AM3270/TR3271 Reference Manual

Details of the CRT Protocol
AM3270 Subdevice Protocols
086705 Tandem Computers Incorporated 4–11
Figure 4-4 shows the format for inbound CRT messages.
Figure 4-4. CRT Protocol Message Format—Inbound Data
015
Length #1
Sent by the Device
S
T
X
E
T
X
B
C
C
Screen Information
(Part #2)
Sent by the Device
A
I
D
Screen Information
(Part #1)
Returned to the Application
S
T
X
C
U
D
E
V
A
I
D
E
T
B
B
C
C
Screen Information
(Part 1)
Screen Information
(Part #2)
Length #2
Count Read
(Length #1 + Length #2)
Applicable File-System
Procedures
For AM3270/CRT protocol, the following file-system procedures are recognized:
AWAITIO DEVICEINFO READ
CANCEL FILEINFO SETMODE
CLOSE OPEN WRITE
CONTROL
File system procedures CONTROL 22, READ, SETMODEs 53, 54, and 59, and WRITE
have special significance for this protocol and are discussed in this section.
CONTROL 22 (Two-Step
READ Operation)
The CONTROL procedure is used to perform device-dependent I/O operations. A
CONTROL 22 initiates a two-step READ operation which is designed to conserve
buffer resources.
A normal file-system READ request requires the file system to allocate buffer space in
the amount specified by the COUNT parameter of the READ call. The space must be
allocated for the lifetime of the READ request. Assuming that for a given
configuration many subdevices exist simultaneously, each with one or more