Understanding ARC Path: A Dive into Legacy Boot Configuration

The Advanced RISC Computing (ARC) path, an integral part of legacy Microsoft operating systems, is a boot configuration path that played a key role in system startups.

This article offers an in-depth exploration of ARC path, providing clarity on its definition, how it operates, and its significance in the historical context of operating systems. Aimed at both enthusiasts and professionals, this comprehensive guide will navigate through the intricacies of ARC path, from basic examples to a deeper look into its specifications and historical relevance.

In this chapter:

  1. What is ARC path?
  2. How it Works
  3. ARC Path Examples
  4. Delve into the Specification
  5. Historical Perspective
  6. References

1. What is ARC path?

ARC path is an acronym for Advanced RISC Computing path. Syntax for naming partitions of a disk on a system, used in the boot.ini file of Microsoft Windows NT. The ARC path specifies the location of the partition that contains the Windows NT operating system files. In other words, the ARC path locates the system partition on the machine.

 ARC Path
ARC Path

Definition and Purpose

  • ARC Path: ARC path is a method of specifying device paths used by the Windows NT bootloader. It is designed to abstract the hardware configuration and provide a consistent interface for the operating system to start up, regardless of the underlying hardware differences.
  • Function: The primary function of the ARC path is to accurately identify the location of system components and resources necessary for the boot process. This includes specifying the paths to storage devices (like hard drives) and their partitions where the operating system and boot files reside.

Context in Operating Systems

  • Usage in Windows NT: In Windows NT-based systems, the ARC path is used in the boot.ini file. This file is critical for the bootloader to understand where the operating system kernel and associated files are located for initiating the boot process.

2. How it Works

The ARC path notation follows a specific structure to pinpoint the location of the boot files within the computer’s hardware architecture. Understanding this structure is key to comprehending how the Windows NT boot process works.

ARC Path Structure

  • Components: An ARC path typically contains elements like multi, disk, rdisk, partition, and a path to the directory where the operating system is located. Each element plays a specific role in directing the bootloader to the correct location.
  • Syntax Example: A common ARC path looks like multi(0)disk(0)rdisk(0)partition(1)\WINDOWS. This notation breaks down as follows:
    • multi(0): Refers to the SCSI bus number (though it can be used for IDE systems as well).
    • disk(0): Indicates the disk controller number.
    • rdisk(0): Specifies the physical hard disk number.
    • partition(1): Points to the partition on the hard disk.
    • \WINDOWS: The directory where the Windows operating system is located.

Boot Process Utilizing ARC Path

  • Boot Sequence: During the boot process, the Windows NT bootloader (NTLDR) reads the boot.ini file, which contains one or more ARC paths. The bootloader uses this information to locate and load the necessary system files to initiate the operating system.
  • Role in Hardware Abstraction: By using ARC paths, the bootloader abstracts the hardware configuration details, allowing the operating system to boot from various types of hardware setups without needing configuration changes.

In summary, the ARC path is an essential part of the boot process in Windows NT-based systems, providing a hardware-independent way to locate and load the operating system. Understanding its structure and function is crucial for comprehending the boot process and system configuration in these legacy operating systems.

3. ARC Path Examples

To illustrate the concept of ARC paths, let’s explore some common examples and dissect their components. These examples demonstrate how the ARC path notation specifies different hardware configurations.

Example 1: Standard Single Hard Disk

  • ARC Path: multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
  • Explanation:
    • multi(0): Refers to the first (or only) SCSI adapter, commonly used even in IDE systems.
    • disk(0): Indicates the primary disk controller.
    • rdisk(0): Specifies the first physical hard disk.
    • partition(1): Points to the first primary partition on the hard disk.
    • \WINDOWS: Specifies that the Windows directory is located in the root of the partition.

Example 2: Dual Boot System with Multiple Partitions

  • ARC Path: multi(0)disk(0)rdisk(0)partition(2)\WINNT
  • Explanation:
    • The elements are similar to Example 1, but partition(2) indicates that the Windows operating system is located on the second primary partition of the first physical hard disk.

Example 3: A typical ARC path on an x86-based computer could be the following:

"Windows NT Server Version 4.00"

In this syntax

  • Multi(0) indicates that the system is using Integrated Device Electronics (IDE), Enhanced Small Device Interface (ESDI), or Small Computer System Interface (SCSI) drives and that it relies on the computer BIOS to load system files using INT 13 BIOS calls. The number in parentheses here should always be 0.
  • Disk(0) is always zero when multi() is used; otherwise, it refers to the physical disk when scsi() is used instead of multi(). (Multi() means that Windows NT must use the system BIOS to load system files, while scsi() means that a SCSI device driver must be loaded to access the boot partition.)
  • Rdisk(0) indicates that the ordinal number for the system disk is 0. This value is usually between 0 and 3.
  • Partition(2) indicates the number of the partition where the system files are located

4. Delve into the Specification

The ARC path specification was part of the broader Advanced RISC Computing specification, which aimed to create a cross-platform open hardware standard.

Structure and Rules

  • Uniform Naming Convention: ARC path uses a uniform naming convention to refer to devices irrespective of the underlying hardware, providing a level of abstraction for the operating system.
  • Boot Configuration: The specification details how boot configurations should be structured, allowing the bootloader to access the necessary files across different hardware setups.

Specification Details

  • Multi: Refers to the SCSI controller and is used even if the system uses IDE drives.
  • Disk: Denotes the disk controller number.
  • Rdisk: Points to the physical hard disk number.
  • Partition: Specifies the partition number on the hard disk where the OS resides.

5. Historical Perspective

The ARC path has a rich history in the evolution of Microsoft’s operating systems, particularly in the transition from monolithic to modular and flexible architectures.

Development and Implementation

  • Origin in RISC Systems: The ARC standard was initially developed for RISC-based systems but was adopted by Microsoft for its Windows NT line, reflecting a shift towards more hardware-agnostic operating systems.
  • Role in Windows NT: ARC paths were crucial in Windows NT’s ability to boot from various types of disk configurations and hardware setups, aiding its reputation as a robust and versatile operating system.

Transition to Newer Technologies

  • Evolution of Boot Processes: With the advent of newer Windows versions, Microsoft moved away from the traditional boot.ini and ARC paths. Technologies like the Boot Configuration Data (BCD) store and Unified Extensible Firmware Interface (UEFI) have since replaced ARC paths in later versions of Windows.
  • Legacy Impact: Despite being superseded by newer technologies, understanding ARC paths provides valuable insights into the evolution of boot processes and system architecture in Windows operating systems.

In conclusion, ARC paths played a significant role in the boot process of Windows NT systems, offering a flexible and hardware-independent method for locating system resources. While they are part of legacy technology, their impact is still evident in the principles that guide current boot management systems.

6. References

  1. Windows NT Workstation Configuration and Maintenance” by Andrew Bettany and Andrew Warren: A resource covering various aspects of Windows NT, including boot configurations.
  2. Advanced RISC Computing Specification,” published by the Advanced Computing Environment (ACE) initiative.
  3. Online Technical Documents and Archives: Various resources from Microsoft and other technical archives discussing legacy Windows systems and their boot processes.
  4. How to mirror the system and boot partition (RAID1)“, Microsoft Learn