T-Kernel Specification (1.B0.02)

2.7. MEMORY 15
which the upper and lower distinction is to be made. In principle, however, the system attribute
portion is assigned from the least significant bit (LSB) toward the most significant bit (MSB), and
implementation-dependent attributes from the MSB toward the LSB. Bits not defining any attribute
must be cleared to 0.
In some cases an object may contain extended information. Extended information is designated when
the object is registered. Information passed in parameters when an object starts execution has no effect
on T-Kernel behavior. Extended information can be read by calling an object status reference system
call.
2.7 Memory
2.7.1 Address Space
Memory addressable space is distinguished as system space (shared space) or task space (user space).
System space can be accessed equally by all tasks, whereas a task space is accessible only by the tasks
belonging to it (see Figure 2.8). Multiple tasks may in some cases belong to the sametask space.
The logical address space of task space and system space depends on the CPU (and MMU) limitations
and is therefore implementation-dependent, but in principle task space should be assigned to low
addresses and system space to high addresses.
Logical addresses (examples)
0x00000000
0x3fffffff
Task space
#1
Task space
#2
. . . Task space
#n
0x40000000
0x7fffffff
System space
Figure 2.8: Address Space
Since interrupt handlers and other task-independent software are not tasks, they do not have a task
space of their own. Instead, while in a task-independent portion they belong to the task executing
just before entering the task-independent portion. This is the same as the task space of the currently
running task returned by tk get tid. When there is no task in RUN state, task space is undefined.
T-Kernel does not create or manage address space. Normally T-Kernel is used along with a subsystem
for handling address space management and the like.
In a system with no MMU (or not using an MMU), essentially task space does not exist.
2.7.2 Nonresident Memory
Memory may be resident or nonresident.
When nonresident memory is accessed, data is copied to that memory from a disk or other storage. It
therefore requires complicated processing such as disk access by a device driver.
Accordingly, when nonresident memory is accessed, the device driver, etc., must be in operational state.
Access is not possible during dispatch disabled or interrupts disabled state, or while a task-independent
portion is executing.
Similarly, in OS internal processing, it is necessary to avoid accessing nonresident memory in a critical
section. One such case would be when the memory address passed in a system call parameter points
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T-Kernel 1.B0.02