1.ISA bus: Industry Standard Architecture bus (aka. AT bus)

          8/16 data bits, 24 address bits (16Meg addressable)

          8-8.33MHz, asynchronous ; 5.55M/s burst ; bus master support

          edge triggered TTL interrupts (IRQs) - no sharing ; low cost

 Comments: ideal for low to mid bandwidth cards, though lack of IRQs can quickly   

           become annoying.


2.MCA bus: Micro Channel Architecture bus ; 16/32 data bit, 32 address bits

           80M/s burst, synchronous ; full bus master capability good bus arbitration ;
           auto configurable ; IBM proprietary (not ISA/EISA/VLB compatible)

 Comments: Since MCA was proprietary, EISA was formed to compete with it.

           EISA gained much more acceptance; MCA is all but dead.


3.EISA bus: Enhanced Industry Standard Architecture bus

           32 data bits, 32 address bits ; 8-8.33MHz, synchronous

           32M/s burst (sustained) ; full bus master capability good bus arbitration ;  
           auto configurable sharable IRQs DMA channels backward compatible
           with ISA ; some acceptance outside of the PC architecture ; high cost

 Comments: EISA is great for high bandwidth bus mastering cards such as SCSI

           host adaptors, but its high cost limits its usefulness for other types of cards.


4.VESA Local Bus

       32 data bits, 32 address bits ; 25-40MHz, asynchronous

       130M/s burst (sustained is closer to 32M/s) ;

       bus master capability will coexist with ISA/EISA slot limited to 2 or

       3 cards typical backward compatible with ISA moderate cost.

 Comments: VLB is great for video cards, but its lack of a good bus arbiter limits its
           usefulness for bus mastering cards and its moderate cost limits its
           usefulness for low to mid bandwidth cards. Since it can coexist with
           EISA/ISA, a combination of all three types of cards usually works best.


5.PCI: Peripheral Component Interconnect local bus

      32 data bits (64 bit option), 32 address bits (64 bit option)up to 33MHz

      synchronous 132M/s burst (sustained) (264M/s with 64 bit option) full bus
      master capability ; good bus arbitration slot limited to 3 or 4 cards typical ;

      auto configurable will coexist with ISA/EISA/MCA as well as another PCI bus
      strong acceptance outside of the PC architecture moderate cost.

 Comments: The newest of the buses, combining the speed of VLB with the advanced

           arbitration of EISA.  Great for both video cards and bus mastering      
           SCSI/network cards.



1.Auto configurable: Allows software to identify the board's requirements and resolve any potential resource conflicts(IRQ/DMA/address/BIOS/etc).

2.Bus master support: Capable of First Party DMA transfers.

3.Full bus master capability: Can support any First Party cycle from any device, including another CPU.

4.Good bus arbitration: Fair bus access during conflicts, no need to back off unless another device needs the bus. This prevents CPU starvation while allowing a single device to use 100% of the available bandwidth. Other buses let a card hold the bus until it decides to release it and attempt to prevent starvation by having an active card voluntarily
release the bus periodically (bus on time) and remain off the bus for a period of time (bus off time) to give other devices, including the CPU, a chance even if they don't want it.

5.16Meg addressable: This limits first party DMA transfers to the lower 16 Meg of address space.There are various software methods to overcome this problem when more than 16 Megs of main memory are available. This has no effect on the ability of the processor to reach all of main memory

6.Backward compatible with ISA: Allows you to place an ISA card in the slot of a more advanced bus. Note, however, that the ISA card does not get any benefit from being in an advanced slot, instead, the slot reverts to an ISA slot. Other slots are unaffected.


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