Radio Shack 26-3801 - Page 34

3o
Symbol
So
,
Si
,
and
IO/M
(Output)
Function
Machine
cycle
status:
IO/M
S, So
Status
1
1
1
1
1
Memory
write
Memory
read
1
I/O
write
1
I/O
read
1
1
Opcode
fetch
1
1
Interrupt
Acknowledge
* Halt
*
X
X
Hold
*
X
X
Reset
*
=
3-state
(high
impedance)
X
=
unspecified
Si
can be
used as
an
advanced
R/W status.
IO/M,
So
and.
Si
become
valid at
the
beginning
of a
machine
cycle and
remain
stable
throughout
the
cycle.
The
falling
edge
of ALE
may
be
used
to
latch
the
state
of
these
lines.
RD
(Output,
3-state)
READ
control:
A
low
level on RD
indicates
the
selected
memory
or I/O
device is
to
be
read and
that the
Data
Bus
is
available
for
the
data
transfer,
3-stated
during
Hold
and
Halt
modes
and
during
RESET.
WR
(Output,
3-state)
WRITE
control:
A
low
level on
WR
indicates
the
data
on
the
Data
Bus is
to
be
written
into
the
selected
memory
or I/O
loca-
tion.
Data
is
set
up
at
the
trailing
edge
of
WR.
3-stated
during
Hold
and
Halt
modes
and
during
RESET.
READY
(Input)
If
READY
is
high
during
a
read or
write
cycle,
it
indicates
that
the
memory or
peripheral
is
ready
to
send
or
receive
data.
If
READY is
low,
the
cpu will
wait
an
integral
number of
clock
cycles for
READY
to go
high
before
completing
the
read or
write
cycle.