T-Kernel Specification (1.B0.02)

222 CHAPTER 5. T-KERNEL/SM
5.2 Address Space Management Functions
Memory access privilege is held as access privilege information for each task. Essentially access privilege
information indicates the right to access at the protection level immediately before an extended SVC is
called. If, for example, a task is running at protection level 3 when it calls an extended SVC, its access
privilege information indicates the right to access at protection level 3. Since the protection level when
an extended SVC is executing is protection level 0, in the case of nested calling of an extended SVC
from another extended SVC, access privilege information in the extended SVC for which a nested call
was made indicates the right to access at protection level 0.
Memory access privilege information is set as follows.
• Immediately after a task is started, its access privilege is that designated when the task was
created.
• When an extended SVC is called, the access privilege at the protection level at which it was
running at the time of the call is set.
• Upon return from the extended SVC, the access privilege reverts to that at the time the extended
SVC was called.
• Executing SetTaskSpace() copies the current access privilege of the target task to the invoking
task.
5.2.1 Address Space Configuration
• ER SetTaskSpace( ID tskid )
Assigns to the invoking task the task space and access privilege information of the task designated
in tskid. As a result, both the invoking task and target task have the same task space and access
privilege information.
Note that this copying of task space information applies only at the time the function is called; if
thereafter the task designated in tskid switches to a different address space and its access privilege
changes, the invoking task is not affected by those changes (its address space and access privilege
do not change accordingly). If the invoking task is calling an extended SVC, on return from the
extended SVC its access privilege reverts to that prior to calling the extended SVC. Its task space,
however, does not revert.
The task ID of the invoking task cannot be designated in tskid. However, if TSK SELF is used to
designate the invoking task, access privilege is set to the currently running protection level; task
space is not switched in this case.
E ID
E NOEXS
E OBJ
tskid is invalid
Object does not exist (the task designated in tskid does not exist)
Invoking task designated by other than TSK SELF
5.2.2 Address Space Checking
The following functions check whether access is allowed to the designated memory space, based on the
current access privilege information. If access cannot be made (no privilege or the memory does not
exist), they return error code E MACV.
—R Check for read access privilege.
—RW Check for read and write access privilege.
—RE Check for read and execute access privilege.
Copyright c○ 2002, 2003 by T-Engine Forum
T-Kernel 1.B0.02