"IBM Mainframe Assembler List" <[email protected]> wrote on
09/19/2023 04:41:14 AM:
> I implemented local 3270 support for Altergo Software's
> transaction processing system Shadow II on OS/VS in the 1970s
> using similar tricks, but I think it also used a start-I/O
> appendage.
>
> To keep track of attention, it marked the UCB as still busy on
> completion of any request, and queued a NOP to wait if a read
> was issued. When Attention came in, the I/O supervisor marked
> the UCB as available and executed any queued request.
>
> The trickiest bit was cancelling the wait for attention if
> another Write was issued. I don't remember the details (this
> was nearly 50 years ago) but I think it involved marking the
> next request as related so that the start-I/O appendage would
> run immediately rather than waiting for the previous request,
> then clearing the UCB busy bit from the start-I/O appendage.
> I think we also marked the queued request so it would be
> cancelled (and marked as purged) rather than executed.
Yes, that is basically what I am doing. Before the WRITE, if a READ
is already queued, then I remove it from the queue.
LA R1,MYCCB SET ADDRESS OF CCB
IF CCBQUEUE,(ON,TF),$NOOP IF ALREADY QUEUED FOR READ
SVC 25 DEQUE CHANNEL QUEUE (HIO)
CF CCBQUEUE CLEAR THE FLAG
ENDIF ENDIF
EXCP (1) START THE I/O OPERATION
WAIT (1) WAIT FOR I/O COMPLETION
IF MYCCB+4(2),(NE,CLC),=X'0C00' IF NOT CE+DE, ERROR
LA R5,16 SET RC VALUE
XR R15,R15 CLEAR REGISTER
IC R15,MYCCB+4 SET RESULT VALUE
B RETERR GO RETURN ERROR
ENDIF ENDIF
At READ time, I queue up a NOOP first.
IF OPT,(NE,CLC),=C'READQ' IF NOT QUERY
IF CCBQUEUE,(NOT,TF),$NOOP THEN NEED I/O QUEUEING
MVC MYCCW+0(1),LCLNOOP USE NOOP FOR CHANNEL QUEUE
MVC MYCCW+6(2),=H'1' WITH I/O LENGTH OF 1
EXCP (1) PUT IN CHANNEL QUEUE
SF CCBQUEUE SET QUEUED FLAG
ENDIF ENDIF
WAIT (1) WAIT FOR ATTN INTERRUPT
CF CCBQUEUE CLEAR QUEUED FLAG
ENDIF ENDIF
Then, in the channel appendage routine, I do the following.
IF X'44'(2),(EQ,CLC),=X'0C00' IF CE+DE
IF 8(R1),EQ,X'03' IF CCW STILL NOOP
MVC X'44'(2),=X'0080' CHANGE TO PCI, STAY QUEUED
ENDIF
ELSE
IF X'44',(ON,TM),X'80' IF INTERRUPT WAS ATTN
MVC X'44'(2),=X'0C00' CHANGE TO CE+DE, DEQUE
ENDIF
ENDIF
B 4(R7) RETURN TO SUPERVISOR
When WAIT for attention interrupt is satisfied, I continue the READ
as follows.
IF OPT,(EQ,CLC),=C'READB'
MVC MYCCW+0(1),LCLREADB PUT COMMAND CODE IN CCW
ELSE
MVC MYCCW+0(1),LCLREADM PUT COMMAND CODE IN CCW
ENDIF
L R0,BUFLEN PUT MAX LENGTH OF BUFFER
STH R0,MYCCW+6 IN CHANNEL COMMAND WORD
EXCP (1) START THE I/O OPERATION
WAIT (1) WAIT FOR I/O COMPLETION
IF MYCCB+4(2),(NE,CLC),=X'0C00' IF NOT CE+DE, ERROR
LA R5,16 SET RC VALUE
XR R15,R15 CLEAR REGISTER
IC R15,MYCCB+4 SET RESULT VALUE
B RETERR GO RETURN ERROR
ENDIF ENDIF
I haven't tested this, yet, though. Does that all look correct?
Sincerely,
Dave Clark
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