What is multi programing?

Answer 1

A batch system is one in which jobs are bundled together with the instructions necessary to allow them to be processed without intervention.

Often jobs of a similar nature can be bundled together to further increase economy

The basic physical layout of the memory of a batch job computer is shown below:

--------------------------------------

| |

| Monitor (permanently resident) |

| |

--------------------------------------

| |

| User Space |

| (compilers, programs, data, etc.) |

| |

--------------------------------------

The monitor is system software that is responsible for interpreting and carrying out the instructions in the batch jobs. When the monitor started a job, it handed over control of the entire computer to the job, which then controlled the computer until it finished.

A sample of several batch jobs might look like:

$JOB user_spec; identify the user for accounting purposes

$FORTRAN; load the FORTRAN compiler

source program cards

$LOAD; load the compiled program

$RUN; run the program

data cards

$EOJ; end of job

$JOB user_spec; identify a new user

$LOAD application

$RUN

data

$EOJ

Often magnetic tapes and drums were used to store intermediate data and compiled programs.

Advantages of batch systems

move much of the work of the operator to the computer

increased performance since it was possible for job to start as soon as the previous job finished

Disadvantages

turn-around time can be large from user standpoint

more difficult to debug program

due to lack of protection scheme, one batch job can affect pending jobs (read too many cards, etc)

a job could corrupt the monitor, thus affecting pending jobs

a job could enter an infinite loop

As mentioned above, one of the major shortcomings of early batch systems was that there was no protection scheme to prevent one job from adversely affecting other jobs.

The solution to this was a simple protection scheme, where certain memory (e.g. where the monitor resides) were made off-limits to user programs. This prevented user programs from corrupting the monitor.

To keep user programs from reading too many (or not enough) cards, the hardware was changed to allow the computer to operate in one of two modes: one for the monitor and one for the user programs. IO could only be performed in monitor mode, so that IO requests from the user programs were passed to the monitor. In this way, the monitor could keep a job from reading past it's on $EOJ card.

To prevent an infinite loop, a timer was added to the system and the $JOB card was modified so that a maximum execution time for the job was passed to the monitor. The computer would interrupt the job and return control to the monitor when this time was exceeded.