Editing Micro Channel architecture (section)

== Design ==
{{Refimprove section|date=July 2024}}
[[File:CHIPS P82C612.jpg|thumbnail|right|CHIPS P82C612 in a [[Plastic leaded chip carrier|PLCC]] package]]
The Micro Channel architecture was designed by engineer Chet Heath.<ref>[https://www.forbes.com/2001/04/09/0409dvorak.html "IBM Wild Duck Flies South"], John C. Dvorak, 4/09/2001</ref><ref>[https://www.ardent-tool.com/tech/Why_MCA.html "Why MCA?"]</ref> A lot of the Micro Channel cards that were developed used the [[Chips and Technologies]] P82C612 MCA interface controller; allowing MCA implementations to become a lot easier.<ref>{{cite web|url=http://www.datasheetarchive.com/P82C612-datasheet.html|title=82C611, 82C612 MicroCHIPS: Micro Channel Interface Parts|publisher=[[Chips and Technologies]]}}</ref>
[[File:MCA NIC IBM 83X9648.jpg|thumb|280px|IBM 83X9648 16-bit network interface card]]

===Overview===
The Micro Channel was primarily a 32-bit bus, but the system also supported a 16-bit mode designed to lower the cost of connectors and logic in [[Intel]]-based machines like the IBM [[PS/2]].

The situation was never that simple, however, as both the 32-bit and 16-bit versions initially had a number of additional optional connectors for memory cards which resulted in a huge number of physically incompatible cards for bus attached memory. In time, memory moved to the CPU's [[local bus]], thereby eliminating the problem. On the upside, signal quality was greatly improved as Micro Channel added ground and power pins and arranged the pins to minimize interference; a ground or a supply was thereby located within 3 pins of every signal.

Another connector extension was included for [[graphics card]]s. This extension was used for analog output from the video card, which was then routed through the system board to the system's own monitor output. The advantage of this was that Micro Channel system boards could have a basic [[VGA]] or [[Multi-Color Graphics Array|MCGA]] graphics system on board, and higher-level graphics ([[XGA]] or other accelerator cards) could then share the same port. The add-on cards were then able to be free of '[[legacy system|legacy]]' VGA modes, making use of the on-board graphics system when needed, and allowing a single system board connector for graphics that could be upgraded.

Micro Channel cards also featured a unique, 16-bit software-readable ID, which formed the basis of an early plug and play system.    The BIOS and/or OS can read IDs, compare against a list of known cards, and perform automatic system configuration to suit. This led to boot failures whereby an older [[BIOS]] would not recognize a newer card, causing an error at startup. In turn, this required IBM to release updated Reference Disks (The [[Nonvolatile BIOS memory|CMOS]] Setup Utility) on a regular basis. A fairly complete list of known IDs is available (see External links section). To accompany these reference disks were ADF files which were read by setup which in turn provided configuration information for the card. The ADF was a simple text file, containing information about the card's memory addressing and interrupts.

Although MCA cards cost nearly double the price of comparable non-MCA cards, the marketing stressed that it was simple for any user to upgrade or add more cards to their PC, thus saving the considerable expense of a technician. In this critical area, Micro Channel architecture's biggest advantage was also its greatest disadvantage, and one of the major reasons for its demise. To add a new card (video, printer, memory, network, modem, etc.) the user simply plugged in the MCA card and inserted a customized [[floppy disk]] (that came with the PC) to blend the new card into the original hardware automatically, rather than bringing in an expensively trained technician who could manually make all the needed changes. All choices for interrupts (an often perplexing problem) and other changes were accomplished automatically by the PC reading the old configuration from the floppy disk, which made necessary changes in software, then wrote the new configuration to the floppy disk. In practice, however, this meant that the user must keep that ''same floppy disk matched to that PC''. For a small company with a few PCs, this was annoying, but practical. But for large organizations with hundreds or even thousands of PCs, permanently matching each PC with its own floppy disk was logistically unlikely or impossible. Without the original, updated floppy disk, no changes could be made to the PC's cards. After this experience repeated itself thousands of times, business leaders realized their dream scenario for upgrade simplicity did not work in the corporate world, and they sought a better process.

=== Data transmission ===
The basic data rate of the Micro Channel was increased from ISA's 8&nbsp;MHz to 10&nbsp;MHz. This may have been a modest increase in terms of clock rate, but the greater bus width, coupled with a dedicated bus controller that utilized [[burst mode (computing)|burst mode]] transfers, meant that effective throughput was up to five times higher than ISA. For faster transfers the [[address bus]] could be reused for data, further increasing the effective width of the bus. While the 10&nbsp;MHz rate allowed 40 [[MB/s]] of throughput at 32-bit width, later models of RS/6000 machines increased the data rate to 20&nbsp;MHz, and the throughput to 80 MB/s.<ref>[https://www-01.ibm.com/common/ssi/rep_ca/9/877/ENUSZG92-0339/index.html RISC System/6000 POWERstation/POWERserver 580]</ref> Some higher throughput functions of the Micro Channel bus were available to RS/6000 platform only, and were not initially supported on cards operating on an Intel platform.<ref>[https://books.google.com/books?id=KzsEAAAAMBAJ&dq=microchannel+rs/6000+differences+intel&pg=PA1 Infoworld March 5, 1990, p1]</ref>

With [[bus mastering]], each card could talk to another directly.  This allowed performance that was independent of the CPU.  One potential drawback of multi-master design was the possible collisions when more than one card would try to bus master, but Micro Channel included an arbitration feature to correct for these situations, and also allowed a master to use a ''[[burst mode (computing)|burst-mode]]''. Micro Channel cards had complete control for up to 12 [[millisecond]]s. This was long enough to permit the maximum number of other devices on the bus to [[data buffer|buffer]] inbound data from over-runnable devices like tape and communications.

Multiple bus-master support and improved arbitration mean that several such devices could coexist and share the system bus. Micro Channel bus-master-capable devices can even use the bus to talk directly to each other ([[peer-to-peer]]) at speeds faster than the system CPU, without any other system intervention. In theory, Micro Channel architecture systems could be expanded, like [[mainframe]]s, with only the addition of intelligent masters, without periodic need to upgrade the central processor.

Arbitration enhancement ensures better system throughput since control is passed more efficiently. Advanced interrupt handling refers to the use of level-sensitive interrupts to handle system requests.  Rather than a dedicated interrupt line, several lines can be shared to provide more possible interrupts, addressing the ISA-bus interrupt line conflict problems.

All interrupt request signals were "public" on Micro Channel architecture permitting any card on the bus to function as an I/O processor for direct service of I/O device interrupts. ISA had limited all such processing to just the system's CPU. Likewise, bus master request and grant signals were public, such that bus attached devices could monitor latency to control internal buffering for I/O processors. These features were not adopted for PCI, requiring all I/O support to come uniquely from the system board processor.

The final major Micro Channel architecture improvement was '''POS''', the ''Programmable Option Select'', which allowed all setup to take place in software. This feature is taken for granted now, but at the time setup was a huge chore for ISA systems. POS was a simple system that included device IDs in firmware, which the drivers in the computer were supposed to interpret. (This type of software-configuration system is known as ''[[plug and play]]'' today.) The feature did not really live up to its promise; the automatic configuration was fine when it worked, but it frequently did not - resulting in an unbootable computer - and resolving the problem by manual intervention was much more difficult than configuring an ISA system, not least because the documentation for the MCA device would tend to assume that the automatic configuration ''would'' work and so did not provide the necessary information to set it up by hand, unlike ISA device documentation which by necessity provided full details (however having to physically remove and check all [[interrupt request|IRQ]] settings, then find and set the new IRQ for a new device—if a suitable one was available—for ISA was no fun at all, and beyond many users... it is obvious why the attempt was made to move to software-arbitrated configuration, and why this was to later succeed in the form of [[Plug and play|PnP]].)