Issue with detecting whether the shift key is pressed and released - c

Question

Im developing a keyboard driver and it works however I'm also
implementing capital letters. Im trying to set a flag for when the shift key
is being pressed or not so the OS will know when to switch to a new scancode.
It can detect if the shift key is being pressed but sadly NOT when the shift
key is released.
Heres my code:
static void keyboard_callback(registers_t *regs) {
uint8_t scancode = port_byte_in(0x60);
// return scancode_to_shift[scancode];
// scancode_to_captial[scancode];
unsigned char c = 0;
if (scancode > SC_MAX) return;
// If shift is pressed.
if (scancode == 0x2A)
{
shift_hold = true;
print_string("Pressed!");
} if (scancode == 0xAA) {
shift_hold = false;
print_string("Worked!");
}
/*
if (scancode == BACKSPACE) {
if (backspace(key_buffer) == true) {
print_backspace();
}
} else if (scancode == ENTER) {
print_string("\n");
execute_command(key_buffer);
key_buffer[0] = '\0';
} else {
if (shift_hold == true)
{
char letter = scancode_to_shift[(int) scancode];
append(key_buffer, letter);
char str[2] = {letter, '\0'};
print_string(str);
}
else if (shift_hold == false)
{
char letter = scancode_to_char[(int) scancode];
append(key_buffer, letter);
char str[2] = {letter, '\0'};
print_string(str);
}
}
*/
}
`
Unless im wrong 0xAA should be the right code for referencing to the right shift key unreleasing.
I tried googling this issue and there is not a lot of information on this sadly..
And other source code for keyboard drivers seem a little too "complex" for me to implement.

Answer 1

Im trying to set a flag for when the shift key is being pressed or not so the OS will know when to switch to a new scancode.
There are 2 shift keys. You need 2 flags (maybe isLeftShiftDown and isRightShiftDown) that are ORed together (like "isAnyShiftDown = isLeftShiftDown | isRightShiftDown") to be able to handle sequences like "left shift pressed, right shift pressed, left shift released" properly.
There is also capslock state; which (for letters and not other keys) makes it more like "isCapital = (isLeftShiftDown | isRightShiftDown) ^ capsLockState". For other keys (e.g. "1" vs. "!") the capslock is ignored.
Then there's "control" and "alt" (and number lock) - e.g. "control+alt+shift+A" is not considered a printable character even though "shift+A" by itself would be.
I tried googling this issue and there is not a lot of information on this sadly..
Try https://wiki.osdev.org/PS/2_Keyboard#Driver_Model .
And other source code for keyboard drivers seem a little too "complex" for me to implement.
Unfortunately (like many things) there's a certain amount of required complexity needed to make it work properly.