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CPU
Zilog
CPU
signal is also generated to protect the system data and sys-
tem stack areas from being accessed by the user program.
In system mode, if the segment number of a user
segment
is
generated
(segments
2
through
63
or 66 through 127) the
address translation is the same as in the normal mode.
The
separation
of
code, data, and stack spaces is deactivated,
MMU M2 is enabled for segments 2 through 63, and MMU
M3
is
enabled
for
segments
66
through
127.
This
allows the
operating system to access any user segment directly.
3.5.2.3
System Access to User Space:
To access a user
seg-
ment,
the
operating system can use a free segment slot and
set up its SDR to point to the same memory area as the
tar-
get
user
segments
SDR.
A
non-segmented operating system
running a non-segmented process can directly access the user
data
and
stack areas by going to system mode and using the
user segment number.
To access the user code
segment,
one
of
the
unused segment slots (e.g.62)
is set up to point to
the code segment.
The SDRs for this slot in M2 and
M3
are
both
set
up
to point to the code segment so that the con-
tents of the NBR do not matter.
3-18
Zilog
3-18
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