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HRM
Zilog
HRM.
A segmented user program
can
run
in
this
configuration,
although
the
Nonsegmented
as,
Segmented USER configuration
is intended for that purpose.
Such a
user
program
has
a
potential address space of 62 code and data segments.
Nonsegmented
as,
Segmented USER
This configuration is intended to run exactly as the
previ-
ous
configuration,
provided that the CPU is in system mode
and the operating system is running in memory segment
o.
In
addition,
code,
data, and stack references are directed to
M1, M2, and M3 respectively, and the contents of the SBR are
used
to select between data and stack references.
However,
if the CPU is in normal mode, MMU M2 is enabled for
segment
numbers
2
to 63, and MMU M3 is enabled for segment numbers
65 to 127.
If a memory reference is made,
a
segment
trap
violation
is
generated
and
the
three MMUs are disabled.
Also, the suppress signal is generated by the CPU to protect
the
system
data and stack areas from being accessed by the
user program.
In system mode, if the segment
number
of
a
user
segment is generated (segments 1 to 63, or 66 to 127),
the address translation is
the
same
as
in
normal
mode.
Separation
of code, data, and stack spaces are deactivated;
MMU M2 is enabled for segments
2
to
63;
and
MMU
M3
is
enabled
for
segments 66 to 127.
This allows the operating
system to directly access any
~ser
segment.
4.9.5.
System Access to User Space:
To access a user
seg-
ment,
the
operating system can use a free segment slot and
set its Segment Descriptor Register to
point
at
the
same
memory area as the target user segment's SDR.
A nonsegmented operating system running a non segmented
user
program can directly access the user' data and stack areas by
switching to system mode and using the user segment
number.
To
access
the user code segment, one- of the unused segment
slots is set to point at
the
code
segment;
for
example,
number
62.
The SDRs for this segment slot in M2 and M3 are
both set to point at the code segment, negating the contents
of the NBR.
A nonsegmented operating system
running
a
segmented
user
program can directly access any prot ion of the user space by
switching directly into segmented mode.
4-69
Zilog
4-69
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