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Mainframe Computers Created a Sense of Wonder in Me—Part 2

October 13, 2013

In my previous post, I wrote about the importance of Assembler Language and system organization to my understanding of the mainframe computers I was using. In this post, I want to discuss access methods and the use of interrupts as a mechanism to flow work through the system. 

Flexible Access Methods

When I wrote programs, I selected just the right access method for the job. At first, we had older methods like SAM and DAM but when VSAM came along, everything changed. VSAM could do it all—sequential, direct, indexed and more, and it was developed to take advantage of virtual storage. It was a breakthrough method and the clarity of the diagnostics was awesome. We worked with current DASD technology to get just the right block sizes to maximize disk utilization but not to expose our job to buffers that were too large. Achieving balance was an important consideration as it still is today.

Interrupts to Implement Multitasking

When I read about interrupts, I realized that this was an ingenious way of making it possible for more than one program to operate at the same time. Interrupts would occur when a supervisor call was made or when I/O happened making it possible to keep the CPU busy with new work while slower actions took place, like data being retrieved from devices. What a marvelous way to create the illusion that hundreds of programs were running at the same time when there were just a few CPUs. It was a stunning and brilliant architectural design, and it worked. It was nothing short of amazing.

Did you have a similar experience? Please comment with your own reflections and experiences.

Posted October 13, 2013| Permalink