Meaning of MEMISOSTART and MEMISOSIZE

[wombat]

routine startup() refers to 2 symbols, MEMISOSTART and MEMISOSIZE:

if ((long)&link_bss_end > (MEMISOSTART + MEMISOSIZE))
            {
                started();
                shutdown();
            }
           
MEMISOSTART and MEMISOSIZE are defined for each camera model.
For A540 they are 0x9222D0 and 0x40000 respectively. Can someone please explain the
meaning of these 2 symbols and how they were obtained?

Is 0x40000 the size of some spare memory where CHDK can be loaded?

Is this the limit of such spare memory?

Regards
W