What is AHCI and how does it different from IDE?

Advanced Host Controller Interface

AHCI stands for Advanced Host Controller Interface. It is an addition to complement the Serial ATA standard. It allows for access of the native functions of the SATA standard for example Native command queuing which allows greater performance by optimizing how controller requests data from the hard drive and hot-swappable which means you can utilize eSATA without shutting the system down capabilities at an operating system level.

AHCI provides a standard system that designers and developers can use to configure, detect, or program SATA/AHCI adapters. Many SATA controllers can enable AHCI either separately or in conjunction with RAID support. Intel recommends choosing RAID mode on their motherboards (which also enables AHCI) rather than the plain AHCI/SATA mode for maximum flexibility, due to the issues caused when the mode is switched once an operating system has already been installed.

AHCI is fully supported out of the box for Microsoft Windows Vista and the Linux operating system from kernel 2.6.19. NetBSD also supports drivers in AHCI mode out of the box in certain versions. If you are installing Microsoft Windows Vista or higher, AHCI is directly supported natively on the operating system installation DVD so you do not have to load AHCI drivers during the installation of the operating system. However, Microsoft's Windows XP does not include native support and therefore you will need to manually install the AHCI drivers during the installation of Windows XP.

How does AHCI work?

Most motherboards have AHCI enabled by default in the Unified Extensible Firmware Interface (UEFI) or BIOS. Older motherboards may have IDE mode enabled by default and would need to be switched to AHCI before the operating system (OS) is installed. AHCI is supported on Windows Vista and later versions of Windows; Linux since version 2.6.19; OS X; and various open source operating systems, such as OpenBSD, NetBSD, and FreeBSD. While Windows Vista and Windows 7 include the AHCI driver, those OSes won't install AHCI if it's not enabled on the boot drive's controller. SATA hard drives and solid-state drives (SSDs) offer several modes of operation: IDE, AHCI or RAID, which usually has AHCI enabled. However, it isn't easy to switch the BIOS setting from IDE mode to AHCI once the OS is installed. Windows has a registry workaround that will allow AHCI to be enabled after the OS is installed. Older OS versions require hardware-specific drivers to support AHCI. As a traditional storage protocol developed for hard drives and tape, AHCI was designed to manage a single storage request queue. For AHCI, this queue has a depth - the number of I/O requests that can be kept waiting to be serviced in a port queue - of 32 commands.

What are the differences between AHCI and IDE?

  • AHCI is a faster mode of operation compared to IDE.

  • AHCI provides a standard system that designers and developers can use to configure, detect, or program SATA/AHCI adapters.

  • IDE interface does not support hot plugging, but AHCI supported for hotplugging.

  • IDE has lacked support for new technology such as native command queuing. On the hand, AHCI supports new technologies such as native command queuing.

  • AHCI involves operation of Serial ATA (SATA) host bus adaptors. And IDE involves the operation of a parallel ATA drive.

  • IDE only has one operating mode. But AHCI is a newer standard that was designed to be backward-compatible with legacy systems that could only "speak IDE".

  • IDE is considered adequate for the average computer user and is the most compatible with other technology, particularly older devices. On the hand, AHCI also offers an improved performance (speed) over IDE.

  • AHCI allows users to utilize advanced features.

  • AHCI is defined by Intel. In contrast, IDE is defined by Western Digital.

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