IBM Personal System/2 50 - Arbitration Bus Priority Assignments

Arbitration
Bus
Priority
Assignments
The
following
figure
shows
the
assignment
of
arbitration
levels.
The
functions
with
the
lowest
arbitration
level
have
the
highest
priority.
ARB
Level
Primary
Assignment
-2
Memory
Refresh
-1
NMI
0
DMA
Channel
0
(Programmable
to
any
arbitration
level)
1
DMA Channel 1
2 DMA
Channel 2
3
DMA
Channel 3
4
DMA
Channel
4
(Programmable
to
any
arbitration
level)
5
DMA
Channel 5
6
DMA Channel
6
7
DMA Channel
7
8-E
Available
F
System
Microprocessor
Figure
3-1.
Arbitration
Bus
Priority
Assignments
Note:
Devices
designed
for
arbitration
level
0
or
1
should
have
limited
bandwidth
or
short
bursts
so
diskette
overruns
can
be
prevented
or
recovered
by
retry
operations.
The
diskette
drive
controller,
on
arbitration
level
2,
can
be
held
inactive
by
devices
on
levels
0
and
1,
by
a
refresh
operation,
and
by
the
previous
controlling
master.
The
diskette
drive
controller
should
not
be
held
inactive
for
more
than
12
microseconds
to
prevent
overrun.
NMI
service
is
executed
at
a
priority
level
higher
than
0,
called
-1,
Memory
refresh
is
prioritized
at
-2,
two
levels
higher
than
0.
Levels
-1
and
-2
are
reached
on
the
system
board
only,
while
the
‘arbitrate/-grant'
signal
(ARB/-GNT))
is
in
the
arbitrate
state.
When
the
centrai
arbitration
control
point
receives
a
level
-1
request
(NMI,
a
system-board
internal
signal),
it
activates
-PREEMPT,
waits
for
the
end
of
transfer,
and
then
places
ARB/-GNT
in
the
arbitrate
state,
which
denies
channel
activity
to
arbitrating
devices.
The
central!
arbitration
control
point
gives
the
grant
to
the
level
-1
request,
and
holds
ARB/-GNT
in
the
arbitrate
state
until
the
operation
is
complete
and
the
NMI
is
reset.
3-4
Model
50
System
Board
—
October
1990