From: joemonk64@gmail.com   
      
   > > > Ok, good point. I don't really understand drivers.    
      
   You do, you just dont think you do.   
      
   MVS has channels.   
   Channels have control units   
   Control Units have devices.   
      
   MVS speaks to channels thru channel control words ("API")   
   Channels talk to devices thru control units ("drivers")   
   control units drive the devices.   
      
   MVS almost never speaks directly to the device. It always talks thru the API   
   ("ccw") to the controller, which then speaks to the device.   
      
   Think of a 3270 terminal. Read Write Read Modified Write Erase. Those are all   
   3270 data stream commands. When MVS wants to talk to a terminal, it sends a   
   read or write to the channel, which then speaks to the 3x74 control unit, and   
   the 3x74 control unit    
   sends then 3270 data stream to the end device.   
      
   In PC terms, think of a SCSI disk. There is no native support in BIOS for a   
   SCSI disk. The SCSI controller driver hooks the bios interrupt, and puts   
   itself first in the chain, so all the disk interrupts go thru the SCSI driver.   
   If it is a SCSI command,    
   then the driver will use the interface (ASPI?) to talk to the disk. Otherwise,   
   the driver will simply jump to the routine in ROM.   
      
   Joe   
      
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