IBM Professional Series User Manual page 362

DS OD
Write Control Character (WCC) and including 3270 orders
following the WCC. Data may be written anywhere on the
screen. The virtual machine must provide the address where
the write is to begin; it uses a SET BUFFER ADDRESS
(SBA) order to do this. Writing can also start at the current
cursor address.
CCW code X'2A' performs a READ BUFFER or a READ
MODIFIED operation, depending on the value of the control
field.
To specify the full screen mode CCW, use the following
assembler language instructions:
DC AL1(CCWCODE),AL3(DATADDR),AL1(FLAGS),AL1(CONTROL),AL2(COUNT)
Where:
CCWCODE
is a CCW code (X'29' or X'2A')
DATADDR
for a write operation, specifies the first byte of the data
stream (WCC) to be written. For a read operation,
specifies the address of the read buffer.
FLAGS
is the standard CCW flag field.
CONTROL
for a write operation, an ERASE/WRITE is performed
by specifying a ccwcode of X'29', if the high-order bit
(X'80') is on.
If
the high-order bit (X'80') is on for a read operation, a
READ MODIFIED is performed; otherwise, a READ
BUFFER is performed. The addition of X'IO' to the
CONTROL field values for ERASE/WRITE making it
X'90' causes the PAl key interrupt to be reflected to the
virtual machine. This replaces the normal PAl key
function of returning the virtual machine to CP mode.
Appendix A. CP/CMS Interface A-23