Tandy 1000 Programmer's Reference Manual page 181

ChaDter 5 I Device Drivers
E
R
RESERVED U
R
0
Error
-
If an error occurs, MS-DOS sets the error code
MS-DOS will call BUILD BPB under the following conditions:
0
If "Media Changed" is returned
0
If "Not Sure" is returned and there are no dirty buffers
The BUILD BPB call also gets a pointer to a one-sector buffer.
What this buffer contains is determined by the NON IBM FOR-
MAT bit in the attribute field. If the bit is zero (device is IBM
format-compatible), then the buffer contains the first sector of
the first FAT. The FAT ID byte is the first byte of this buffer.
Note: The BPB must be the same, as far as location of the FAT is
concerned, for all possible media because this
first
FAT sector
must be read before the actual BPB is returned. If the NON
IBM FORMAT bit is set, then the pointer points to one sector of
scratch space (which may be used for anything).
accordingly.
B D
0
ERROR CODE (bit 15 on)
S N
179