Slotting the Images
I mentioned earlier that the amount of memory available to each of the various instrument tasks was limited, but just how limited may be surprising to some, given that it is now commonplace to find 500 megabytes or even a gigabyte (or more) in a desktop PC. The initial memory allocation to both the SSI and RAC/OM tasks for image data storage was 230K short of a full 10 megabytes (10,255,360 bytes, to be exact). There was discussion of increasing this after the spacecraft landed, which meant that any memory management scheme had to be flexible, but this was the design baseline. The default storage scheme needed to be able to handle at least four SSI images (or two pairs, consisting of one image for each "eye") and at least four RAC/OM images, all in the same memory space. The odd size meant that it wouldn't be possible to squeeze in more than four full-size SSI images, at least not initially.
In embedded systems, the use of dynamic memory allocation is usually considered to be a Really Bad Idea. To avoid issues with fragmented memory, memory leaks, null pointers, mystery crashes, and the possibility of losing the mission completely, the use of dynamic memory allocation (C's
malloc function and its kin) was forbidden by the flight software coding rules. This meant that the imaging software had to manage the image data itself within whatever amount of memory was assigned to it, and it had to be robust and reliable.
The solution was the use of a set of functions that ...