Batch

Batch

1. Definition

A batch operating system executes a group (batch) of jobs that are collected, ordered and submitted together, without any interaction between the user and the running job. The user prepares the job (program + data + control statements), hands it to the operator, and receives the output afterwards. Jobs with similar resource needs are kept in the same batch to reduce setup overhead.

Era: second and third generation mainframes (IBM 1401, IBM 360) — the "resident monitor" executed batch after batch automatically.

2. How a Batch System Works

  PUNCH CARDS / JOB FILE
        │  (job = program + data + JCL control cards)
        ▼
  ┌────────────────────┐        ┌──────────────────────────────┐
  │ OPERATOR collects  │        │  RESIDENT MONITOR (in RAM)   │
  │ & sorts similar    │───────►│  1. reads next job (JCL)     │
  │ jobs into a BATCH  │        │  2. loads it into memory     │
  └────────────────────┘        │  3. runs it, traps its end   │
                                │  4. writes output to spool   │
                                │  5. loads the NEXT job       │
                                └──────────────┬───────────────┘
                                               ▼
                                 OUTPUT (printer / file) later

Part	Role
Job	A unit of work: program, data and control statements
JCL (Job Control Language)	Commands telling the monitor what resources the job needs (e.g. $JOB`, `$RUN, $END)
Resident monitor	Always-resident supervisor that loads, runs and ends jobs automatically
Job pool / queue	Set of jobs waiting to be scheduled
Spooling	Reading input and writing output to disk so slow peripherals overlap with computation
Scheduler	Chooses which job runs next (FCFS, SJF, priority)

3. Advantages and Disadvantages

Advantages	Disadvantages
No operator intervention between jobs — high throughput for large, repetitive volumes	No interaction — the user cannot change or correct a running job
CPU idle time between jobs is minimised (automatic job-to-job transition)	Long turnaround time — hours or days; output arrives much later
Good for bulk processing: payroll, billing, bank statements, ETL, scientific runs	Difficult debugging — an error means resubmitting the whole job
Scheduling is simple and cheap; resources are planned in advance	No priority for urgent jobs; a long job delays everything behind it
Can use spooling to overlap I/O with computation	CPU sits idle during I/O in a single-programmed batch system
Straightforward accounting of resources per job	Poor utilisation of interactive hardware; unsuitable for desktop use

4. Scheduling Example (FCFS in a batch)

Jobs are queued in arrival order and run one after another.

Job	Arrival	Burst	Start	Completion	Turnaround	Waiting
J1	0	5	0	5	5	0
J2	0	3	5	8	8	5
J3	0	8	8	16	16	8

Average turnaround = 29/3 ≈ 9.67, average waiting = 13/3 ≈ 4.33.
Now run the shortest job first in the same batch (J2, J1, J3): TATs are 3, 8, 16 → average 9.0 — a small scheduling change improves the average, which is the whole point of batch scheduling policies.

5. Improvements Built on Batch

• Multiprogramming — several jobs resident at once, so the CPU switches to another job while one waits for I/O; CPU utilisation rises sharply.
• Spooling — print/read jobs queue on disk, decoupling slow devices from the CPU.
• Job classes and priorities — fast lanes for short or urgent jobs (used in mainframe job entry subsystems, e.g. JES).
• Modern descendants — scheduled batch processing is still everywhere: nightly bank reconciliation, payroll, report generation, campus placement data loads, and cluster job schedulers (SLURM, PBS, Hadoop/Spark jobs) that run queue-based, non-interactive work.

6. Batch vs Time-sharing (quick contrast)

Basis	Batch OS	Time-sharing OS
Interaction	None during execution	Continuous, interactive
User presence	Submit and come back later	Logged in, working live
Turnaround	Hours/days	Seconds
Scheduling goal	Throughput and utilisation	Short response time and fairness
Typical use	Payroll, billing, bulk data	Desktops, servers, terminals

7. Exam Points

• Define batch operating system and explain its working with a diagram — 5-7 marks, frequent.
• Advantages and disadvantages of a batch OS — 5 marks, very frequent.
• What is JCL? Give a one-line example of its purpose — 2-3 marks.
• Numerical: compute average turnaround and waiting time for a small batch of jobs — 5 marks.
• How do multiprogramming and spooling improve a batch system? — 3-5 marks.
• Differentiate batch and time-sharing operating systems — 5 marks.