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CPU
Zilog
CPU
7.
All CTC channels stop counting and all
channel
inter-
rupt
enable
bits are cleared.
All CTC control regis-
ters must be initialized.
8.
All port mask registers in the Z80B PIO are cleared
to
inhibit all port data bits.
All "ready" handshake sig-
nals are inactive.
Mode
I
of the PIO is selected, port
interrupt
enable
flip-flops
are
reset,
port output
registers are reset, and the vector
address
registers
are not reset.
3.4.2.
Non-maskable
Interrupts:
Non-maskable
interrupts
cannot
be
disabled
(masked) by software and typically are
reserved for external events that require
immediate
atten-
tion.
There are three sources of NMI in the System 8000:
1.
A manual NMI can be generated in the System
8000
with
the
START
pushbutton on the front panel.
This can be
disabled by turning the POWER/LOCK key
switch
on
the
front panel to the LOCK position.
When the START push-
button is pressed immediately after a RESET has occured
(manual or power-up), the power-up diagnostics firmware
is invoked.
At the conclusion of this
diagnostic,
if
no errors have been recorded, the words "POWER UP DIAG-
NOSTICS COMPLETE" appear on the console
and
the
ZEUS
operating
system
is
automatically
booted within the
next 45 seconds.
2.
A power-fail NMI is generated to the CPU if the
System
8000
power
supply
detects an oncoming power failure.
The CPU then provides an orderly shut-down of the
sys-
tem.
3.
An NMI can be generated to the
CPU
if
a
double
bit
non-correctable
ECC
error has been flagged by the ECC
Controller, and if the Clear ECC Error bit(D3)
of
the
SCR
has
been
set
to enable an ECC NMI.
This bit is
initially cleared at the
SCR
on
a
system
reset
or
power-up.
3.4.3.
NMI Identification:
If
an
interrupt
or
trap
is
detected
by the Z8001A CPU, the subsequent initial instruc-
tion fetch is initiated, but aborted.
The next CPU
machine
cycle
is the interrupt acknowledge transaction.
This tran-
saction acknowledges an interrupt or trap and reads a 16-bit
identifier word from the device that generated the interrupt
(in this case an NMI source).
The
identifier
word,
along
3-10
Zilog
3-10
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