ardent-blue
Experienced Member
- Joined
- Jan 2, 2015
- Messages
- 479
Did you mean to say "would MCA be able to allocate only part of its bandwidth to the ISA-derived card while it is active" ? There are a few different transfer types, all of which are derivatives of the basic transfer.Even if the expansion card is little more than ISA adapted to MCA, does that mean it fully saturates an MCA bus, or would MCA be able to allocate only part of its bandwidth to the ISA card while it is active?
MCA - Basic Transfer Procedure gives more details. All cards, regardless of performance, sit on the MCA expansion bus. Some cards are busmasters, and they can transfer data to/from other cards. Some are PIO, meaning that the system master works that card. Now your curiosity about bandwidth is one that I have NEVER seen quantified. So, I'll toss out some generalities [we're all in this together...].
An ISA-derived card will most likely NOT be a busmaster. Though ISA did have the capability, it required specific things to work dependably... To head off the exceptions, if you have a single manufacturer with their implementation, a busmaster could work on ISA. But running a mix of system and busmaster might not work as intended.
Now we look at PIO. This card will most likely be the embodiment of the ISA-derived card. If the design was ported over without taking into consideration of the faster speed and reduced timing of MCA, this card could fare badly at anything over the 10MHz basic transfer. -OR- this card could need the extended cycles. Consider a SCSI-1 device on a SCSI-2 controller, something just fine at 5MHz can't respond at 10MHz so you have slowed the effective bandwidth of the SCSI bus down. It doesn't saturate the SCSI bus, it just takes longer.
MCA allocates bus access according to priority and in some cases, fairness. It can handle the slowpoke without dragging the whole bus down. But if the ISA-derived card is sloppy, it might not arbitrate as fast as the other cards. Or it might try taking too much bus time [7.8mS once granted, maybe extended to 15.6mS]. Once I tried to figure out bus overhead, and using the 10MHz basic transfer, I scientifically guess that the arbitration process adds 2% overhead to data transfer performance.
To be mercifully terse, MCA would allocate bus access to the ISA-derived card just like all cards on the MCA bus.