It always bypasses I/O redirection, which sometimes may not be desirable.Programmers writing applications for IBM PC compatibles can bypass the MS-DOS keyboard functions and choose from two hardware-dependent techniques for keyboard input.
The first method is to call the ROM BIOS keyboard driver using Int 16H. For example, the following sequence reads a single character from the keyboard input buffer and returns it in the AL register:
mov ah,0 ; function 0=read keyboard
int 16h ; transfer to ROM BIOS
Int 16H Function 00H also returns the keyboard scan code in the AH register, allowing the program to detect key codes that are not ordinarily returned by MS-DOS. Other Int 16H services return the keyboard status (that is, whether a character is waiting) or the keyboard shift state (from the ROM BIOS data area 0000:0417H). For a more detailed explanation of ROM BIOS keyboard functions, see Section III of this book, "IBM ROM BIOS and Mouse Functions Reference."
You should consider carefully before building ROM BIOS dependence into an application. Although this technique allows you to bypass any I/O redirection that may be in effect, ways exist to do this without introducing dependence on the ROM BIOS. And there are real disadvantages to calling the ROM BIOS keyboard driver:
It always bypasses I/O redirection, which sometimes may not be desirable.
It is dependent on IBM PC compatibility and does not work correctly, unchanged, on some older machines such as the Hewlett-Packard TouchScreen or the Wang Professional Computer.
It may introduce complicated interactions with TSR utilities.
The other and more hardware-dependent method of keyboard input on an IBM PC is to write a new handler for ROM BIOS Int 09H and service the keyboard controller's interrupts directly. This involves translation of scan codes to ASCII characters and maintenance of the type-ahead buffer. In ordinary PC applications, there is no reason to take over keyboard I/O at this level; therefore, I will not discuss this method further here. If you are curious about the techniques that would be required, the best reference is the listing for the ROM BIOS Int 09H handler in the IBM PC or PC/AT technical reference manual.