ARCHIVED: What is MicroChannel Architecture (MCA)?

MicroChannel Architecture is IBM's answer to the failings of its older PC bus, which is now referred to as the ISA (Industry Standard Architecture) bus. The problems with the ISA bus grew as systems became faster and more powerful. The problems included: slow bus speed, a limited number of interrupts, lack of busmaster support, complex configuration and poor electrical grounding. Most of the problems were not significant early in the history of DOS computing, but came to the surface as the range of tasks and peripherals grew.

MicroChannel is a complete diversion from the original bus. Expansion cards are not interchangeable from the ISA to the MCA bus. The decision to break from the previous design and start clean eliminated the problems of the older cards along with preventing the use of a large installed base of cards. The decision also enabled IBM to break from the Intel CPU and build a bus capable of being used in machines with a variety of processors. Currently it is available from IBM in the upper end of its PS/2 line, the RS/6000 workstations, the AS/400 series and in the new 370 architecture machines. This one bus architecture covers an entire line of computing products, from PCs to mainframes.

Benefits of the MicroChannel Architecture include Programmable Option Select (POS), support for multiple busmasters, advanced interrupt handling, better grounding and bus arbitration. POS means that on a properly designed adapter for the MCA bus, all configuration can be done with software on a closed machine. This is a big step forward from the days of juggling jumpers in an ISA bus PC. A busmaster is a device which is capable of taking over control of the system bus of a computer. Multiple busmaster support and improved arbitration means that several such devices can coexist and share the system bus. MCA busmasters can even use the bus to talk directly to each other at speeds faster than the system CPU, without any other system intervention. Arbitration enhancement also provides that we have 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. It helps reduce the chance of lost interrupts, yields more flexible configuration and a more stable system. Better grounding helps eliminate one of the major hardware causes of system crashes - the generation of unwanted signals by an adapter board.

What do these things mean to a user? Well, it may only mean a more stable and easily configured system. Installing most option boards is as easy as plugging in the board and running a software setup. The shared interrupts can mean that modems on COM3 or COM4 work without any special setup. If you are a high-end system user, you may have more potential for benefit from the MCA bus. High-end systems have a greater need for fast I/O, which is better supported in the MCA bus. When compared to the Enhanced Industry Standard Architecture (EISA) bus, Computer Technology Review (April, 1992) had the following to say: "..the MCA bus, in its present incarnation (20 MHz), can transfer data at a theoretical maximum of 66MB/sec - twice as fast as EISA. In addition, its implementation of busmastering is technically superior: arbitration for the bus is faster, and the maximum block transfer size is 2KB, twice that of EISA. The only disadvantages of the MCA bus are political -it belongs to IBM, and is not as easily licensed as EISA." And as Winn L. Rosch said in the January 22, 1992 issue of PC Magazine, " ...with one of the top-performing MCA memory boards you can expect a loss of 20 to 25 percent of the system board memory throughput. In contrast, with memory boards for a 33-MHz EISA machines, there might be as much as a 75 percent throughput loss." The performance differences are real, but depending on what you do, they may not have much visible impact on your work.

What do these things cost for an end user? This answer depends on the exact system and peripherals being discussed. 386SX level MCA bus systems are available at prices comparable to the name-brand ISA bus clones. As we look at high-end machines, the difference grows significantly. Option boards are priced higher (with few exceptions) than ISA bus boards. The margin on many boards tends to be about $50-$100 over a similar ISA bus board. Busmastering boards tend to be somewhat more expensive. In the end, users must weigh the added cost of an MCA bus machine against how the benefits of the bus affect their needs.

This is document acae in the Knowledge Base.
Last modified on 2010-03-31 00:00:00.

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