> On 15 Aug, 2016, at 07:29, V. Sigma <[email protected]> wrote: > > Does the slow controller for the SCSI CD drive mean that loading stuff off > CDs takes forever though? That's what I'm worried about...
Not so you’d notice. The SCSI chip in most of the 68K Macs is the same (https://en.wikipedia.org/wiki/NCR_5380) and the Mac can usually get at least 1MB/sec through it. The 5380 uses the most basic “asynchronous” mode of the SCSI bus, so it proceeds at a pace amenable to the slowest device - usually the host computer. Although the 5380 does actually support DMA transfers independently of the host CPU, this functionality is inexplicably not used on the Mac, which instead uses it in PIO mode. The CPU babysits each packet transfer once signalled by an interrupt, and this is fast enough to keep up with the asynchronous SCSI protocol. For such old machines, on which software and data is much smaller than we’re used to today, 1MB/sec is actually pretty fast; for context, it means reading a full CD-ROM in about ten minutes, which is rather faster than a double- or quad-speed drive typical of the era could actually manage. Some of the better Quadra models had a “Fast SCSI” chip instead, usually a 53C94 or 53C96, which supported 10MB/sec or 20MB/sec synchronous modes. This *did* have the DMA functionality wired up, mainly because PIO doesn’t work reliably with synchronous SCSI modes; the main advantage of DMA however was that it allowed more efficient use of the CPU by increasing amounts of software which had learned about background I/O. The 5380 is “slow” in comparison to the 53C9x series, but remained adequate for basic Mac software. - Jonathan Morton -- -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/ --- You received this message because you are subscribed to the Google Groups "Vintage Macs" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
