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CPU
Zilog
CPU
4.8.3.
Segmented User Program:
A
segmented
user
program
uses M2 and M3 to provide an address space consisting of 124
segments without separating code,
data,
and
stack
areas.
The
high
order
segment line SN
6
on M2 and M3 is grounded
(logic
0)
and M2 and M3 only see segments
0
through 63.
When the segmented user control bit is set high (logic 1) in
the
SCR register, with the CPU in Normal mode, either M2 or
M3 is enabled for
address
translation.
This
depends
on
whether the Z800lA CPU SN
6
line is logic 0 (OFF) or logic 1
(ON).
If SN
6
is logic
0,
MMU M2 is enabled.
If
SN
6
is
logic
1,
M3 is enabled.
So even though MMU M3 is used for
translating USER segments above 63, the Upper
Range
Select
bit (URS) in the MMUs will always be zero.
Segment numbers 0,1,64 and
65
are reserved for the operating
system
since it requires SDR number 0 of M2 and M3.
When a
segmented user program is run under a non-segmented
operat-
ing
system,
external
hardware
prohibits access to system
segments in normal mode.
4.8.4.
MMU Configuration:
The
MMU
configuration
on
the
System
8000
CPU
board
is
organized in part by the board
jumpers El through Ell
in
the
memory
management
control
logic,
and
in
part by the operating system software.
The
hardware jumpers configure the MMU select logic for
a
non-
segmented operating system.
The operating system configures the hardware
System
Confi-
guration
Register
correctly for segmented or non-segmented
user support before running the corresponding type
of
pro-
cess.
NOTE
The E1 through Ell jumpers are
factory
installed
and should not be changed in the field.
4.9.
System Configuration Register
The CPU communicates with the system configuration
register
(SCR)
as
a
programmable 8-bit I/O port (Figure 4-7).
The
lower nibble (00-03) can be
written
for
different
system
functions,
while
the upper nibble (04-07) reads the switch
selectable system console baud rate.
The upper nibble
bits
04
and D5 are used to choose one of four baud rates for the
system console by setting the rocker DIP switches 1 and 4 of
U70.
The selectable baud rates are 300,1200,9600, and 19200
baud (Table 2-9).
The board is shipped from the factory set
for
9600
baud.
It
is
important that terminal baud rates
4-14
Zilog
4-14
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