quit task is int 0x38


yield task is int 0x39
eax is cleared on yield


syscall is int 0x30
system call number in eax
args in ebx, ecx, edx, esi, edi
result in eax
modifies ecx, edx
modifies eax even if no value is returned
see table 1

file system calls have units of bytes unless otherwise specified
functions returning handles or pointers use 0 to indicate error
see keys.txt for the return type of the "get key" system call
the edx register of "start task" is a pointer to a null-terminated string.
  a pointer to a readonly copy of this string is put into the new task's edx.

invalid system call numbers change eax to -1, and have no other effect.


table 1:

 function       | eax | eax out         | ebx            | ecx         | edx         | esi    | edi
----------------|-----|-----------------|----------------|-------------|-------------|--------|-----
 open file      | 0x0 | handle          | drive number   | path        |             |        |
 close file     | 0x1 |                 | handle         |             |             |        |
 file read      | 0x2 | read            | handle         | file offset | count       | buffer |
 get file size  | 0x3 | size            | handle         |             |             |        |
 start task     | 0x4 | handle          | drive number   | path        | passed sz   |        |
 log string     | 0x5 |                 | sz string      |             |             |        |
 get key        | 0x6 | keycode         |                |             |             |        |
 allocate ram   | 0x7 | start pointer   | pages          |             |             |        |
 memory info    | 0x8 | see table 2     | see table 2    |             |             |        |
 wait for task  | 0x9 |                 | handle         |             |             |        |
 enumerate dir  | 0xa | count           | drive number   | path        | see table 3 | max    |
 vga print at   | 0xb |                 | row << 8 | col | sz string   |             |        |
 count of dir   | 0xc | number of files | drive number   | path        |             |        |
 clear screen   | 0xd |                 |                |             |             |        |
 set color      | 0xe |                 | VGA color code |             |             |        |


table 2:
all values are in units of pages
eax -1 indicates unrecognized ebx

 ebx | eax out
-----|--------------------------
 0x0 | kernel dynamic area size
 0x1 | kernel dynamic area left
 0x2 | total userspace size
 0x3 | total userspace left
 0x4 | this process memory left


table 3:
edx of "enumerate dir" is a pointer to a buffer where an array of "directory info"s can be put.
esi is the maximum number of "directory info"s to be placed in the buffer.
the "directory info" structure is 128 bytes long. The first 100 bytes are a null-terminated string
  indicating the name of the file represented by this entry. The next four bytes are a 32-bit integer
  indicating the size of the file represented in bytes. The next byte's lowest bit is a one if this
  entry is a directory, and a zero if this entry is not. The remainder of the structure is reserved.