Persistent object oriented architectures have been researched for many years, deriving initially from the Manchester University Atlas machine. In reality however, few actual implementations of persistent architectures exist. In the first half of this paper an examination of four well known designs is examined, namely the SYSTEM/38, MONADS, MUTABOR, and the Rekursiv. Each machine’s object management model is explained, along with an analysis of the design decisions made. Following this, a discussion concerning the ideal persistent architecture is presented, suggesting design decisions which should be considered in any future persistent architecture. 1 Historical background The idea of architectural support for persistent programming derives ultimately from the work of Tom Kilburn and others [1] on the Manchester University Atlas computer. They introduced the idea of what was termed a single level store: a notion that is now more familiar to us as virtual memory. During the 1950s machines used a variety of different store technologies: Williams Tubes, mercury delay lines, magnetic cores and moving magnetic devices. Although these media differed considerably in their response times, they were all used in the same conceptual fashion, as the primary store of the

Linux 2.5.x has good support now for user-mode device drivers --- XFree being the biggest and most obvious --- but also there is support for user-mode input devices and for devices that hang off the parallel port. The motivations for user-mode device drivers are many: .

Now that Linux has fast system calls, good (and getting better) threading, and cheap context switches, it's possible to write device drivers that live in user space for whole new classes of devices. Of course, some device drivers (Xfree, in particular) have always run in user space, with a little bit of kernel support. With a little bit more kernel support (a way to set up and tear down DMA safely, and a generalised way to be informed of and control interrupts) almost any PCI bus-mastering device could have a user-mode device driver.