Radio Shack 26-3801 Service Manual 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.