Today, Apple released Mac OS 7.6.1 in three forms: four disk images for owners of Mac OS 7.6, five disk images for owners of PowerBook 3400s, and a full Mac OS CD-ROM. The disk images versions are freely available for downloading from Apple's Internet sites; getting physical copies is more complicated, but CD-ROM (for owners of the latest Macs) or floppy disk versions can be obtained through Apple's Mac OS Up-To-Date program (800/335-9258). For further information, see the files that accompany the online version of the update or URLs below.
Online, the Mac OS 7.6.1 Update is available as a net install or as disk images totalling about 6.5 MB. The 7.6.1 disk images use Apple's new NDIF format, so you must use the newly-revised DiskCopy 6.1.2 (itself a 1.1 MB download) to use the disk image (utilities like ShrinkWrap don't yet support Apple's new format). You don't need DiskCopy to use the net install version.
Currently, Mac OS 7.6.1 Update is only available for U.S. English system software. Apple says localized versions of the update should be available within 90 days.
So What Is It? Mac OS 7.6.1 is mostly an incremental OS update to support new Apple hardware, like the PowerBook 3400 and the Power Mac 4400, 5500, 6500, 7300, 8600 and 9600 models. It's not intended to offer new features or add items released since Mac OS 7.6, like Open Transport 1.1.2 or Macintosh Runtime for Java. However, it also contains a handful of subtle fixes that can be useful for owners of Mac OS 7.6. (For a detailed overview of Mac OS 7.6, see TidBITS-363). Remember: unless you buy the full CD-ROM version of 7.6.1, you must already own Mac OS 7.6 to upgrade to Mac OS 7.6.1.
About Those Type 11 Errors... The big talk about Mac OS 7.6.1 is the elimination of nearly all Type 11 crashes on PowerPC-based Macintoshes, which at first glance seems like a spectacular thing. Unfortunately, this has been widely misinterpreted in discussion forums and some press reports as a giant leap in the stability of Mac OS 7.6.1 over previous releases.
Here's the real story: before Mac OS 7.6.1, most crashes in Power-PC native code were mapped to the error number 11, which stands for a generic fatal error. Frequently, the Macintosh had another error code that accurately described the problem, but because the crash happened in PowerPC code the Mac couldn't do anything more precise, and users saw the number 11. The big change in Mac OS 7.6.1 is that errors in PowerPC code now map to the correct error numbers. This doesn't mean the crashes have gone away, but rather that the system software can now report them accurately.
So what? If the crash is still going to happen, what does it matter if a different error number is reported? The difference is in how the Mac can handle those errors. Without a low-level debugger like MacsBug installed, a Type 11 error forces the immediate restart of your computer: there's no opportunity to save work in other programs. Under Mac OS 7.6.1, most of these errors will simply cause the offending application to quit (resulting in a familiar "application has unexpectedly quit" dialog) rather than a complete restart of the machine. You should still restart your Mac after such an error (there's no telling what the crashed application left in memory), but now you'll be able to save work in other applications, eject disks, or make a quick backup copy of an important file before you restart. Yes, it's still a crash, but in many cases it'll be a nicer crash.
What Else Is There? Mac OS 7.6.1 also includes CFM-68K 4.0 (see TidBITS-369), which lets 68K Macs run software (like Cyberdog, Microsoft Internet Explorer 3.0.1, AOL 3.0, and LaserWriter 8.4) that requires the Code Fragment Manager. Support for CFM-68K was explicitly removed from Mac OS 7.6 due to potential serious problems, giving 68K owners little incentive to upgrade. Including CFM-68K 4.0 in Mac OS 7.6.1 lets Apple have a single version of the System software that offers comparable features for all 32-bit clean Macs (from the Mac IIci onwards). Mac OS 7.6.1 also includes Apple CD-ROM 5.3.3 (which supports high-speed IDE CD-ROM drives), Apple System Profiler 1.1.4, Apple Video Player 1.6 (now scriptable!), and improved software for PowerBook PC Cards.
Mac OS 7.6.1 also includes a library called ObjectSupportLib 1.2, which is important if you do any Macintosh scripting, or need to run scripts on your Mac. In recent months there has been a cacophony of versions, reversions, and regressions of ObjectSupportLib, which would be funny if all the shenanigans hadn't caused so much confusion. Complete, gory details are available in an Apple Tech Note, but the bottom line is that you should use ObjectSupportLib 1.2, which is also available with many third-party products like Eudora and Internet Explorer.
Miscellaneous Changes -- Mac OS 7.6.1 includes a few other changes and bug fixes, including updates (and a larger memory allocation) in the Process Manager, improved IDE driver support, serial communications improvements in some Performa models, a fix in the DR (68K) emulator from 7.5.5 that was accidently left out of Mac OS 7.6, and anyone who still has to use old 400K MFS floppy disks will find that they're read-only under Mac OS 7.6.1. Apple has made a complete list of updated components, changes, and known problems in Mac OS 7.6.1 available in a Tech Note, including workarounds for problems with LaserWriter 8.3.4 (use version 8.4) and using Virtual Memory with DayStar 040 Upgrade cards.
If you have a PowerBook 3400 and want to use both Mac OS 7.6.1 and Open Transport 1.1.2, install Open Transport 1.1.2 over System 7.6, then install the 3400s version of the 7.6.1 update, or you'll wind up with the wrong versions of some network resources.
Should You Install 7.6.1? If you own Mac OS 7.6 or one of the new machines Mac OS 7.6.1 is explicitly designed to support, then getting Mac OS 7.6.1 is probably a good idea. If you use or support a range of Macintoshes (including older 68K machines along with newer models) and need to have one comprehensive system release that will work well on all those machines, then the 7.6.1 Update is worth some thought if you've already upgraded to Mac OS 7.6. Otherwise, if you're looking for new features, bells, and whistles, it's probably best to wait for Mac OS 8 (Tempo), due to ship in July.