# Why PCI

### General overview

The PC-architecture has several BUS-Systems to choose from:

ISA
  cheap, slow (usualy 8Mhz), standard, many cards available>
EISA
  expensive, fast, some cards available>
MCA
  ex-ibm-proprietary, fast but not very wide-spread
VESA-Local-Bus
  based on ISA, cheap, fast, some cards available, not very standard>
PCI-Local-Bus
  expensive, fast, some cards available, the up-coming standard>

ISA/VESA-Local-Bus had some problems with high-frequenzy, and was not very relyable, but due to its low price and higher performance sold very well.

EISA was reliable, but rather expensive, more for power-users, and servers, not for the average user.

PCI now has the advantage, that it is like EISA not proprietary, fast as EISA (or even faster), 64bit-wide. This will be important with the i586 (they would rather like to read as Pentium).

PCI is not like ISA/Local-Bus prozessor-dependend. This means you can use the winner-1000-PCI in an Alpha-driven-PCI-board as well as in a i486/i586-driven PCI-Board).

PCI allows cheaper production of onboard-components, no glue-logic needed.

### Performance

taken from Craig Sutphin's Pro-PCI-Propaganda

> Unlike some local buses, which are aimed at speeding up graphics alone, the PCI Local Bus is a total system solution, providing increased performance for networks, disk drives, full-motion video, graphics and the full range of high-speed peripherals. At 33 MHz, the synchronous PCI Local Bus transfers 32 bits of data at up to 132 Mbytes/sec. A transparent 64-bit extension of the 32-bit data and address buses can double the bus bandwidth (264 Mbytes/sec) and offer forward and backwards compatibility for 32 and 64-bit PCI Local Bus peripherals. Because it's processor-independent, the PCI Local Bus is optimized for I/O functions, enabling the local bus to operate concurrent with the processor/memory subsystem. For users of high-end desktop PCs, PCI makes high reliability, high performance and ease of use more affordable than ever, no trivial task at 33 MHz bus-clock rates. Variable length linear or toggle mode bursting for both reads and writes improves write dependent graphics performance. By comprehending the loading and frequency requirements of the local bus at the component level, buffers and glue logic are eliminated.

### The onboard-SCSI-II-chip NCR53c810

One very nice feature of the PCI-Boards is the onboard-SCSI-II-chip, which is said to be as fast as the EISA-Adaptec-1742, but much cheaper. Drivers for DOS/OS2 are available. Linux does not have the driver yet, but Drew Eckard is working on it, the iX Multiuser Multitasking Magazine is supporting the driver development. It seems to be not yet stable-enough for release but people are already testing it... It is not easy to write the driver, because it is radicaly different from the normal NCR-Chip.

Until it is finished Linux-users have to disable the chip and use a cheap ISA-Card...

Drew said about the SCSI on PCI:

> Currently, your only PCI SCSI option that stands a chance of working is the Buslogic 946. It purports to be Adaptec 1540 compatable, like the EISA/VESA/ISA boards in the series.

> I'm working on getting the Linux NCR53c810 driver stable, some one else is doing the same thing for one of the BSD flavors. Fast busmaster, often included on motherboard implementations, supposedly available for $ 100 in card for sans BIOS (many of the PCI boards that don't include the NCR onboard still have the NCR BIOS).

> This covers the majority of PCI SCSI adapters on the market (Nexstor, Chaintech, Gigabyte, FIC, etc). Adaptec is shipping (see below) a FAST WIDE version of the AIC-7770 with a PCI interface, AIR is using it on their Pentium boards, but I haven't seen any board level product with it.

> Scott Ferris is working on the AIC-7770 driver under Linux, but I don't know of anyone doing the same thing under one of the BSDs.

> Emulux has a propriety FAST+WIDE PCI controller, it's unlikely that it will show up as supported under one of the BSDs or Linux anytime soon since the NCR based controllers are cheaper and more prevalant (even included on many mainboards which don't have a real price difference versus non SCSI equipped boards) and the Buslogic controllers are compatable with the 1540 so I doubt anyone will buy them.

> Forex is shipping a PCI SCSI adapter, I don't have details.

James Soutter (J.K.Soutter1@lut.ac.uk) asked me to add the following information on Fast-Wide-SCSI-2:

> Fast Wide SCSI-2 is sometimes incorrectly called SCSI-3. It differs from the normal Fast SCSI-2 (like the Adapted 1542B?) because it uses a 16 bit data bus rather than the more usual 8 bit bus. This improves the maximum transfer rate from 10 MB/s to 20 MB/s but requires the use of special Fast Wide SCSI-2 drives. The added performance of Fast Wide SCSI-2 will not necessarily improve the speed of your system. Most hard disk drives have a maximum internal transfer rate of less than 10 MB/s and so one drive alone can not flood a FAST SCSI-2 bus. In Segate's Oct 1993 product overview, only one Fast Wide SCSI-2 drive has an internal transfer rate of more than 10 MB/s (the ST12450W). Most of the drives have a maximum internal transfer rate of 6 MB/s or less, although the ST12450W is not the only exception to the rule. In conclusion, Fast Wide SCSI is designed for the file server market and will not necessarily benefit a single user workstation style system.

> Rather than buying a PCI system with a SCSI interface on the motherboard, or rather than waiting for the NCR driver, you could purchase a separate PCI based SCSI card. According to Drew, the only PCI SCSI option that stands a chance of working is the Buslogic 946. It purports to be Adaptec 1540 compatible, like the EISA/VESA/ISA boards in the series.

> Drew commented that other PCI based SCSI controllers are unlikely to be supported under Linux or the BSDs because the NCR based controllers are cheaper and more prevalent.

Ernst Kloecker (ernst@cs.tu-berlin.de) wrote:

> Talus Corporation has finished a NS/FIP driver for PCI boards with NCR SCSI. It will be shipping very soon, might even be fee because a third party might buy it of them and donate the driver to NeXT.

Not every PCI-Board has got the chip. ASUS does, and one of the J-Bond-boards does, too. Some vendors provide an alternative as you can read in Drews text...

The NCR-Chip is clever enough to work with drives formatted by other controllers, and should be no problem.