Micro Channel Architecture is an interface between a computer and
multiple computers and its expansion cards and their associated devices. MCA
was a distinct break from previous bus architectures such as Industry Standard
Architecture. The pin connections in MCA are smaller than other bus interfaces.
For this and other reasons, MCA does not support other bus architectures.
Although MCA offers a number of improvements over other bus architectures, its
proprietary, nonstandard aspects did not encourage other manufacturers to adopt
it.
It has influenced other bus designs and it is
still in use in PS/2s and in some minicomputer systems. The MCA bus was IBM's
attempt to replace the ISA bus with something "bigger and better".
When the 80386DX was introduced in the mid-80s with its 32-bit data bus, IBM
decided to create a bus to match this width. MCA is 32 bits wide, and offers
several significant improvements over ISA. The MCA bus has some pretty
impressive features considering that it was introduced in 1987, a full seven
years before the PCI bus made similar features common on the Pc. In some ways
it was ahead of its time, because back then the ISA bus really wasn't a major
performance limiting factor:
32 Bit Bus Width: The MCA bus features a full 32 bit bus width, the same width as
the VESA and PCI local buses. It had far superior throughput to the ISA bus.
Bus Mastering: The MCA bus supported bus mastering adapters for greater
efficiency, including proper bus arbitration.
Plug and Play: MCA automatically
configured adapter cards, so there was no need to fiddle with jumpers. This was
eight years before Windows 95 brought PnP into the mainstream!
MCA had a great deal of potential. Unfortunately, IBM made two
decisions that would doom MCA to utter failure in the marketplace. First, they made
MCA incompatible with ISA, this means ISA cards will not work at all in an MCA
system, one of the few categories of PCs for which this is true. The PC market
is very sensitive to backwards-compatibility issues, as evidenced by the number
of older standards that persist to this day. Second, IBM decided to make the
MCA bus proprietary. It in fact did this with ISA as well, however in 1981 IBM
could afford to flex its muscles in this manner, while by this time the clone
makers were starting to come into their own and weren't interested in bending
to IBM's wishes.
These two factors, combined with the increased cost of MCA
systems, led to the demise of the MCA bus. With the PS/2 now discontinued, MCA
is dead on the PC platform, though it is still used by IBM on some of its RISC
6000 UNIX servers. It is one of the classical examples in the field of
computing of how non-technical issues often dominate over technical ones. But
one of MCA's disadvantages is that it has poor DMA controller circuitry.
Features of Micro Channel
Architecture
I/O data transfers of 8-, 16-, 24-, or 32-bits within a 64KB address space (16-bit address
width).
• Memory data transfers of 8-, 16-, 24-, or 32-bits within a 16MB
(24-bit address width) or 4GB (32-bit address width) address space.
• An arbitration procedure that enables up to 15 devices and the
system master to bid for control of the channel.
• A basic transfer procedure that allows data transfers between
masters and slaves.
• A direct memory access (DMA) procedure that supports multiple
DMA channels. Additionally, this procedure allows a device to transfer data in
bursts.
• An optional streaming data procedure that provides a faster
data-transfer rate than the basic transfer procedure and allows 64-bit data
transfers.
• Address- and data-parity enable and detect procedures.
• Interrupt sharing on all levels.
• A flexible system-configuration procedure that uses programmable
registers.
• An adapter interface to the channel using:
o A 16-bit connector with a 24-bit address bus and a 16-bit data
bus
o A 32-bit connector with a
32-bit address bus and a 32-bit data bus
o An optional matched-memory
extension
o An optional video extension.
• Support for audio signal transfer (audio voltage-sum node).
• Support for both synchronous and asynchronous data transfer.
• An exception condition reporting procedure.
• Improved electromagnetic characteristics.