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Unit 1: Introduction to Operating System

program, recorded timing and accounting information, and then went back to begin processing Notes
of another user program, and so on, as long as there were programs waiting in the input queue
to be executed.

The first generation saw the evolution from hands-on operation to closed shop operation to the
development of mono-programmed operating systems. At the same time, the development of

programming languages was moving away from the basic machine languages; first to assembly
language, and later to procedure oriented languages, the most significant being the development

of FORTRAN by John W. Backus in 1956. Several problems remained, however, the most obvious

was the inefficient use of system resources, which was most evident when the CPU waited while
the relatively slower, mechanical I/O devices were reading or writing program data. In addition,
system protection was a problem because the operating system kernel was not protected from
being overwritten by an erroneous application program.
Moreover, other user programs in the queue were not protected from destruction by executing
programs.

1.5.3 Second Generation (1956-1964)

The second generation of computer hardware was most notably characterised by transistors
replacing vacuum tubes as the hardware component technology. In addition, some very important
changes in hardware and software architectures occurred during this period. For the most part,

computer systems remained card and tape-oriented systems. Significant use of random access
devices, that is, disks, did not appear until towards the end of the second generation. Program
processing was, for the most part, provided by large centralised computers operated under
mono-programmed batch processing operating systems.
The most significant innovations addressed the problem of excessive central processor delay

due to waiting for input/output operations. Recall that programs were executed by processing
the machine instructions in a strictly sequential order. As a result, the CPU, with its high speed
electronic component, was often forced to wait for completion of I/O operations which involved
mechanical devices (card readers and tape drives) that were order of magnitude slower. This
problem led to the introduction of the data channel, an integral and special-purpose computer
with its own instruction set, registers, and control unit designed to process input/output
operations separately and asynchronously from the operation of the computer’s main CPU near

the end of the first generation, and its widespread adoption in the second generation.
The data channel allowed some I/O to be buffered. That is, a program’s input data could be
read “ahead” from data cards or tape into a special block of memory called a buffer. Then, when
the user’s program came to an input statement, the data could be transferred from the buffer
locations at the faster main memory access speed rather than the slower I/O device speed.
Similarly, a program’s output could be written another buffer and later moved from the buffer
to the printer, tape, or card punch. What made this all work was the data channel’s ability to
work asynchronously and concurrently with the main processor. Thus, the slower mechanical
I/O could be happening concurrently with main program processing. This process was called
I/O overlap.

The data channel was controlled by a channel program set up by the operating system I/O
control routines and initiated by a special instruction executed by the CPU. Then, the channel
independently processed data to or from the buffer. This provided communication from the CPU
to the data channel to initiate an I/O operation. It remained for the channel to communicate
to the CPU such events as data errors and the completion of a transmission. At fi rst, this
communication was handled by polling – the CPU stopped its work periodically and polled the
channel to determine if there is any message.

Polling was obviously inefficient (imagine stopping your work periodically to go to the post

office to see if an expected letter has arrived) and led to another significant innovation of the

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