The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

5.4 THE DIRECTORY RECORD STRUCTURE
The disk directory contains the information sufficient to access all files on the disk.
We have already shown that disk space allocation is defined in the Granule Allocation
Table. We have also revealed in the previous section how the operating system uses file
hash codes stored in the Hash Index Table to locate the Directory Entry Code for each
file. Each DEC refers to a specific directory entry record. A directory record is 32bytes
in length. Thus, each directory entry sector contains eight directory entry
records.
The HIT was shown to contain a maximum of 256 Directory Entry Codes. Since there are
eight entries per sector, the maximum number of directory entry sectors is 32 (256
divided by 8). If we add one sector for the GAT and one for the HIT, we discover that the
maximum length of the entire directory can be 34 sectors. The directory must be contained
completely on a single cylinder. Therefore, the exact length of the directory and hence
the number of directory entries is highly dependent on the size of a cylinder. For
example, an 18-sector per cylinder formatted disk will have 16 directory entries and
hence 16 times 8 or 128 directory entries. Consult the section on the HIT for the formula
calculating the number of directory sectors.
=====================================================================
| |
| SECTORS PER DIRECTORY FILES AVAILABLE PER DIRECTORY |
| CYLINDER RECORDS TOTAL SYSTEM DISK DATA DISK |
| ----------- ---------- ------- --------- ---------- |
| 5" SDEN-1 10 8 64 48 62 |
| 5" SDEN-2 20 18 144 128 142 |
| 5" DDEN-1 18 16 128 112 126 |
| 5" DDEN-2 36 32 256 240 254 |
| 8" SDEN-1 16 14 112 96 110 |
| 8" SDEN-2 32 30 240 224 238 |
| 8" DDEN-1 30 28 224 208 222 |
| 8" DDEN-2 60 32 256 240 254 |
| - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
| 5" HARD- 128 32 256 240 254 |
| |
| 5" HARD- 64*2 32*2=64 256*2=512 240*1+ 254*2=508 |
| 254*1=494 |
| 5" HARD- 32*4 30*4=120 240*4=960 224*1+ 238*4=952 |
| 238*3=938 |
| |
| Note: Hard drive values show total entries for all partitions. |
| "" denotes the number of logical drives. |
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Figure 5-7: Directory entries for various media
The first two directory entries of the first eight directory entry sectors are reserved
for system overlays on a SYSTEM disk. A DATA disk reserves only the first directory entry
of the first two directory entry sectors. The total capacity of files is equal to the
number of directory sectors times eight (since 256/32 = 8). The quantity available for
use will always be reduced by 16 on a SYSTEM disk or by two on a DATA disk to account for
those entries reserved for the operating system. Figure 5-7 shows the record capacity
(file capacity) of each floppy format type. The dash suffix on the density indicator
represents the number of sides formatted. The figure also lists representative values for
5 megabyte winchester drives (typical ST-506 compatible: 4 heads, 32 sectors per track,
153 tracks per head).
5-9