Operating systems and hardware evolution: what made multiprogramming (concurrent residence and progress of multiple jobs) practically possible on mainstream systems?

Introduction / Context:
Multiprogramming—keeping multiple programs in memory and allowing them to make progress seemingly at once—was a landmark capability in computing. It relied on both hardware advancements (I/O subsystems) and software sophistication (operating systems). This question asks you to identify the necessary enablers.

Given Data / Assumptions:

• “Multiprogramming” refers to overlapping CPU and I/O among several jobs.
• Hardware supports asynchronous I/O operations decoupled from CPU execution.
• An OS exists to schedule and control these activities.

Concept / Approach:
To realize multiprogramming, two pillars were required: (1) I/O devices and controllers capable of operating in parallel with the CPU (e.g., DMA, channels), so the CPU need not busy-wait; and (2) an operating system to manage memory, schedule jobs, handle interrupts, and coordinate context switches. Without either, true multiprogramming would stall or be unmanageable.

Step-by-Step Solution:
1) Identify hardware need: independent I/O so CPU can compute while devices transfer data.2) Identify software need: OS to manage ready/blocked states, interrupts, and resource allocation.3) Conclude that both hardware capability and OS software are required.

Verification / Alternative check:
Historical systems (e.g., IBM 7090 → 360 series) introduced channel I/O and sophisticated OSes (e.g., MVT/MVS), demonstrating that only with both elements could multiprogramming flourish, improving utilization and throughput.

Why Other Options Are Wrong:

• Only I/O hardware: Without OS control, concurrency degenerates into chaos.
• Only OS: Without independent I/O, CPU remains a bottleneck waiting for devices.
• Neither: Contradicted by practice and theory.
• None of the above: Incorrect because both are necessary.

Common Pitfalls:
Confusing multiprogramming with multiprocessing (multiple CPUs). Multiprogramming can occur on a single CPU by overlapping CPU and I/O, but it requires both I/O autonomy and OS scheduling.

Final Answer:
both (a) and (b).