporary storage for function calls, and variables within a function's stack frame and are typically accessed relative to the stack pointer (esp).

.text:004027E2 and eax, 0FFFFh

This instruction performs a bitwise AND operation between the value in the eax register and the constant `0FFFFh` (hexadecimal for 65535 in decimal).
- Each bit in eax is compared with the corresponding bit in `0FFFFh`.
- If both bits are 1, the result bit is 1. Otherwise, the result bit is 0.
- Crucially, since the higher 16 bits of `0FFFFh` are all 0s, they will not affect the lower 16 bits of eax because AND with 0 always results in 0.
Example:
Let's say the value in eax is initially `0000C3BAh` (hexadecimal). This represents a 32-bit binary value where the higher 16 bits might contain irrelevant data.
- eax (before AND): 0000 C3BA
  | |
  higher 16 bits lower 16 bits
- `0FFFFh` (constant): 0000 FFFF (hexadecimal) - This has all bits set to 1 in the lower 16 bits.
Performing the AND operation:
- - Each bit in eax is ANDed with the corresponding bit in `0FFFFh`.
- - If the corresponding bits in both eax and `0FFFFh` are 1, the result bit is 1. Otherwise, the result bit is 0.
- - Since the higher 16 bits of `0FFFFh` are all 0s, ANDing them with any value in eax will always result in 0, effectively leaving the lower 16 bits of eax unchanged.
The `GetVersion` function might return a larger value than what's relevant for identifying the operating system version. This AND operation isolates the lower 16 bits of the value in eax and sets all higher bits to zero.
Operating system versions are typically represented by smaller integer values, so the assumption is that the lower 16 bits contain the necessary information. This filtering ensures the following comparisons work correctly.

.text:004027E7 cmp eax, 6

If the filtered value in eax is greater than `6`, it likely indicates a version higher than Windows Vista (version 6). The following jump instructions will determine the specific course of action based on the comparison result.
Click here to see OS Version Details

This comparison is the first step in identifying the operating system version based on the filtered value in eax. Since the higher 16 bits are cleared, the lower 16 bits now represent a smaller integer that might correspond to the OS version. Here's how the code proceeds based on the comparison:

This instruction compares the value in eax with the constant `6`.
- If eax (after AND) is greater than `6`, the code likely jumps to the instruction labeled `loc_402817` using the `jg` (jump if greater) instruction in the next line. This suggests the code has different logic to handle operating system versions higher than Windows Vista.
- If eax (after AND) is equal to `6`, the code likely jumps to the instruction labeled `loc_40280C` using the `jz` (jump if zero) instruction two lines below. This indicates the OS version is likely Windows Vista.
- If eax (after AND) is less than `6`, the code continues to the next instruction, which is a subtraction from eax. This suggests the code might be trying to identify older Windows versions (e.g., 9x) based on the remaining value in eax.

.text:004027EA jg short loc_402817

This instruction performs a jump if greater (jg) instruction. Here's why it's used:
- `jg` stands for "jump if greater."
- `short loc_402817` specifies the target location (instruction labeled `loc_402817`) for the jump, indicating a short jump within the same code block.
As explained earlier, if the value in eax (after AND) is greater than `6`, the program jumps to the instruction labeled `loc_402817`. This suggests the code has different logic to handle operating system versions higher than Windows Vista. The specific logic for those versions would be implemented at the jump target (`loc_402817`).

.text:004027EC jz short loc_40280C

This instruction performs a jump if zero (jz) instruction. Here's why it's used:
- `jz` stands for "jump if zero."
If the value in eax (after the AND operation) is exactly equal to `6`, the program jumps to the instruction labeled `loc_40280C`. This indicates the operating system version is likely Windows Vista (version 6). The code for handling Windows Vista might be located at that jump target (`loc_40280C`).

.text:004027EE sub eax, 4

This instruction subtracts the value `4` from the eax register.
- We've identified that the initial comparisons handled versions higher than or equal to Windows Vista (version 6). This code path now deals with older Windows versions, likely Windows 9x.
- Subtracting `4` from eax might be a way to manipulate the lower bits of the filtered value obtained after the AND operation to identify the specific version.
  
  Here's a breakdown of the logic behind the subtraction:
- Example (Hypothetical):
  
  Imagine (this is not guaranteed behavior) that the original value returned by `GetVersion` for a specific Windows 95 version has the following encoding in its lower 16 bits after the AND operation:

.text:004027F1 jz short loc_402801

This instruction performs a "jump if zero" operation. It checks the value in the eax register after the subtraction in the previous instruction. If the value in eax is indeed zero at this point, the code jumps to the instruction labeled `loc_402801`. Here's why this jump might be important:
- This jump depends on the outcome of the subtraction. If a specific pattern of bits resulted in zero after the subtraction (depending on the encoding scheme), the code might jump to handle a particular Windows 9x version (potentially) at `loc_402801`.

.text:004027F3 dec eax

This instruction simply subtracts 1 (decrements) from the value in the eax register.
- Since the previous jump (`jz`) might not have been triggered, this decrementing could be part of a loop that manipulates eax further. The goal might be to isolate relevant bits in eax for subsequent comparisons used to identify specific Windows 9x versions.

.text:004027F4 jnz short loc_402825

This instruction performs a "jump if not zero" operation.
- `jnz` stands for "jump if not zero."
- `short loc_402825` specifies the target location (instruction labeled `loc_402825`) for the jump, indicating a short jump within the same code block.
If the value in eax after the decrement is not zero, the program jumps to the instruction labeled `loc_402825`. This suggests that further logic for identifying specific Windows 9x versions might be located at that jump target, depending on the value remaining in eax.

In-depth Explanation (MOV vs. LEA)

MOV (Move): Imagine MOV as a copy-paste operation. It directly copies a value from one location (source) to another location (destination register). This value can be a number, a character, or even the address of another memory location.
- Think of it this way: You have a book (source) and copy a specific page (value) into your notes (destination register). MOV simply takes the information and puts it in the new location.
LEA (Load Effective Address): Think of LEA as a "find the address" instruction. It calculates the memory address based on a starting point (like the stack pointer) and offsets, but it doesn't actually bring the data from that address. It simply stores the calculated address itself in a register.

.text:004027F6 lea edx, [esp+5A0h+Buffer]

This instruction uses the LEA (Load Effective Address) operation. LEA calculates the memory address based on a base address and one or more offsets, but it doesn't load the value from that address into a register.
- edx: This is the destination register where the calculated address will be stored.
- This instruction calculates the address of the memory location on the stack where the filtered version information (pcbBuffer) is stored. The address is then stored in the edx register. This address will likely be used in subsequent instructions for comparisons or function calls to determine the specific OS version.

.text:004027FA mov edi, offset aWin2000 ; "Win2000"

This instruction uses the MOV (Move) operation.
- MOV Operation:
  - mov: This is the instruction that performs the move operation.
  - edi: This is the destination register where the value will be copied.
  - offset aWin2000: This specifies the source of the value.
    - offset: This keyword indicates that the instruction refers to the memory address of a label or variable, not the data itself.
    - aWin2000: This is likely a label (instruction name) that precedes an actual string value "Win2000" stored in memory. So, the offset points to the memory location where "Win2000" is stored.
- Comment: ; "Win2000": This is a comment that clarifies the purpose of the instruction, indicating that the offset points to the string "Win2000". However, the comment itself is not part of the machine code instruction.
- This instruction copies the memory address of the string "Win2000" (pointed to by the label aWin2000) into the edi register. This suggests the code might be preparing to compare the information retrieved using LEA in the previous instruction (likely version-related) with "Win2000" to see if it matches Windows 2000.

.text:004027FF jmp short loc_402844

This instruction performs an unconditional jump (JMP) operation.
- Jump Operation:
  - jmp: This is the instruction that performs the jump.
  - short: This keyword indicates a short jump within the same code block (usually limited to a certain instruction range).
  - loc_402844: This is the label (instruction name) of the target location where the program will continue execution.
- This jump likely signifies that none of the previous comparisons identified a specific Windows 9x version (based on the limited information we see here). The code jumps to a different location (loc_402844) to handle this scenario. The logic at that jump target might involve handling an unknown version or performing further checks.

Conclusion

By understanding how to use the GetVersion() function and interpret assembly instructions, you can gain valuable insights into how programs interact with the operating system.

x86 Assembly Simplified using GetVersion() win32 api call

Conclusion

THE END