@candle_86: Interesting find about AGTL, but I'm guessing this wouldn't be the name of the bus? It seems to be the electrical s […]
@candle_86: Interesting find about AGTL, but I'm guessing this wouldn't be the name of the bus? It seems to be the electrical specification, like the ISA bus uses TTL? So if I understand correctly, you wouldn't call the PPro-P3 bus AGTL for the same reason you wouldn't call the 8086 bus TTL, right?
@Scali: When you say "...the CPU, FPU and memory could run at much faster speeds on another bus", how should you refer to these buses? I agree that e.g. a 20MHz 286 must be decoupled from the 8MHz ISA bus, but I'm unclear on how exactly is this done. The 286 machines I have have an oscillator that's double the CPU speed, so it makes me think the bus they use - whatever it's called - runs at double the CPU speed and effectively the CPU has a 0.5x multiplier. But finding a name for that non-ISA CPU bus eludes me for these early machines.
So what I have now, for the eras I'm interested in, are:
- 8086/8088: ISA @ 4.77MHz, ? for later turbo XT boards
- 286: ?
- 386: ? @ 12-40MHz
- 486: Maybe VLB? @ 16-50MHz
- Pentium 1: ? @ 50/60/66MHz
- Pentium Pro: FSB @ 60/66MHz
- Pentium II: FSB @ 66/100MHz
- Pentium III: FSB @ 100/133MHz
The 8086/8088, all the way to the Pentium 4 may have an ISA bus, but on the 486/Pentium they are most certainly on a bridge chip connected to south bridge of the FSB (Front Side Bus) For all intents everything 486+ has a Front-Side Bus, but the physical CPU only communicates accross it to the bridge chips, not expansion cards. The chipset, which the North Bridge connects the RAM to the CPU, and the South Bridge connects the PCI/ISA bridge to the North Bridge.
This has a diagram: https://en.wikipedia.org/wiki/Northbridge_(computing)
The only weird "buses" on the 80x86 chips are the names the FSB go by and the link between the North Bridge and South Bridge chips. With the Sandy bridge chipsets, the northbridge was integrated into the CPU. AMD did the "integrate the memory controller" bit first. You'll see DMI(P4), QPI(Servers), and so forth. Of note, the i820 chipset is the odd man out, when Intel was attempting to force RAMBUS on the P4 generation and every single i820 MTH (to connect non RAMBUS RAM) failed.
AGTL+ is the actual electrical technology that the FSB uses, and isn't the bus name used by marketing itself. QPI is Intel's answer to HyperTransport, while previously DMI (Direct Media Interface) was the bus between the north and south bridge since 2004. For all intents, when you want to talk about computer buses, it goes "connectivity" with a port or connector eg Serial, Parallel, ISA, PCI, VLB, PCI Express, AGP, USB, Firewire, Thunderbolt. Or internal buses with protocols (eg QPI/DMI/HT.) AMD used Alpha's EV6 before Hypertransport.
Prior to the 486, you had some competing replacements for ISA. So you had ISA-PnP, EISA, and MCA which were not real improvements, just extensions or in the case of MCA, IBM trying to use it's brand as leverage to regain control over something they can license. Which in a way is ironic because that's exactly what Intel did to AMD (AMD couldn't make Pentum clones, but their 386 and 486 chips worked intel boards), and nVidia (which is why you don't see any nVidia chipsets for intel after 2008.) When AMD bought ATI in 2006, that locked nVidia completely out of the motherboard game.
Anyway, that's why on new machines you often see things like "PCI Express to PCI bridge" "PCI to PCI bridge" and "PCI to ISA Bridge" much in the same way there are bridge adapters for SATA/PATA and USB/Serial/Parallel ports.
VLB was short lived and found only on 486's. It was also the reason why PCI came out (which was also on 486's, but all Pentium systems) Because VLB operated at the FSB speed, while PCI was set at 33Mhz. At the time VLB was 66Mhz and only used as an extension of the motherboard (eg the hard drive and video card were on it) where as in PCI motherboards, the hard drive controller became integrated (I can not ever recall seeing a PCI hard drive controller since all Pentium systems seemed to have all the Super IO integrated.) Sound cards on the other hand stayed on ISA for a super-long time.
I'm not sure what the 8087/80287/80387's were connected by if they were even used (I've never seen a separate FPU chip installed on any system, the sockets were always empty.)