usenet@Apple.COM (USENET Administrator) (02/16/91)
In article <49151@apple.Apple.COM> cappella@Apple.COM (Mike Cappella) writes: > ...The basic problem >has to do with the floppy hardware being very dumb and intolerant of >interrupts. Typical systems have the 60HZ clock at a high priority >interrupt level (i.e. splhi), but we have ours set to spl1. This keeps >the floppy hardware from being interrupted in time critical portions. From: rick@crowbar.apple.com (Rick Auricchio) Path: crowbar!rick Though Mike is right the other time issues he points out, the floppy hardware (on all but the IIfx) is not exactly as he described. The floppy hardware can't be interrupted by anything, hence it runs at high spl during the entire sector data transfer time, dropping spl during seek, intersector gap, etc. As a result, not only do we see a time shift but other interrupts get poor service (e.g. network). As for the 60HZ being spl1, that's probably because the Mac OS doesn't use 60HZ for timing, but rather they always read the hardware clock. The 60HZ interrupt isn't important enough to Mac OS to run higher. The IIfx, because it has an IOP for the floppy (650x CPU + RAM), offloads the timing-critical code from the main CPU. -- Rick Auricchio rick@apple.COM Mooney N894AR (408) 974-4227 Apple Computer Inc, A/UX Engineering,10300 Bubb Rd, MS 50-UX Cupertino CA 95014 Work is for people who don't know how to fly. My opinion is my own. My employer? They use a windsock and a fire extinguisher.