When writing to a path block the Draw module uses the word after the end of path marker to store the size of the buffer. As data is written and the the end of path marker is moved this value is updated, and the SWI returns a pointer to the new position of the end of path marker. This allows a second SWI to be called to append to the path, with no need for the program to keep track of the free space in the buffer.
Hence before calling any SWIs that write a path to a buffer the first two words should be initialised:
Offset Value
0 0 (end of path marker)
4 size of buffer
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