Jon Udell, Senior Technical Editor
Since the dawn of microcomputing, users and developers have jousted with one another to defend the honor of their chosen operating systems. The battle still rages; the dust hasn't even begun to settle. New contenders will exploit mainstream RISC workstations built around MIPS, Alpha, and PowerPC processors even as they ride the Intel performance escalator. But the grounds of the operating-system debate are subtly shifting. Microsoft, IBM, USL (Unix Systems Laboratories), Sun Microsystems, and others are rapidly converging on a set of common design themes--microkernels, objects, and personalities. The battle is no longer about whether to layer object-oriented services and emulation subsystems (i.e., personalities) on a small kernel. Everyone's doing that. The question isn't whether to build
an operating system in this style but how to do the job right.
MICROKERNELS
In Windows NT, layered subsystems communicate by passing messages through a microkernel. But NT doesn't follow the pure microkernel doctrine, which holds that all nonessential services should run in the processor's nonprivileged (user) mode. IBM, USL, and others say that NT's executive, a layer above the NT microkernel that runs security, I/O, and other services in privileged (kernel) mode, compromises NT's claim to be a microkernel-based system. Microsoft, however, notes that NT's privileged-mode executive subsystems communicate with each other and with the kernel by passing messages, just as its user-mode emulation subsystems do.
IBM's Mach-based Workplace OS, meanwhile, will adhere to the pure microkernel doctrine, relegating the pager, the scheduler, the security system, the file systems, and even major parts of its device drivers to user mode. With this approach, says IBM, its microkernel will be especially val
uable as a base that OEMs can customize for specific purposes. USL, however, says that its Chorus microkernel, which can run services in kernel mode or user mode, gives the best of both worlds. It can locate services in kernel mode for performance or in user mode for flexibility.
In "Small Kernels Hit It Big," Peter D. Varhol explores these and other issues across a range of microkernel-based systems. And in "The Chorus Microkernel," Dick Pountain takes a close look at the advanced technology chosen by USL as the foundation for future Unixes.
OBJECTS
As applications supporting Microsoft's OLE 2.0 begin to roll out, mainstream users are getting a glimpse of an object-oriented, document-centered style of computing in which applications function as components. Apple, IBM, and partners are countering with OpenDoc, a portable compound-document standard that will bring OLE-like benefits to a broader range of platforms than are supported by OLE. Apple says that OpenDoc's object technology, which
relies on IBM's groundbreaking System Object Model, or SOM, offers developers and users the full power of object-oriented programming--including inheritance--while remaining language-neutral. Microsoft says that OLE 2.0's Compound Object Model, which is closely aligned with C++ yet does not support inheritance, will nevertheless yield better results by requiring developers to articulate interfaces precisely and consistently.
On the horizon looms Taligent, an objects-from-the-ground-up system that IBM and Apple say will redefine computing. Meanwhile NextStep, available now on Intel and Motorola platforms, delivers the distributed-object technology that the others are all still talking about. In "Objects on the March," Peter Wayner explores some of the key issues in object and distributed-object computing.
PERSONALITIES
But will it run 1-2-3? For the new breed of operating systems, the answer is almost certainly yes, even on non-Intel hardware, thanks to a hybrid emulation strategy that offse
ts the inherent inefficiency of pure processor emulation by implementing GUI libraries in native RISC code. Applications lean heavily on GUI libraries nowadays; Windows and Mac libraries are appearing as "personalities" on a variety of new operating systems.
In "Personality Plus," Frank Hayes investigates how Microsoft's Windows NT and IBM's Workplace OS implement personalities. Frank also explores popular third-party solutions like Sun's Wabi (Windows Application Binary Interface), Insignia Solutions' SoftWindows, as well as Quorum Software Systems' Equal.
The Great OS Debate
