re ide vs scsi dma and detach in article apr julian uwo ca wlsmith valve heart rri uwo ca wayne smith writes in article richk apr gozer grebyn com richk grebyn com richard krehbiel writes can anyone explain in fairly simple terms why if i get os i might need an scsi controler rather than an ide will performance suffer that much for a mb or so drive if i don t have a tape drive or cd rom any help would be appreciated so when you ve got multi tasking you want to increase performance by increasing the amount of overlapping you do one way is with dma or bus mastering either of these make it possible for i o devices to move their data into and out of memory without interrupting the cpu the alternative is for the cpu to move the data there are several scsi interface cards that allow dma and bus mastering how do you do bus mastering on the isa bus ide however is defined by the standard at interface created for the ibm pc at which requires the cpu to move all the data bytes with no dma if we re talking isa at bus here then you can only have dma channel active at any one time presumably transferring data from a single device so even though you can have at least devices on a scsi bus explain how all of those devices can to dma transfers through a single scsi card to the isa at bus at the same time think it s the scsi card doing the dma transfers not the disks the scsi card can do dma transfers containing data from any of the scsi devices it is attached when it wants to an important feature of scsi is the ability to detach a device this frees the scsi bus for other devices this is typically used in a multi tasking os to start transfers on several devices while each device is seeking the data the bus is free for other commands and data transfers when the devices are ready to transfer the data they can aquire the bus and send the data on an ide bus when you start a transfer the bus is busy until the disk has seeked the data and transfered it this is typically a ms second lock out for other processes wanting the bus irrespective of transfer time also i m still trying to track down a copy of ibm s at reference book but from their pc technical manual page the fdd adapter is buffered on the i o bus and uses the system board direct memory access dma for record data transfers i expect to see something similar for the pc at hdd adapter so the lowly low density original pc fdd card used dma and the pc at hdd controller doesn t that makes real sense guy dawson hoskyns group plc guyd hoskyns co uk tel hoskyns uk guyd austin ibm com tel ibm austin usa
