Technical data

3-2 Theory of Operation
The Model 400t has the following components:
• One DIO-II (Desktop computer Input/Output) bus slot for a monochrome or
color graphics controller board
• One ISA subset bus slot or HP-IB interface slot for a network or HP-IB con-
troller board
• Multiple interface ports
• Up to two internal disk drives
The Model 400dl is identical to the Model 400t with the exception of the CPU board.
The Model 400dl CPU board does not have some of the interface connectors and the
ISA and SGC logic that are available on the Model 400t CPU board. The Model 400dl
CPU board limits the Model 400dl configuration to a monochrome graphics controller
and an 802.3 LAN interface port, with no provisions for internal or external disk
drives, an ISA subset slot, or a HP-IB slot.
The Model 400s has the same core processor and features as the Model 400t, but is
more expandable. The Model 400s has the following features:
• More internal storage options that the Model 400t
• Two DIO-II interface slots standard
• Capability for an additional 3-slot DIO-II interface backplane or 4-slot ISA
AT-compatible backplane
3.1.1 Domain/OS Virtual Memory Operating System
The Domain operating system provides two important functions for the system:
• Runs several programs at the same time
• Runs programs that would otherwise be too large to fit in main memory
To support these functions, the operating system breaks each object (for example, pro
-
gram, file, directory) into “pages” of 4096 bytes and assigns each page a virtual address
(a unique identifier that the operating system uses to keep track of each page). The
operating system can then use the virtual addresses to place pages from one or more
objects at “physical addresses” in the system’s memory. By swapping pages in and out
of memory as required by the different objects, the operating system makes the limited
number of physical address locations appear virtually unlimited to the objects.