prindle@nadc@caip.RUTGERS.EDU (02/04/86)
From: prindle@NADC There are a couple of curious bugs in the 1571 disk drive DOS: 1. If the drive is in the 1541 mode, it can read any file wholly contained on the first side of a diskette formatted in 1571 mode - it can also write a file if there is room for it on the first side. However, doing a "validate" operation (v0 command), which updates the BAM, also screws up the "single/ double" side flag and makes the diskette appear to be in 1541 format (i.e. attempting to access the second side in 1571 mode will fail). Of course, now that you know this, and if it happens by accident, you can always use a disk sector editor to go fix up Track 18, sector 0 so the flag will again indicate double sided. Doing a "validate" in 1571 mode works ok, so I suspect this is less of a bug and more of a necessity for 1541 compatibility. 2. If the drive is in 1571 mode (double sided), attempting to write a new file using any channel other than 0 or 1 causes the drive to write much more slowly once it is writing on the second side of the diskette (ie. the disk is more than half full); the head constantly bounces back and forth between the track being written and track 18. It appears that the BAM is being updated once for every sector written instead of once for each track written as it is on the front side or if channel 1 is used. The file seems to still be written correctly, just *very* slowly. This is evident with many word processors (those which do not use PRG files), since they do not use channel 1 (having been warned not to in the 1541 manual). You can demonstrate this for yourself using the following BASIC program (remember, if you are using a C64 or the 128 in C64 mode, do an OPEN 15,8,15,"U0>M1":CLOSE15 first and make sure you have a 1571 formatted diskette in the drive) 10 open 2,8,2,"file1,s,w" 20 a$="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" 30 a$=a$+a$: a$=a$+a$ 40 for i=1to800 50 print#2,a$; 55 print i; 60 next 70 close 2 Now run this once on an empty diskette and you should get about a 400 block file on the front side of the diskette. Change the file name to "file2,s,w" and run it again. This time, after it writes about 250 blocks and fills up the front side, it will lapse into super slow mode. If you scratch both files or reformat the diskette, then repeat this exercise with statement 10 changed to open 2,8,1,"file1,s,w" (i.e. use channel 1, the write only channel), it will work without the slowdown when it writes the second file. Commodore customer service is aware of this and is informing engineering of the problem (a lot of good it'll do if they go bankrupt). 3. It is reported that the undocumented command "U0>H1", when issued to the 1571 when it is in 1541 mode, will cause subsequent 1541 operations to access the second side of the diskette as if it were a single sided 1541 diskette. While this may be true, if the diskette in the drive at the time has nothing written on the second side (i.e. it is either brand new or has only been for- matted by a 1541 in the past), the drive, upon getting this command, goes into a self destruct mode with much gnashing of internal moving parts and a lot of horrible sounding noise! If you must use this wierd mode, it would probably be best to only try it on a diskette that had been formatted on both sides, preferrably by the 1571 itself in 1571 mode. But formatting each side in 1541 format in this way yields a very strang diskette indeed: the second side cannot be read on a 1541 at all, and only on the 1571 by using 1541 mode and giving this strange command. So, you're probably better off not doing this, but just using 1571 double sided mode all the time you don't care about 1541 compatibility. Note that to get into 1571 mode from a C64, you *must* issue the "U0>M1" command to the command channel. 4. When the 1571 (in 1571 mode) tries to read a 1541 formatted diskette with nothing recorded on the second side, it acts like it is getting read errors for about 5-10 seconds before successfully reading (e.g. the directory). There is nothing wrong with this, I suppose, but it makes commercial software for the 128 (distributed on single sided disks for 1541 compatibility) take a bit long to load. Frank Prindle Prindle@NADC