---
title: "Logical and physical address bits"
subject: "Operating System"
chapter: 4
key: "semester_6/operating_system:residence-monitor-address-bits"
source: https://www.bctengineer.com/q/semester_6/operating_system/residence-monitor-address-bits
site: BCT Engineering
---

# Logical and physical address bits

Operating System · Chapter 4: Memory Management

- Asked in: 2071 Magh, 5+5 marks
- Video: [L-5.10: Question Explanation on Logical address and Physical address space | Operating System](https://www.youtube.com/watch?v=30P73tWmU0s) — Gate Smashers
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/operating_system/residence-monitor-address-bits
- Whole chapter: https://www.bctengineer.com/md/semester_6/operating_system/chapter-4

## Question

What is residence monitor? Consider logical address spaces of eight pages of 1024 words, each mapped onto a physical memory of 32 frames then.
a) How many bits are in logical address?
b) How many bits are in physical address?

## Short answer

A resident monitor is the earliest OS form that stays permanently in memory to sequence jobs automatically. Logical address: 13 bits (3 page + 10 offset). Physical address: 15 bits (5 frame + 10 offset).

## Answer

#### Resident monitor
The **resident monitor** is the earliest form of an operating system, used in batch systems. It is the small part of the OS that stays permanently ("resides") in main memory and automatically sequences jobs so that the CPU does not wait for an operator between jobs. Its parts:
- **Control-card interpreter:** reads and executes the control cards (e.g. \$JOB, \$LOAD, \$RUN, \$END) that tell it what to do.
- **Loader:** loads a user program into memory.
- **Device drivers:** handle the I/O devices used by the system and the user program.

Memory is divided into the monitor area and the user-program area; hardware (a boundary register) protects the monitor. When one job ends, control returns to the monitor, which loads the next job.

```text
 +----------------+
 | Resident       |  always in memory
 | monitor        |
 +----------------+  <- boundary
 | User program   |
 | area           |
 +----------------+
```

#### a) Bits in logical address
- Logical address space = 8 pages × 1024 words = 8192 words = 2¹³ words.
- Page number needs 3 bits (8 = 2³), offset needs 10 bits (1024 = 2¹⁰).
- Logical address = 3 + 10 = **13 bits**.

#### b) Bits in physical address
- Physical memory = 32 frames × 1024 words = 32768 words = 2¹⁵ words.
- Frame number needs 5 bits (32 = 2⁵), offset 10 bits.
- Physical address = 5 + 10 = **15 bits**.

**Answer: logical address = 13 bits; physical address = 15 bits.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the resident monitor?
   A. A hardware timer
   B. Earliest OS that stays in memory and sequences batch jobs
   C. A virtual memory manager
   D. A disk driver
   Answer: B. It auto-sequences jobs without operator. (Key point: Resident monitor)

2. Which are parts of the resident monitor?
   A. Scheduler, pager, shell
   B. Control-card interpreter, loader, device drivers
   C. Compiler and linker
   D. TLB and MMU
   Answer: B. They read control cards, load programs and drive I/O. (Key point: Monitor parts)

3. 8 pages of 1024 words: logical address bits?
   A. 10
   B. 16
   C. 13
   D. 15
   Answer: C. 3 bits page + 10 bits offset. (Key point: Logical bits)

4. 32 frames of 1024 words: physical address bits?
   A. 32
   B. 10
   C. 15
   D. 13
   Answer: C. 5 bits frame + 10 bits offset. (Key point: Physical bits)

## Related in Operating System

- [Page fault; virtual to physical address](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apage-fault-64k-virtual-32k-physical) — Operating System, chapter 4
- [Page fault, demand paging, address translation](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apage-fault-demand-paging-address) — Operating System, chapter 4
- [Page faults with 3 frames: LRU, FIFO, OPT](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apage-fault-circumstances-lru-fifo-optimal) — Operating System, chapter 4, asked 2×
- [Thrashing; page faults on 3 frames](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Athrashing-page-faults-fifo-opt-lru-lfu) — Operating System, chapter 4
- [Segmentation; page table size; TLB](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Asegmentation-address-bits-tlb) — Operating System, chapter 4

## Related in other subjects

- [Memory-mapped I/O reduces address space](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Ammio-address-space) — Computer Organization and Architecture, chapter 7
- [Unique/non-unique decoding, memory mapped I/O](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aunique-nonunique-decoding-memory-mapped-io) — Microprocessor, chapter 4
- [Memory-mapped I/O design for 8085](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Amemmapped-io-design-1000h) — Instrumentation II, chapter 2
- [Isolated I/O vs memory-mapped I/O](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aisolated-vs-mmio) — Computer Organization and Architecture, chapter 7, asked 2×