DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the invention will now be described, by way of a non-limiting example, with reference to the drawings, in which FIGS.
4, 8, 9, 22 to 24 and 48 are of major relevance to the present invention.
Hardware Overview FIGS.
1 to 3 show three different hardware configurations of computer systems embodying the invention.
Referring firstly to FIG.
1, a host computer 10 has its own backplane in the form of a VME bus 12 which provides general purpose communications between various circuit boards of the computer, such as processor, memory and disk controller boards.
To this known configuration, and within a standard housing 14 for the computer 10, there is added a board on which is provided a renderer 16 and a video processor 18, a Futurebus+ 20, and a front-end board 22.
The renderer 16 is connected to the VME bus 12 and the Futurebus+ 20, and also communicates with the video processor 18, which in turn drives an external color monitor 24 having a high-resolution of, for example 1280.
times.
1024 pixels.
The front-end board 22 is also connected to the Futurebus+ 20 and can communicate with a selection of peripherals, which are illustrated collectively by the block 26.
The configuration of FIG.
1 is of use when the host computer 10 has a VME backplane 12 and there is sufficient room in the computer housing 14 for the renderer 16, video processor 18, Futurebus+ 20 and front-end board 22, and may be used, for example, with a `Sun Workstation`.
In the case where the computer housing 14 is physically too small, or where the host computer 10 does not have a VME or Futurebus+ backplane, the configuration of FIG.
2 may be employed.
In FIG.
2, a separate housing 28 is used for the renderer 16, video processor 18, front-end board 22 and Futurebus+ 20, as described above, together with a VME bus 12 and a remote interface 30.
In the host computer housing 14, a host interface 32 is connected to the backplane 34 of the host computer 10, which may be of VME, Qbus, Sbus, Multibus II, MCA, PC/AT, etc
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I/O Systems 
