Added documentation.

doc/shells.md

@@ -1,22 +0,0 @@
-# Shells
-
-Turaco-DOS reserves a portion of main memory for a "shell", a
-program which exists to load and run other programs, and handle
-their return values. This shell might be a command line, or some
-other program that manages other programs, such as a batch file
-interpreter.
-
-The active shell has a "resident" portion of memory, which stays
-in memory as long as the shell is active, and a "transient" portion
-of memory which is only used while the shell is not running an
-external program.
-
-When the shell runs a program, and that program exits, its main
-function returns a two-byte value; this value is stored in a
-specific memory address, and used by the shell as the "status"
-code for the program's exit. Like Unix, a zero value is considered
-a normal exit, with any positive value being some type of error.
-
-Additionaly, when running a program, the shell can provide the
-program with a sequence of arguments, which populate the two
-arguments to the main() function.

doc/syscalls.md

@@ -0,0 +1,10 @@
+# System Calls
+
+The system API is exposed to user code using the COP interrupt.
+The high byte of the C accumulator is loaded with a function number;
+the other registers are loaded with call-specific data (or nothing),
+and any return values are placed in the same registers.
+
+What follows is a list of the system calls, their numbers, and
+register arguments they take, and what if any values they return.
+

include/kernel/file.h

@@ -50,6 +50,10 @@ int file_init(char *pathname, struct file *file);
 
 int file_open(struct file *file);
 
+int file_ioctl(struct file *file,
+               unsigned short operation,
+               void *arg);
+
 // Closes an open file, given a unique ID.
 int file_close(struct file *file);