Documentation of kernel API

doc/syscalls.md

@@ -435,3 +435,18 @@ the current directory in the drive with the volume name specified in the
 `drive` argument, and puts that string into the buffer pointed to by
 `dest`. On success, `C` will contain `STATUS_OK`. On failure, `C` will
 containe a negative value indicating the error type.
+
+ ## `0x18`: Set Current Directory
+
+ Arguments:
+ - `C`: `0x18`
+ - `X`: Bank of string
+ - `Y`: Address of string
+
+Return Values:
+ - `C`: Status code
+
+This call attempts to change the current directory on a the drive specified
+in the supplied `NULL`-terminated string to the directory listed in that
+string. On success, `C` will contain `STATUS_OK`. On failure, `C` will
+containe a negative value indicating the error type.