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*** D71 (Electronic form of a double-sided 1571 disk)
*** Document revision: 1.5
*** Last updated: Nov 7, 2008
*** Compiler/Editor: Peter Schepers
*** Contributors/sources: unknown
Similar to the D64 (1541), the 1571 drive can operate in either
single-sided (1541 compatible) mode or double-sided (1571) mode. In this
document I will be dealing with the double-sided mode only. For the
breakdown of the single-sided mode, read the D64.TXT document.
The D71 has 70 tracks, double that of the 1541, with a DOS file size of
349696 bytes. If the error byte block (1366 bytes) is attached, this makes
the file size 351062 bytes. The track range and offsets into the D71 files
are as follows:
Track Sec/trk # Sectors
-------------- ------- ---------
1-17 (side 0) 21 357
18-24 (side 0) 19 133
25-30 (side 0) 18 108
31-35 (side 0) 17 85
36-52 (side 1) 21 357
53-59 (side 1) 19 133
60-65 (side 1) 18 108
66-70 (side 1) 17 85
---
total 1366
Track #Sect #SectorsIn D71 Offset Track #Sect #SectorsIn D71 Offset
----- ----- ---------- ---------- ----- ----- ---------- ----------
1 21 0 $00000 36 21 683 $2AB00
2 21 21 $01500 37 21 704 $2C000
3 21 42 $02A00 38 21 725 $2D500
4 21 63 $03F00 39 21 746 $2EA00
5 21 84 $05400 40 21 767 $2FF00
6 21 105 $06900 41 21 788 $31400
7 21 126 $07E00 42 21 809 $32900
8 21 147 $09300 43 21 830 $33E00
9 21 168 $0A800 44 21 851 $35300
10 21 189 $0BD00 45 21 872 $36800
11 21 210 $0D200 46 21 893 $37D00
12 21 231 $0E700 47 21 914 $39200
13 21 252 $0FC00 48 21 935 $3A700
14 21 273 $11100 49 21 956 $3BC00
15 21 294 $12600 50 21 977 $3D100
16 21 315 $13B00 51 21 998 $3E600
17 21 336 $15000 52 21 1019 $3FB00
18 19 357 $16500 53 19 1040 $41000
19 19 376 $17800 54 19 1059 $42300
20 19 395 $18B00 55 19 1078 $43600
21 19 414 $19E00 56 19 1097 $44900
22 19 433 $1B100 57 19 1116 $45C00
23 19 452 $1C400 58 19 1135 $46F00
24 19 471 $1D700 59 19 1154 $48200
25 18 490 $1EA00 60 18 1173 $49500
26 18 508 $1FC00 61 18 1191 $4A700
27 18 526 $20E00 62 18 1209 $4B900
28 18 544 $22000 63 18 1227 $4CB00
29 18 562 $23200 64 18 1245 $4DD00
30 18 580 $24400 65 18 1263 $4EF00
31 17 598 $25600 66 17 1281 $50100
32 17 615 $26700 67 17 1298 $51200
33 17 632 $27800 68 17 1315 $52300
34 17 649 $28900 69 17 1332 $53400
35 17 666 $29A00 70 17 1349 $54500
The directory structure is the same as a D64/1541. All the same filetypes
apply, the directory still only holds 144 files per disk and should only
exist on track 18.
The first two bytes of the sector ($12/$04 or 18/4) indicate the location
of the next track/sector of the directory. If the track value is set to
$00, then it is the last sector of the directory. It is possible, however
unlikely, that the directory may *not* be competely on track 18 (some disks
do exist like this). Just follow the chain anyhow.
When the directory is done, the track value will be $00. The sector link
should contain a value of $FF, meaning the whole sector is allocated, but
the actual value doesn't matter. The drive will return all the available
entries anyways. This is a breakdown of a standard directory sector and
entry:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII
----------------------------------------------- ----------------
00: 12 04 82 11 00 4A 45 54 20 53 45 54 20 57 49 4C <20><><EFBFBD>.<2E>JET<45>SET<45>WIL
10: 4C 59 A0 A0 A0 00 00 00 00 00 00 00 00 00 2B 00 LY<4C><59><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>+<2B>
20: 00 00 82 0F 01 4A 53 57 20 31 A0 A0 A0 A0 A0 A0 <20><><EFBFBD>..JSW<53>1<EFBFBD><31><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
30: A0 A0 A0 A0 A0 00 00 00 00 00 00 00 00 00 BF 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
40: 00 00 82 06 03 53 4F 4E 20 4F 46 20 42 4C 41 47 <20><><EFBFBD>..SON<4F>OF<4F>BLAG
50: 47 45 52 A0 A0 00 00 00 00 00 00 00 00 00 AE 00 GER<45><52><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
60: 00 00 82 15 0D 50 4F 54 54 59 20 50 49 47 45 4F <20><><EFBFBD>..POTTY<54>PIGEO
70: 4E A0 A0 A0 A0 00 00 00 00 00 00 00 00 00 A2 00 N<><4E><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
Bytes: $00-1F: First directory entry
00-01: Track/Sector location of next directory sector ($00/$FF if
its the last sector)
02: File type.
Typical values for this location are:
$00 - Scratched (deleted file entry)
80 - DEL
81 - SEQ
82 - PRG
83 - USR
84 - REL
Bit 0-3: The actual filetype
000 (0) - DEL
001 (1) - SEQ
010 (2) - PRG
011 (3) - USR
100 (4) - REL
Values 5-15 are illegal, but if used will produce
very strange results. The 1541 is inconsistent in
how it treats these bits. Some routines use all 4
bits, others ignore bit 3, resulting in values
from 0-7.
Bit 4: Not used
Bit 5: Used only during SAVE-@ replacement
Bit 6: Locked flag (Set produces ">" locked files)
Bit 7: Closed flag (Not set produces "*", or "splat"
files)
03-04: Track/sector location of first sector of file
05-14: 16 character filename (in PETASCII, padded with $A0)
15-16: Track/Sector location of side-sector block (REL file only)
17: REL file record length (REL file only)
18-1D: Unused (except with GEOS disks)
1C-1D: Track/sector of replacement file (only used during an
@SAVE or an @OPEN command)
1E-1F: File size in sectors, low/high byte order ($1E+$1F*256).
The approx. filesize in bytes is <= #sectors * 254
20-3F: Second dir entry. From now on the first two bytes of each
entry in this sector should be $00/$00, as they are
unused.
40-5F: Third dir entry
60-7F: Fourth dir entry
80-9F: Fifth dir entry
A0-BF: Sixth dir entry
C0-DF: Seventh dir entry
E0-FF: Eighth dir entry
When the 1571 is in is native ("1571") mode, files are stored with a
sector interleave of 6, rather than 10 which the 1541 (and the 1571 in
"1541" mode) uses. The directory still uses an interleave of 3.
*** Non-Standard & Long Directories
Most Commdore floppy disk drives use a single dedicated directory track
where all filenames are stored. This limits the number of files stored on a
disk based on the number of sectors on the directory track. There are some
disk images that contain more files than would normally be allowed. This
requires extending the directory off the default directory track by
changing the last directory sector pointer to a new track, allocating the
new sectors in the BAM, and manually placing (or moving existing) file
entries there. The directory of an extended disk can be read and the files
that reside there can be loaded without problems on a real drive. However,
this is still a very dangerous practice as writing to the extended portion
of the directory will cause directory corruption in the non-extended part.
Many of the floppy drives core ROM routines ignore the track value that the
directory is on and assume the default directory track for operations.
To explain: assume that the directory has been extended from track 18 to
track 19/6 and that the directory is full except for a few slots on 19/6.
When saving a new file, the drive DOS will find an empty file slot at 19/6
offset $40 and correctly write the filename and a few other things into
this slot. When the file is done being saved the final file information
will be written to 18/6 offset $40 instead of 19/6 causing some directory
corruption to the entry at 18/6. Also, the BAM entries for the sectors
occupied by the new file will not be saved and the new file will be left as
a SPLAT (*) file.
Attempts to validate the disk will result in those files residing off the
directory track to not be allocated in the BAM, and could also send the
drive into an endless loop. The default directory track is assumed for all
sector reads when validating so if the directory goes to 19/6, then the
validate code will read 18/6 instead. If 18/6 is part of the normal
directory chain then the validate routine will loop endlessly.
*** Bam layout
The BAM is somewhat different as it now has to take 35 new tracks into
account. In order to do this, most of the extra BAM information is stored
on track 53/0, and the remaining sectors on track 53 are marked in the BAM
as allocated. This does mean that except for one allocated sector on track
53, the rest of the track is unused and wasted. (Track 53 is the equivalent
to track 18, but on the flip side of the disk). Here is a dump of the first
BAM sector...
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII
----------------------------------------------- ----------------
00: 12 01 41 80 12 FF F9 17 15 FF FF 1F 15 FF FF 1F ..A<>.<2E><>..<2E><>..<2E><>.
10: 15 FF FF 1F 15 FF FF 1F 15 FF FF 1F 15 FF FF 1F .<2E><>..<2E><>..<2E><>..<2E><>.
20: 15 FF FF 1F 15 FF FF 1F 15 FF FF 1F 15 FF FF 1F .<2E><>..<2E><>..<2E><>..<2E><>.
30: 15 FF FF 1F 15 FF FF 1F 15 FF FF 1F 15 FF FF 1F .<2E><>..<2E><>..<2E><>..<2E><>.
40: 15 FF FF 1F 15 FF FF 1F 11 FC FF 07 13 FF FF 07 .<2E><>..<2E><>..<2E><>..<2E><>.
50: 13 FF FF 07 13 FF FF 07 13 FF FF 07 13 FF FF 07 .<2E><>..<2E><>..<2E><>..<2E><>.
60: 13 FF FF 07 12 FF FF 03 12 FF FF 03 12 FF FF 03 .<2E><>..<2E><>..<2E><>..<2E><>.
70: 12 FF FF 03 12 FF FF 03 12 FF FF 03 11 FF FF 01 .<2E><>..<2E><>..<2E><>..<2E><>.
80: 11 FF FF 01 11 FF FF 01 11 FF FF 01 11 FF FF 01 .<2E><>..<2E><>..<2E><>..<2E><>.
90: A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
A0: A0 A0 30 30 A0 32 41 A0 A0 A0 A0 00 00 00 00 00 <20><>00<30>2A<32><41><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 15 15 15 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>...
E0: 15 15 15 15 15 15 15 15 15 15 15 15 15 15 00 13 ................
F0: 13 13 13 13 13 12 12 12 12 12 12 11 11 11 11 11 ................
Bytes:$00-01: Track/Sector location of the first directory sector (should
be set to 18/1 but it doesn't matter, and don't trust what
is there, always go to 18/1 for first directory entry)
02: Disk DOS version type (see note below)
$41 ('A') = 1541
03: Double-sided flag
$00 - Single sided disk
$80 - Double sided disk
04-8F: BAM entries for each track, in groups of four bytes per
track, starting on track 1.
90-9F: Disk Name (padded with $A0)
A0-A1: Filled with $A0
A2-A3: Disk ID
A4: Usually $A0
A5-A6: DOS type, usually "2A"
A7-AA: Filled with $A0
AB-DC: Not used ($00's)
DD-FF: Free sector count for tracks 36-70 (1 byte/track).
The "free sector" entries for tracks 36-70 are likely included here in
the first BAM sector due to some memory restrictions in the 1571 drive.
There is only enough memory available for one BAM sector, but in order to
generate the "blocks free" value at the end of a directory listing, the
drive needs to know the extra track "free sector" values. It does make
working with the BAM a little more difficult, though.
These are the values that would normally be with the 4-byte BAM entry,
but the rest of the entry is contained on 53/0.
Note: If the DOS version byte is set to anything other than $41 or $00,
then we have what is called "soft write protection". Any attempt to write
to the disk will return the "DOS Version" error code 73. The 1571 is simply
telling you that it thinks the disk format version is incorrect.
The BAM entries require some explanation. Take the first entry at bytes
$04-$07 ($12 $FF $F9 $17). The first byte ($12) is the number of free
sectors on that track. Since we are looking at the track 1 entry, this
means it has 18 (decimal) free sectors.
The next three bytes represent the bitmap of which sectors are used/free.
Since it is 3 bytes (8 bits/byte) we have 24 bits of storage. Remember that
at most, each track only has 21 sectors, so there are a few unused bits.
These entries must be viewed in binary to make any sense. We will use the
first entry (track 1) at bytes 04-07:
FF=11111111, F9=11111001, 17=00010111
In order to make any sense from the binary notation, flip the bits around.
111111 11112222
01234567 89012345 67890123
--------------------------
11111111 10011111 11101000
^ ^
sector 0 sector 20
Since we are on the first track, we have 21 sectors, and only use up to
the bit 20 position. If a bit is on (1), the sector is free. Therefore,
track 1 has sectors 9,10 and 19 used, all the rest are free.
In order to complete the BAM, we must check 53/0.
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII
----------------------------------------------- ----------------
00: FF FF 1F FF FF 1F FF FF 1F FF FF 1F FF FF 1F FF <20><>.<2E><>.<2E><>.<2E><>.<2E><>.<2E>
10: FF 1F FF FF 1F FF FF 1F FF FF 1F FF FF 1F FF FF <20>.<2E><>.<2E><>.<2E><>.<2E><>.<2E><>
20: 1F FF FF 1F FF FF 1F FF FF 1F FF FF 1F FF FF 1F .<2E><>.<2E><>.<2E><>.<2E><>.<2E><>.
30: FF FF 1F 00 00 00 FF FF 07 FF FF 07 FF FF 07 FF <20><>.<2E><><EFBFBD><EFBFBD><EFBFBD>.<2E><>.<2E><>.<2E>
40: FF 07 FF FF 07 FF FF 07 FF FF 03 FF FF 03 FF FF <20>.<2E><>.<2E><>.<2E><>.<2E><>.<2E><>
50: 03 FF FF 03 FF FF 03 FF FF 03 FF FF 01 FF FF 01 .<2E><>.<2E><>.<2E><>.<2E><>.<2E><>.
60: FF FF 01 FF FF 01 FF FF 01 00 00 00 00 00 00 00 <20><>.<2E><>.<2E><>.<2E><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>
Each track from 36-70 has 3 byte entries, starting at address $00.
Byte: $00-02: FF FF 1F - BAM map for track 36
03-05: FF FF 1F - BAM map for track 37
...
33-35: 00 00 00 - BAM map for track 53
...
66-68: FF FF 01 - BAM map for track 70
69-FF: - Not used
You can break down the entries for tracks 36-70 the same way as track 1,
just combine the free sector bytes from 18/0 and the BAM usage from 53 to
get the full 4-byte entry.
Just like a D64, you can attach error bytes to the file, for sector error
information. This block is 1366 bytes long, 1 byte for each of the 1366
sectors in the image. With the error bytes, the file size is 351062 bytes.
-------------------------------------------------------------------------------
*** REL files
The REL filetype requires some extra explaining. It was designed to make
access to data *anywhere* on the disk very fast. Take a look at this
directory entry...
00: 00 00 84 11 02 41 44 44 49 54 49 4F 4E 41 4C 20 <20><><EFBFBD>..ADDITIONAL<41>
10: 49 4E 46 4F A0 11 0C FE 00 00 00 00 00 00 61 01 INFO<46>..<2E><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>a.
The third byte ($84) indicates this entry is a REL file and that the
three normally empty entries at offset $15, $16 and $17 are now used as
they are explained above. It's the track/sector chain that this entry
points to, called the SIDE SECTOR, which is of interest here (in this case,
17/12). If you check the D64 document for a REL file and do the
calculations, you will find that the maximum file size of the REL file is
720 data sectors.
The side sector layout is the same as the D64.
00: 02 11 00 FE 11 0C 07 13 04 09 00 00 00 00 00 00
10: 11 02 11 0D 11 03 11 0E 11 04 11 0F 11 05 11 10
Bytes: $00: Track location of next side-sector ($00 if last sector)
01: Sector location of next side-sector
02: Side-sector block number (first sector is $00, the next is
$01, then $02, etc)
03: REL file RECORD size (from directory entry)
04-0F: Track/sector locations of the six other side-sectors. Note
the first entry is this very sector we have listed here.
The next is the next t/s listed at the beginning of the
sector. All of this information must be correct. If one of
these chains is $00/$00, then we have no more side sectors.
Also, all of these (up to six) side sectors must have the
same values in this range.
10-FF: T/S chains of *each* sector of the data portion. When we
get a $00/$00, we are at the end of the file.
---------------------------------------------------------------------------
*** Overall Good/Bad of D71 Files:
Good
----
* Most emualators support these.
* Supports *all* filenames, even those with 00's in them
* Filenames are padded with the standard $A0 character
* Supports GEOS files
* Supports REL files
* Allows full directory customization
* Because it is a random-access device, it supports fast-loaders and
random sector access
* Cluster slack-space loss is minimized since the file is a larger fixed
size
* Has a label (description) field
* With the inclusion of error bytes, you have support for basic
copy-protection
* Files on a disk can easily be re-written, as long as there is free
blocks
Bad
---
* The format doesn't contain *all* the info from the 1571 disk (no sector
header info like ID bytes, checksums). This renders some of the
original special-loaders and copy-protection useless.
* You don't *really* know the file size of the contained C64 files in
bytes, only blocks
* It can't store C64s FRZ files due to FRZ files needing a special flag
that a D71 can't store
* It is not an expandable filesize, like LNX or T64
* Unless most of the space on a D71 disk is used, you do end up with lost
space
* Directory limited to 144 files maximum
* Cannot have loadable files with the same names
* Has no recognizeable file signature (unlike most other formats). The
only reliable way to know if a file is a D71 is by its size
* It is too easy for people to muck up the standard layout
* It is much more difficult to support fully, as you really need to
emulate the 1571 DOS (sector interleave, REL support, GEOS interleave)
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