Re: OS/400 topics

I don't know what you consider a mainstream OS - this one has been around in 
basically the same form since 1980 - over 25 years.

I recommend a couple books, both by Frank Soltis, whose PhD thesis had a lot to 
do with the development of OS/400. The first one is Inside the AS/400, Second 
edition and the second newer one is Fortress Rochester. There is also 
http://www-03.ibm.com/servers/enable/site/porting/iseries/ where you can get 
some information about the architecture, etc.

OS/400 is actually not the operating system at the chip level - there is an 
abstraction layer that isolates OS/400 from the machine operations. This 
abstraction layer is sometimes called the machine interface (MI) and OS/400, as 
well as user applications, write to the abstraction layer, not the true OS 
below this layer. This is partly why the system can execute code that is over 
20 years old with minimum fuss.

As to process scheduling, it is pre-emptive, I believe, in your terms. 
Processes must relinquish use of the CPU based on a priority scheme, of which 
there are 255 levels (256?). 100 of these are assignable to user-level 
processes (jobs) while the rest are reserved for system functions and all have 
precedence. There is also a time slice involved in the scheduling, so that 
processes must yield regularly to other processes in the queue.

There is probably what could be called a kernel, but we real-world developers 
never deal with it. There is something called System Licensed Internal Code 
(SLIC) that is below the abstraction layer. Nothing there is accessible except 
through the well-defined and rigidly-controlled interface (MI).

You might also look at http://www.research.ibm.com/journal/ - there is a 
Systems Journal, and you can search there for OS/400 and get some hits.

Hope this is helpful - may I ask where you got interested in this?

Vern
-------------- Original message -------------- 
From: "N W" <artofzen1@xxxxxxxxxxx> 

Hello, I'm looking for technical data on the OS/400. 

I would like to know how does it compare to mainstream OS, if it is a batch 
processing system or interactive, and a little bit about his heritage. 

Some of the information that I need deals with the Name and type of Kernel, 
Process Scheduling, Scheduling Algorithms and Policies (Pre-emptive, Non 
Pre-emptive), Critical Sections, Mutex and Semaphores, Managing Tasks and how 
they are applied into OS/400. It is a real challenge to find information on 
these topics. I only know that QJOBSCD is the Job Scheduler. 

I have already e-mailed IBM and received an uncomplete answer. I know it is 
very 
difficult (almost impossible) to have some of this information. However, it 
would be very appreciated if you can at least answer one of these questions, 
or 
refer me to someone who knows about it. You are one of my last hopes. 

Thanks very much to anyone that answers, 

N. Wong 

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