Xerox 550 Reference Manual page 136

I/o subchannel (i. e., data transfers between the device
and data buffers, updating the memory byte address and
byte count, and functions as specified by the control flags)
are performed ina norma I manner.
For input operations, the Skip flag (in conjunction with
data chaining) provides the capabi lity to selectively read
portions of a record.
For output operations, the lOP wi" generate and transmit
zeros (X1OOI) unti I the byte count is reduced to zero. Thus,
for example,
if
the lOCO contains a Punch Binary order, a
byte count of 120, and the S flag is coded as a 1, a blank
card may be punched without accessing main memory
for data.
WRITE KEY
This four-bit field (bit positions 40-43), if required, may
be coded with an appropriate write key. During input op-
erations and providing the Skip control flag is coded as aD,
the lOP will access main memory and furnish a memory unit
with up to four bytes of data or information accompanied
with a four-bit write key. If the write key matches the
preassigned write lock for the memory word location ac-
cessed, or if either the key or lock has a value of 0000,
the memory unit accepts and stores the information. If the
write key does not match the write lock, and neither the
key nor the lock has a value of 0000, the memory unit re-
jects the information, does not disturb the previous content,
and transmits a Write Lock Violation (WLV) signal to the
lOP. The write key/write lock relationship is compared
every time a memory word location is accessed for storing
data or information. (Note: The write key/write lock re-
lationship may change during an input operation when the
byte address is incremented (or decremented) across a mem-
ory page boundary.)
As long as the write key matches the write lock for each
memory word location accessed, or the value of either the
lock or the key is 0000, the input operation is performed
as specified by the other parameters within this lOCO; or
the input operation is terminated by an "unusual end" con-
dition which can not be inhibited (i. e., memory address
error, control check fault, or lOP memory error).
If
the HTE control flag is coded as a 1 when a WLV signal
is received, the I/O operation is terminated -immediately.
If
either the ICE or IUE control flag is coded as a
1,
an
I/o interrupt is requested.
If the HTE control flag is coded as a 0 when a WLV signal
is received, the I/O operation continues in a normal man-
ner, even though the data or information may be rejected
by a memory uni t.
When the lOP receives a WLV signal, the WLV bit within
the status information register is set to 1 and remains set
until a new
VO
operation is initiated within this I/o sub-
channel by an SIO instruction. Thus, after the first WLV
signal has been recorded, subsequent WLV signals have no
further effect upon the WLV bit. The status of the WLV
bit is reported when the BP executes an SIO, TIO, TDV,
HIO, or AIO instruction.
The contents of the write key fie Id is not required and may
be ignored when the write key/write lock memory protection
feature is not operative (i. e., during any output operation
or during any input operation, if the Skip control flag of
the current lOCO is coded as a 1).
BYTE COUNT
This 16-bit field (bit positions 48-63), if required, may be
coded to specify the total number of data or information
bytes that are to be transmitted by the current lOCO.
The minimum number of bytes is 1 and the maximum is
65,356 bytes (16,384 words). When the lOCO is fetched,
the content of the byte count field is loaded into a byte
count register within the appropriate I/o subchannel.
Thereafter, the content of the byte count register is decre-
mented by one for each byte transmitted and then tested
for a zero byte count condition. (Note: As a consequence
of decrementi ng before testi ng for a zero byte count condi-
tion' an initial byte count value of 0 is interpreted as
65,356 bytes.) Unless the I/o operation is terminated
(e. g., as the result of detecting an "unusual end
ll
) ,
data
is transmitted unti I the byte count is reduced to zero. At
any time, the progress of the I/O operation may be ascer-
tained by evaluating the current byte count which is
furnished as status information when the BP executes an
SIO, no, HIO, or TDV instruction. (That is, current byte
count is equal to the number of bytes remaining to be trans-
mitted and Inltlai byte count minus current byte count is
equal to the number of bytes transmitted.) When the byte
count is reduced to zero, the MIOP may perform the fol-
lowing functions:
1.
Transmit a "count done" signal to the device controller/
device if data chaining is not specified.
2. Request an I/O interrupt,
if
the IZC flag is coded
as a 1.
3. Fetch the next lOCO, if the data chain flag is coded
asa1.
Depending upon the characteristics of the I/O device,
certain I/O orders (e. g., Rewind for magnetic tape units)
may nat require a byte count field. In such case, the byte
count field is ignored. Refer to an appropriate Xerox pe-
ripheral equipment reference manual for further detai Is.
CONTROL IDeO
A control lOCO may contain either a Transfer in Channel
or a Stop command.
Input/Output Processor (lOP) Fundamentals 129