The DOS3Call API function in Windows 3.x must be called from assembly language. It is typically used to perform file I/O. In Win32, you should replace assembly language code that calls DOS3Call with the appropriate Win32 file I/O calls. Other (non-file) INT 21H functions should be replaced with the portable Windows API call as shown in the following table.
| INT 21H subfunction | MS-DOS operation | Win32 API equivalent |
| 0EH | Select Disk | SetCurrentDirectory |
| 19H | Get Current Disk | GetCurrentDirectory |
| 2AH | Get Date | GetDateAndTime |
| 2BH | Set Date | SetDateAndTime |
| 2CH | Get Time | GetDateAndTime |
| 2DH | Set Time | SetDateAndTime |
| 36H | Get Disk Free Space | GetDiskFreeSpace |
| 39H | Create Directory | CreateDirectory |
| 3AH | Remove Directory | RemoveDirectory |
| 3BH | Set Current Directory | SetCurrentDirectory |
| 3CH | Create Handle | CreateFile |
| 3DH | Open Handle | CreateFile |
| 3EH | Close Handle | CloseHandle |
| 3FH | Read Handle | ReadFile |
| 40H | Write Handle | WriteFile |
| 41H | Delete File | DeleteFile |
| 42H | Move File Pointer | SetFilePointer |
| 43H | Get File Attributes | GetAttributesFile |
| 43H | Set File Attributes | SetAttributesFile |
| 47H | Get Current Directory | GetCurrentDirectory |
| 4EH | Find First File | FindFirstFile |
| 4FH | Find Next File | FindNextFile |
| 56H | Change Directory Entry | MoveFile |
| 57H | Get Date/Time of File | GetDateAndTimeFile |
| 57H | Set Date/Time of File | SetDataAndTimeFile |
| 59H | Get Extended Error | GetLastError |
| 5AH | Create Unique File | GetTempFileName |
| 5BH | Create New File | CreateFile |
| 5CH | Lock | LockFile |
| 5CH | Unlock | UnlockFile |
| 67H | Set Handle Count | SetHandleCount |