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task interruption

Created by: Siemens_MI at: 10/4/2011 4:08 PM (4 Replies)

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10/4/2011 4:08 PM	Rate (0)
Siemens_MI Member Joined: 6/7/2010 Last visit: 3/27/2023 Posts: 11 Rating: (0)	The online help for Step7 gives the following description of what happens during an interrupt: "the block currently being executed is interrupted at a command boundary and a different organization block that is assigned to the particular event is called." Does anyone know what a 'command bounday' is?

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10/4/2011 5:07 PM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 5/31/2023 Posts: 1524 Rating: (522)	Well when it comes to pre-emptive interrupt scheduling, which is what is happening here, the current task will be suspended (pre-empted) at a convenient point in its execution. This is what they are referring to when they say "command boundary". Now what constitutes a command boundary is hard to say without seeing the scheduler code (firmware). The scheduler on the real time operating system I designed for our own PLC, would suspend the task at the end of the current CPU machine code instruction. The Siemens PLC might do the same. However, it could theoretically be suspended anywhere between a single machine code instruction and a complete block (made up of many machine code instructions), or any point in between. My personal opinion is that it will probably finish the current STL instruction, and then it will be suspended and the interrupt then handled. Just for your information, this is known in computer science as a "context switch". This happens like this:- 1) The current task is suspended at a convenient point (the command boundary). 2) The current values of all CPU address registers, status regsiters, data registers, accumulators, program counter etc. are saved in some static memory. 3) The interupt is then handled and any interrupt code is run to completion. 4) Once interrupt has be handled, the task that was suspended is resumed. To do this all the CPU registers are restored to the saved values and the task is restarted from the exact place it was suspended, and as the registers have been restored, it will continue as if never interrupted.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
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10/4/2011 7:11 PM	Rate (0)
Siemens_MI Member Joined: 6/7/2010 Last visit: 3/27/2023 Posts: 11 Rating: (0)	Thanks for the information. I was hoping to find out what specifically the 'command boundary' referred to - in terms of op codes.

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10/5/2011 4:24 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 6/9/2023 Posts: 2969 Rating: (1025)	smiffy My personal opinion is that it will probably finish the current STL instruction, and then it will be suspended and the interrupt then handled I'd agree with this assumption bysmiffy, the currently active STL command line will have to finish first before the interrupt kicks in. The naming and handling of thisstems back to good old S5 days where(in certain CPU's) you could configure to have either"Interrupts at command boundaries" (which is now the only wayS7 does it) or "Interrupts at Blockboundaries" (in which case the interrupt had to wait until the whole Block was finished or another Block call was programmed to takeplace within it) As for Siemens_MI's question "what specifically the 'command boundary' referred to - in terms of op codes" I'm not too sure what exactlyyou'd like to know, but perhaps the helps anyhow.
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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10/5/2011 3:58 PM	Rate (0)
Siemens_MI Member Joined: 6/7/2010 Last visit: 3/27/2023 Posts: 11 Rating: (0)	That helps, and I appreciate giving me the historical context about the S5 options. That's pretty amazing that you couldcontrol the entry point for the interrupt in S5! I'm going to assume the interrupt will allow the current STL statement to finish execution and design around that. Thanks.

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