6.1.8 ANSI Characters

Windows and Windows applications call the ToAscii function to translate vir-tual-key codes to ANSI characters. The keyboard driver translate these key
codes by checking the state of the virtual-shift keys (such as the VK_SHIFT, VK_CONTROL, and VK_MENU) and mapping the virtual-key code and shift state to the appropriate character. For example, on most U.S. keyboards
if the VK_SHIFT key is up, the VK_2 virtual key code maps to the number 2 (ANSI 50); if VK_SHIFT is down, VK_2 maps to the at sign (@) (ANSI 64).
For foreign-language keyboards, the keyboard also checks for a previously
typed dead key before translating the virtual-key code.

Windows passes the address of the key-state array to the ToAscii function, giving the keyboard driver access to the current state of all virtual keys including the shift keys. The key-state array also contains the shift state of the mouse buttons (that is, the corresponding virtual-key codes VK_LBUTTON, VK_RBUTTON, VK_CANCEL, and VK_MBUTTON).

The keyboard driver must provide translation tables to map virtual keys to ANSI characters. Since the meaning of a virtual key depends on the state of the shift keys, the driver should provide tables for each possible shift-key combination (for example, no shift key, VK_SHIFT, VK_CONTROL, and VK_MENU and VK_CONTROL combined). Windows does not require that all virtual keys map to ANSI characters for all shift-key combinations. Therefore, not all translation tables need to map the complete virtual-key set.

The keyboard type and language also affects the translation of virtual-key codes to ANSI characters. The keyboard driver must adapt its translation tables for a specific-language by loading translation tables from the corresponding language-specific library.