FileStore Bitmaps
Here's how to do the calculation: we need to work out which bits are set in the bytes from the bitmap sectors. We can immediately discount all the zero bytes as these have no bits set meaning that there are no free sectors. The only non-zero bytes in the bitmap sectors for floppy drive 4 are the 7 listed here.
ANDing each these numbers with 1,2,4,8,16,32,64 and 128 tells us, where the answer isn't zero, that there is a free sector, so the sum of set bits for these 7 bytes is 3+4+3+8+2+2+1 = 23 free sectors on our floppy disc.
23 * 256 = 5,888 which is the free space reported by *FREE.