>> The spec, as I know it, is 10 loads per bus.  A slot is considered
>> two loads.  This is the reason why you usually only see 4 PCI slots
>> in a system.
>
>I don't know which specification you might be thinking of, but the PCIMG
>PCI/ISA specification only allows for 4 PCI slots.  Period.

PCIMG is not the same as the PCI spec.  PCIMG (a spec for Single Board
Computers to access backplanes) only gives you 4 IDSEL lines, correct.  The
PCI spec, however, allows for up to 10 chip loads on a bus and I have Pluto
MBs here that in fact have 8, on-board, devices on a given bus.

Copyright (c) 1992-1997 FreeBSD Inc.
Copyright (c) 1982, 1986, 1989, 1991, 1993
	The Regents of the University of California. All rights reserved.
FreeBSD 3.0-CURRENT #16: Fri Oct  2 13:17:53 MDT 1998
    kelly@rocky101.plutotech.com:/usr/src/sys/compile/DEVELOPMENT
CPU: Pentium Pro (300.01-MHz 686-class CPU)
  Origin = "GenuineIntel"  Id = 0x633  Stepping=3
  Features=0x80fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,MMX>
real memory  = 67108864 (65536K bytes)
avail memory = 63188992 (61708K bytes)
Enabled SERR# and IOCHK# NMIs
Probing for devices on the ISA bus:
sc0 at 0x60-0x6f irq 1 on motherboard
sc0: VGA color <16 virtual consoles, flags=0x0>
nsu0 at 0x3f8-0x3ff irq 4 flags 0x10080 on isa
nsu0: type 16550A
nsu1 at 0x2f8-0x2ff irq 3 flags 0x10080 on isa
nsu1: type 16550A
psm0 at 0x60-0x64 irq 12 on motherboard
psm0: device ID 0
mbintr0 at ? irq 9 on isa
wdog0 at ? maddr 0xdfff0 msize 16 on isa
Disabling Watchdog Timer
iic0 at ? maddr 0xdfff0 msize 16 on isa
weather0 at ? maddr 0xdfff0 msize 16 on isa
power0 at ? maddr 0xdfff0 msize 16 on isa
	Power Supply 1: DC good
	Power Supply 2: DC bad
dled0 at ? maddr 0xdfff0 msize 16 on isa
pled0 at ? maddr 0xdfff0 msize 16 on isa
sysstat0 at ? maddr 0xdfff0 msize 16 on isa
	wdog_reset = 0, forced_reset = 0
sercon0 at ? maddr 0xdfff0 msize 16 on isa
smpte0 at ? maddr 0xdfff0 msize 16 on isa
xilinx0 at ? maddr 0xdfff0 msize 16 on isa
nvram0 at ? maddr 0xd0000 msize 1008 on isa
nvram0: size is 8192 bytes
bpeeprom0 not found
npx0 on motherboard
npx0: INT 16 interface
Probing for devices on PCI bus 0:
bridge0: <DEC 21150 PCI-PCI bridge> rev 0x02 on pci0.20.0
bridge1: <DEC 21150 PCI-PCI bridge> rev 0x02 on pci0.19.0
bridge2: <DEC 21150 PCI-PCI bridge> rev 0x02 on pci0.18.0
de0: <Digital DC21143 Fast Ethernet> rev 0x30 int a irq 10 on pci0.17.0
de0: waking device from sleep/snooze mode
de0: 21143 Eval. Board w/ SYM PHY 21143 [10-100Mb/s] pass 3.0
de0: address 00:e0:12:00:00:90
vga0: <VGA-compatible display device> rev 0x64 int a irq 255 on pci0.12.0
chip0: <Intel 82371AB PCI to ISA bridge> rev 0x01 on pci0.7.0
chip1: <Intel 82371AB USB host controller> rev 0x01 int d irq 255 on pci0.7.2
chip2: <Intel 82371AB power management> rev 0x01 on pci0.7.3
chip3: <Host to PCI bridge (vendor=8086 device=7192)> rev 0x02 on pci0.0.0
Probing for devices on PCI bus 3:
acv_probe: cannot match device_id 0x3
eram0: <External RAM & Parity Engine> rev 0x00 int a irq 10 on pci3.15.0
	eram0: phys_size = 32 MB
	set 9080 threshold register to 0x800
ahc0: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci3.14.0
ahc0: Using left over BIOS settings
ahc0: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc1: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci3.13.0
ahc1: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc1: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc2: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci3.10.0
ahc2: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc2: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc3: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci3.9.0
ahc3: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc3: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc4: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci3.6.0
ahc4: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc4: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc5: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci3.5.0
ahc5: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc5: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc6: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci3.2.0
ahc6: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc6: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc7: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci3.1.0
ahc7: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc7: Host Adapter Bios disabled.  Using default SCSI device parameters
Probing for devices on PCI bus 2:
acv_probe: cannot match device_id 0x2
eram1: <External RAM> rev 0x00 int a irq 10 on pci2.14.0
	eram1: phys_size = 32 MB
ahc8: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci2.13.0
ahc8: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc8: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc9: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci2.12.0
ahc9: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc9: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc10: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci2.9.0
ahc10: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc10: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc11: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci2.8.0
ahc11: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc11: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc12: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci2.5.0
ahc12: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc12: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc13: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci2.4.0
ahc13: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc13: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc14: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci2.1.0
ahc14: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc14: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc15: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci2.0.0
ahc15: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc15: Host Adapter Bios disabled.  Using default SCSI device parameters
Probing for devices on PCI bus 1:
avioII0: <Pluto A/V I/O II (270 Mbps)> rev 0x00 int a irq 11 on pci1.14.0
	AVIO I/O memory size is 0x1000000, limiting to 0x800000
acv_probe: cannot match device_id 0x2
eram2: <External RAM> rev 0x00 int a irq 10 on pci1.13.0
	eram2: phys_size = 32 MB
ahc16: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci1.12.0
ahc16: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc16: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc17: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci1.11.0
ahc17: aic7880 Single Channel A, SCSI Id=7, 16/255 SCBs
ahc17: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc18: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 15 on pci1.8.0
ahc18: aic7880 Wide Channel A, SCSI Id=7, 16/255 SCBs
ahc18: Host Adapter Bios disabled.  Using default SCSI device parameters
ahc19: <Adaptec aic7880 Ultra SCSI adapter> rev 0x00 int a irq 14 on pci1.7.0
ahc19: aic7880 Wide Channel A, SCSI Id=7, 16/255 SCBs
ahc19: Host Adapter Bios disabled.  Using default SCSI device parameters
raid: Pseudo disk driver for user RAID
de0: enabling 100baseTX port
da1 at ahc1 bus 0 target 0 lun 0
da1: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da1: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da1: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da3 at ahc3 bus 0 target 0 lun 0
da3: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da3: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da3: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da5 at ahc5 bus 0 target 0 lun 0
da5: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da5: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da5: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da7 at ahc7 bus 0 target 0 lun 0
da7: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da7: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da7: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da9 at ahc9 bus 0 target 0 lun 0
da9: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da9: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da9: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da11 at ahc11 bus 0 target 0 lun 0
da11: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da11: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da11: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da0 at ahc0 bus 0 target 0 lun 0
da0: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da0: 10.0MB/s transfers (10.0MHz, offset 15), Tagged Queueing Enabled
da0: 8683MB (17783112 512 byte sectors: 64H 32S/T 8683C)
da2 at ahc2 bus 0 target 0 lun 0
da2: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da2: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da2: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da4 at ahc4 bus 0 target 0 lun 0
da4: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da4: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da4: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da6 at ahc6 bus 0 target 0 lun 0
da6: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da6: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da6: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da8 at ahc8 bus 0 target 0 lun 0
da8: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da8: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da8: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da10 at ahc10 bus 0 target 0 lun 0
da10: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da10: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da10: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)
da12 at ahc12 bus 0 target 0 lun 0
da12: <SEAGATE ST19171N 0023> Fixed Direct Access SCSI2 device 
da12: 20.0MB/s transfers (20.0MHz, offset 15), Tagged Queueing Enabled
da12: 8683MB (17783112 512 byte sectors: 255H 63S/T 1106C)

>Perhaps there are other specs that allow more slots, but my reading in
>various trade journals has revealed that some/most/many experts feel
>that putting any more than four devices on a PCI bus will risk serious
>reliability problems too.

More than four 'slots' (i.e. 8 loads + 1 for the PCI-Host adapter) 
is certainly a problem unless you are running something like compact
PCI.  But you can have more devices assuming they are 'on-board'
and only constitute a single bus load each.

>CompactPCI (also as defined by PICMG) allows for a maximum of 8 slots on
>the backplane (without having a PCI-PCI bridge), and of course one of
>those slots it always the "system" slot for the SBC.  (This is one of
>the reasons that operating systems which support PCI based motherboards
>may not instantly also support CompactPCI systems containing otherwise
>nearly equivalent hardware.

Because they bogusly limit how many devices they will probe per bus?

--
Justin