doc:cbm:disk:rel
*** REL (RELative file layout)
*** Document revision: 1.1
*** Last updated: March 11, 2004
*** Compiler/Editor: Peter Schepers
*** Contributors/sources: Immers/Neufeld "Inside Commodore DOS"
This is a filetype native to CBM drive (random access) devices. On the
surface it seems similar to all other filetypes, especially SEQ, but was
designed to make access to data *anywhere* in the file very fast.
We start by examining a REL file directory entry...
00: 00 00 84 11 02 41 44 44 49 54 49 4F 4E 41 4C 20 ˙˙Ñ..ADDITIONAL˙
10: 49 4E 46 4F A0 11 0C FE 00 00 00 00 00 00 61 01 INFO†..˛˙˙˙˙˙˙a.
Bit:$00-01: Track/Sector location of next directory sector ($00 $00 if
not the first entry in the 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 first side-sector block (REL file
only)
17: REL file record length (REL file only, max. value 254)
18-1D: Unused (except with GEOS disks)
1E-1F: File size in sectors, low/high byte order ($1E+$1F*256).
The approx. filesize in bytes is <= #sectors * 254
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 sector chain that this entry points to
(called the SIDE SECTORS) which are of interest here (in this case,
#17/#12). Here is a dump of that sector...
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII
----------------------------------------------- ----------------
0000: 0C 13 00 FE 11 0C 0C 13 06 09 00 00 00 00 00 00 ˙˙˙˛˙˙˙˙˙˙˙˙˙˙˙˙
0010: 11 02 11 0D 11 03 11 0E 11 04 11 0F 11 05 11 10 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0020: 11 06 11 11 11 07 11 12 11 08 11 13 11 09 11 14 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0030: 11 0A 11 0B 10 00 10 0A 10 14 10 08 10 12 10 06 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0040: 10 10 10 04 10 0E 10 02 10 0C 10 01 10 0B 10 03 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0050: 10 0D 10 05 10 0F 10 07 10 11 10 09 10 13 0F 07 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0060: 0F 11 0F 05 0F 0F 0F 03 0F 0D 0F 01 0F 0B 0F 00 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0070: 0F 0A 0F 14 0F 08 0F 12 0F 06 0F 10 0F 04 0F 0E ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0080: 0F 02 0F 0C 0F 09 0F 13 0E 07 0E 11 0E 05 0E 0F ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
0090: 0E 03 0E 0D 0E 01 0E 0B 0E 00 0E 0A 0E 14 0E 08 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
00A0: 0E 12 0E 06 0E 10 0E 04 0E 0E 0E 02 0E 0C 0E 09 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
00B0: 0E 13 0D 07 0D 11 0D 05 0D 0F 0D 03 0D 0D 0D 01 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
00C0: 0D 0B 0D 00 0D 0A 0D 14 0D 08 0D 12 0D 06 0D 10 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
00D0: 0D 04 0D 0E 0D 02 0D 0C 0D 09 0D 13 0C 07 0C 11 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
00E0: 0C 05 0C 0F 0C 03 0C 0D 0C 01 0C 0B 0C 00 0C 0A ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
00F0: 0C 14 0C 08 0C 12 0C 06 0C 10 0C 04 0C 0E 0C 02 ˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙˙
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, max. value 254)
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 chain.
If the speed advantage regarding this type file file isn't obvious yet,
consider the following scenario... If we need to access record 4000, its
only a couple of calculations to see how many bytes into the file it is...
4000 * "record length" (254) = byte offset
Once we know this, we can calculate how many sectors into the file the
record is...
byte offset / 254 = # of sectors into REL file
The divisor value "254" in the above formula is the number of bytes
useable in each sector (256 bytes less the 2 bytes used for the forward t/s
pointer) and has no relation to the "max REL record length".
Now that we know the number of sectors, we can look it up in our
side-sector tables to see where the record is. The speed of this system is
truly amazing, given the era of the C64, and a floppy drive.
doc/cbm/disk/rel.txt · Last modified: 2020/10/26 16:39 by eek