The loader's turbo stage, which clears itself as its last act: an LD (HL),0 and an LDIR run zeros from here to the end of the block. The LDIR's own first byte is the last it clears; the processor then fetches the instruction again (an LDIR repeats by going back to itself), finds a NOP, and goes on through the LDIR's second byte (OR B) to the RET after it, which takes the last piece's return address off the stack: START. So 488 bytes are cleared, and the last two are not zeros. (Read from the loader's code in a snapshot taken where the turbo stage starts.)

The code names offset 289 (BUFFER_DC00) only as the end of the 512 bytes below it that SORT_AND_DRAW_BEHIND clears: pages $DA and $DB.