Now that we have the ability to perform I/O we can think about communicating with a device. We will start with the beginnings of an operator console. We will use a locally attached 3270 device as the our console. We will need to have a UCB defined for the device.
To begin we will start with providing the ability to write to the console. To do this we need to define two new control blocks. The first is the Console Communication Block (CCB) and the other is the Console Message Buffer (CMB).
The CCB defines a console and is the anchor for the console buffers.
@CCB DSECT , CONSOLE COMMUNICATIONS BUFFER CCBNEXT DS A NEXT CONSOLE CCBMBQ DS A MESSAGE BUFFER QUEUE CCBFMQ DS A FREE MESSAGE QUEUE CCBECB DS F MESSAGE AVAILABLE ECB CCBDEV DS XL2 CONSOLE DEVICE ADDRESS CCBQLEN DS XL2 QUEUE LENGTH
It begins with a pointer to the next CCB but for now we will only support one console. CCBMBQ points to Console Message Buffers (CMB’s) that are queued to the console. The CMB’s contain the message text to be written to the console. CCBFMQ is a queue of free message buffers for the console. When a message is queued to the console a free buffer is removed from the free buffer queue and added to the message buffer queue. CCBECB is an ECB that will be posted when a message is placed on the message queue. CCBDEV is the device address of the console. CCBQLEN is the total number of message buffers allocated to the console.
@CMB DSECT , CONSOLE COMMUNICATIONS BUFFER CMBNEXT DS A NEXT BUFFER CMBTEXT DS CL79 TEXT CMBFLAG DS X FLAG
The Console Message Buffer consists of a pointer to the next buffer on the queue. We allow for a message length of 79 bytes.
We define the CCB and CMB’s in module TXXCCB. We will define the message buffers in this module.
000000 1 TXXCCB CSECT ,
2 @CCB DSECT=N,DEV=010.
000000 3+CCB010 DS 0F
000000 00000000 4+ DC A(0)
000004 00000000 5+ DC A(0)
000008 00000014 6+ DC A(@M0101)
00000C 00000000 7+ DC F'0'
000010 0010 8+ DC XL2'010'
000012 0040 9+ DC AL2(64)
10+*
000014 0000006840404040 12+@M0101 DC A(@M0102),CL79' ',X'00'
000068 000000BC40404040 13+@M0102 DC A(@M0103),CL79' ',X'00'
.
.
.
00146C 000014C040404040 74+@M01063 DC A(@M01064),CL79' ',X'00'
0014C0 0000000040404040 75+@M01064 DC A(@M01065),CL79' ',X'00'
00000 76+@M01065 EQU 0
77 END ,
Here is the assembly listing for TXXCCB which is the expansion of the @CCB macro. We define 64 message buffers and they are all placed on the free buffer queue.
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