ISA/PCI/AGP Bus Bandwidth Questions

[nformer]

I'm trying to find out the bandwidth of the 3 primary PC busses - ISA, PCI, and AGP. More importantly, I'm trying to learn WHY each bus has the bandwidth that it does. I am aware of the speeds (in MHZ) of the busses, but it obviously isn't nearly as simple as that.

I was presented with a logic problem: since the PCI bus runs at 33Mhz, very basic logic would dictate that the maximum bandwidth of the bus would be 33 Megabits/sec. Yet, one can run a 100 Mbit ethernet card on the PCI bus. How does this work?

Any help, in the form of direct replies or links to specific sources of information, would be greatly appreciated.

Paul
prhod626@wt.net

 

[Vegito]

Let me just throw some numbers around..
I thought PCI was 33Mhz at 133MB @ 32bits, AGP was 66Mhz @ 533MB @ 32bit or something like that.. not sure about ISA..

THats in megabits / sec


AGP 1x is 267, 2x is 533
USB is 1.5
IEEE 1394 is 50? not sure

ISA i think is 8

 

[xtreme2k]

lets start in PCI
33MHz with a 32bit BUS width this means
33 x 32 / 8 = 132MByte/s <- maximum bandwidth of PCI 33/32

AGP 1x
66MHz with a 32bit BUS width
66 x 32 / 8 = 264MByte/s <- maximum theoritical bandwidth of AGP 1x

AGP 2x
66MHz with a 32bit BUS width transfering twice per clock(per Hz)
66 x 32 / 8 x 2 = 528MByte/s

AGP 4x
66MHz with a 32bit BUS width transferring four times per clock (per Hz)
66 x 32 / 8 x 4 = 1056MByte/s

i forgot the bus speed and bus width of ISA

your logic of 33MHz must give 33Mbit/s is simply wrong
if it is only transferring 1bit of data per clock, then you would be right
however, 32bit bus width means 32bit of data is transferred per clock
giving PCI 33/32 a bandwidth of 33MHz x 32bit / 8bits per byte = 132MByte/s


edit: spelling

 

[rockstar]

Awesome explanation, xtreme2k. I always wondered how that worked.
Thanks for the knowledge!
-rockstar

 

[Buddha Bart]

ISA is 16bit @ 7mhz (it was designed to be 8mhz, but no one ever implemented it at that)

bart

 

[LocutusX]

ISA was 8-bit/16-bit... my first modem was 8-bit... was a half-size card with only half the number of lines =)