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dos #36
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======================================
DOS & Don'ts -- Part 36
by James Gregory Weiler
======================================
======================================
Part 4: Directory blocks
======================================
Each directory block consists of two
parts: the directory link in bytes 0
and 1, and the file directory section.
The directory link gives the track
and block number of the block where
the directory continues.
The file directory section, bytes 2
through 255, has storage space for
eight file names, each using thirty
bytes. See map 4, below. The disk
has 18 directory blocks, so you can
store 144 (18 X 8) files on a disk.
======================================
Map 4: A single directory sector.
byte/contents
--------------------------------------
0/Track of next directory block.
Normally eighteen.
1/Block number of the next
directory block. It normally
equals this block plus three
(or this block minus 14 if
this block is greater than 15).
2-31 / File name 1
34-63 / File name 2
66-95 / File name 3
98-127 / File name 4 (see Map 5)
130-159 / File name 5 (far below)
162-191 / File name 6
194-223 / File name 7
226-255 / File name 8
======================================
======================================
Part 5:directory file entries
======================================
All of the 144 file name areas in
the directory share the same
structure. See map 5, far below.
The first of the thirty bytes (byte
zero) is the file type byte.
The value of this byte defines the
file type of this file entry.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
THE FILE TYPE BYTE:
byte file
value type typical file contents
- - - - - - - - - - - - - - - - - - -
0 DEL none -- file scratched
129 SEQ text/data
130 PRG program/text/ml/data
131 USR text/data
132 REL text/data
1-127 - file not closed
properly
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
A file type byte with a value less
than 128 means that this file was not
properly closed. When the disk is
validated, these "invalid" files will
disappear.
Bytes one and two are the track and
block numbers of the first data block
of the file.
Bytes three through eighteen contain
the file's name. If the name is less
than sixteen bytes long, this area
will be padded with shifted spaces.
Bytes nineteen and twenty are only
used by RELative files. They point to
the file's first side sector. Side
sectors keep track of where every part
of a relative file is so you can read
any record directly, without scanning
all the way through the file.
Byte twenty-one, also used only for
relative files, is the record size.
Bytes twenty-eight and twenty-nine
tell us the number of blocks in the
file. The block count can be figured
by multiplying the contents of byte 29
by 256 and adding the results to the
contents of byte 28.
======================================
Map 5: A single directory entry.
byte contents
--------------------------------------
0 File type byte.
1 Track of first data block.
2 Block # of first data block.
3-18 File name followed by shifted
spaces.
19 Track of first side sector block
20 Block # of first side sector
block.
21 Record size.
22-25 Unused
26-27 Block of replacement file when
using save"@0:name" -- the save
and replace syntax.
28-29 Number of blocks in the file.
=======< continued in Part 37 >=======