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FUNCTIONAL DESCRIPTION
2.3.6 HOLD and HALT States
The 8085A uses the T
HOLD
state
to
momentarily
cease executing machine cycles, allowing
ex-
ternal devices
to
gain control
of
the bus and
peform DMA cycles. The processor internally
latches the state
of
the HOLD line and the un-
masked interrupts during
ClK
=
"1"
of
every
T
HALT
state. If the internal latched HOLD signal
is high during
ClK
=
"1"
of any T
HALT
state, the
CPU
will
exit
T
HALT
and enter T
HOLD
on the
following
ClK
=
"1".
As shown in Figure
2-21
this
will
occur even
if
a valid interrupt occurs
simultaneously with the
HOLD signal.
The state
of
the HOLD and the unmasked inter-
rupt lines is latched internally during
ClK
= 1
of each T
HOLD
state as well as during T
HALT
states. If the internal latched HOLD signal is
low during
ClK
=
1,
the
CPU
will exit T
HOLD
and
enter T
HALT
on the following
ClK
=
1.
SIGNALS
To
THOLD
CT.>*
THOLDCTsI*
ClK
u-
'----1
ALE
HOLD
HLDA
~
INTERRUPT
The 8085A accepts the
first
unmasked, enabled
interrupt
sampled; thereafter, all
interrupt
sampling is inhibited. The interrupt thus ac-
cepted will inevitably be executed when the
CPU
exits the HOLD state, even at the expense
of
holding
off
higher-priority
interrupts
(including
TRAP).
(See
Figure
2-22.)
When the
CPU
is not
in
T
HALT
or T
HOLD,
it
inter-
nally latches the
HOLD line only during
ClK
=
1
of
the last state before T 3 (T 2 or T
WAIT)
and dur-
ing
ClK
=
·1
of
the last state before T 5 (T 4
of
a
six T-state
M1). If the internal latched HOLD
signal is high during the next
ClK
=
1,
the
CPU
will enter T
HOLD
after the following clock. When
the
CPU
is not in T
HALT
or
THOLD,
it
will internally
latch the state
of
the I-Inmasked interupts only
during
ClK
of
the next
to
the last state before
each
M1
• T
1
.
THOLD
CTa)*
T
HOLD
THOLD
T,
~
~
L-.r-
L-1
-
II
L
\\
\\
I INTERRUPTS SAMPLED
1 HERE REGARDLESS
1
__
OF HOLD
1
1\
1
--It'H-
1
--t,s-!
START
OF
INTERRUPT
___
CYCLE DELAYED
*SIGNIFIES
THAT
T.·T
a
MAY
TAKE PLACE INSIDE THE 8085A EVEN WHILE
THE
PROCESSOR
IS
IN A HOLD STATE. BY
HOLD
FIGURE
2·21
HOLD
VS
INTERRUPT - NON HALT
2-17