Cover by Toby Segaran, Jeff Hammerbacher

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The Three Tasks

In the VxWorks RTOS environment used for Phoenix, there really isn't anything that is synonymous to what a typical computer user might think of as individual programs. Nothing is loaded from a disk (there are no disks), and everything the computer will ever do is loaded into memory when the system first starts. It's actually all just one big program with a lot of smaller subprograms running more or less at the same time. These smaller subprogram activities are referred to as tasks, or threads, and they execute based on the availability of resources such as timed events or I/O (input/output) devices, and their assigned priority in the greater scheme of things (high-priority tasks get the chance to run more often than low-priority activities).

It was obvious from the outset that a minimum of two tasks would be needed for the surface image processing, one for each of the cameras. The SSI and RAC/OM were very different beasts, with different command sets and different operating characteristics. The SSI used all new controller hardware and incorporated CCD imagers identical to those used on the Mars Rovers. The RAC and OM imagers were originally built by the Max Planck Institute in Germany, and had been around for a while (one of the original designs was flown on the Huygens probe that landed on Titan). The RAC/OM controller hardware was actually a flight spare unit from the ill-fated Mars '98 mission, which apparently met a tragic end when its descent engines shut off ...

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