keycodes are 32-bit integers.

the low byte indicates the key itself. for printable characters (and keys with
reasonable translations to ascii control codes), this is the ascii code. for
other ones, something in the range of 0x80 to 0xff is used, seen in table 1.

the top 24 bits indicate several flags. these are seen in table 2, where bit 0
is the lowest bit of the second lowest byte of the keycode, and bit 23 is the
highest bit of the highest byte of the keycode.

the "get key" system call returns 0 if there is not a key available. it is
recommended to make the system call and, if it returns 0, yield to the scheduler
and then loop back to making the system call. this way your task does not block
the cpu while waiting for keyboard input. see the "get_key_char" function in
user.c of the "knob" library for an example of this.


table 1:

 code | key
------|-----
 0x80 |
 .... |
 0xff |

table 2:

bit  0: left shift
bit  1: right shift
bit  2: caps lock
bit  3: insert
bit  4: num lock
bit  5: scroll lock
bit  6: left alt
bit  7: right alt
bit  8: left control
bit  9: right control
bit 10: left meta
bit 11: right meta

bits 12-23 are reserved for future versions. in this version, they should be set
to zero when giving a keycode, and should be ignored when recieving a keycode.