OSWORD &7F Memory Corruption
                      ============================

OSWORD &7F incorrectly assumes it owns the absolute workspace instead of
just the NMI workspace. Consequently, it corrupts certain memory locations
in page &10. This can cause problems with other software that owns the
absolute workspace and then calls OSWORD &7F. These are the locations
corrupted by various versions of DFS. Calling software should not touch
these locations and should not rely on their contents afterwards.

OSWORD &7F also corrupts many locations in zero page at &B0-&CF, but it is
quite easy to stack all 32 locations before calling OSWORD &7F and then
restore them all afterwards.

Acorn 0.90               Acorn DFS 1.20              Acorn DFS 2.10
----------               --------------              --------------
1072/3/4/5=address       1072/3/4/5=address          1087=7F
1080=01                  1080=01                     1088=04 drive 0/2 flag
10C9=00                  1082=FF                     1089    drive 1/3 flag
                                                     108B must be FF - drive 0 flag
                                                     108C must be FF - drive 1 flag
                                                     108D must be FF - drive 2 flag
                                                     108E must be FF - drive 3 flag
10D6=00                  1083=00                     10D3 b7=I own NMI
                         1085=b7 set                 10D6=Tube presence flag
                         10C8=00                     10DE must be b7=0 - drive 0 flag
                         10D3=A9                     10DF must be b7=0 - drive 1 flag
                         10D5=00                     10E0 must be b7=0 - drive 2 flag
                                                     10E1 must be b7=0 - drive 3 flag

Watford DFS 1.10/1.30/1.43/1.44        Watford DDFS 1.54
-------------------------------        -----------------
1042=00                                1030=02
1043=FF                                1036=80
1044 must be 00                        1038=63
104A=last drive used                   103F=0A
1078=drive 0 flag                      1042=00
1079=drive 1 flag                      1043=FF
107A=drive 2 flag                      1044 must be 00
107B=drive 3 flag                      104A=last drive used
1088=00/FF                             1078=drive 0 flag
1089=00/01                             1079=drive 1 flag
108A=00                                107A=drive 2 flag
                                       107B=drive 3 flag
                                       1088=88
                                       1089=00/01
                                       108A=00
                                       10A0/1/2/3=addr
                                       10A4=00
                                       10A5=01
                                       10A6=00
                                       10A7=01
                                       10A8=02
                                       10A9=03
                                       10AA=94