Int 21.7303 returns one too many in count of total clusters

[ecm-pushbx]

This is where function 7303h uses dpb_size or dpb_xsize:

kernel/kernel/dosfns.c

Line 948 in 5de2eb1

(ISFAT32(dpbp) ? dpbp->dpb_xsize : dpbp->dpb_size);

But function 36h uses dpb_xsize minus one:

kernel/kernel/dosfns.c

Line 835 in 5de2eb1

ntotal = dpbp->dpb_xsize - 1;

Or dpb_size minus 1 on FAT12/FAT16:

kernel/kernel/dosfns.c

Line 859 in 5de2eb1

*nc = dpbp->dpb_size - 1;

The field is marked as "number of clusters plus 1":

kernel/hdr/dcb.h

Line 49 in 5de2eb1

UWORD dpb_size; /* # of clusters+1 on media */

In other words the DPB field gives the maximum valid cluster number (0 and 1 are reserved), plus one gives amount of FAT entries, plus one minus two = minus one gives amount of data clusters.