UNIX for Dummies Questions & Answers

I'm not sure where to post this but I'm having some trouble with the directories in CP/M... I'm sorry about the length but I'm totally confused... I've seen that the directory entry in CP/M contains the following:

1 byte User Code
8 bytes Filename
3 bytes File extension
1 byte Extension
2 bytes reserved
16 bytes Disk block numbers

First of all, what are those 2 bytes reserved for? Will that be used if the file occupies a large number of blocks thus supporting as an extension to that 1 byte "Extension field"? Secondly I don't understand how this works... Is it stored something like this?

1 | FileA | exe | 1 | --- | 1 2 3 4 5 6 7 8 9 ... 16 |
1 | FileA | exe | 2 | --- | 17 18 19 ...............33 |
...
...

I read that the Extension field is used to keep track of the order of the storage as to which blocks come first. The disk block numbers point to blocks of size 1 KB... So what is the largest disk that the CP/M could support anyways? Was it just a floppy disk or something? What happens if I choose a block size more than 1KB say 512 KB or something? I did a rough calculation messing with other parameters and got some 2048 GB which I think is just too huge to be true... I could've even made the block size as 1 MB but what is the limiting factor here?

The first byte is E5 if the file is deleted, this stems from the original formatting chips that filled blank disks with E5 (single-density) double-density were filled with F6.

When the entry is read into memory this first byte holds the disk number.

The whole thing is 32 bytes long and has room for 16 blocks to be recorded, if the file exceeds that length then another directory entry is added.

I'm a bit rusty but CPM groups sectors into allocation units, but a directory entry must still be able to say exactly how many 128 byte sectors it refers to.

Apparently those two reserved bytes are truely not used.

But I think you are missing a sector count.

0 db user type
1-8 db file name
9-11 db file type
12 db extent
13-14 db not used
15 db sector count
16-31 allocation map

So a disk can have a maximum of 256 allocation units, which may contain 2^N 128 byte sectors.

CPM is really designed for floppies, originally single sided, single density 8" disks.

Two pieces of information are not held on the disk.....

1. The starting sector, eg typically the operation system is held outside of the file format just in the sectors at the start of the disk.

2. The interleave map, as CPM is designed for 128 byte sectors and todays floppies use 512 byte sectors, a single real sector will contain multiple CPM sectors. Also, there is a translation from logical sector to real sector to allow the computer to process one sector while the disk is turning to the next one. Eg sectors may be ordered......

1 3 5 7 2 4 6 8


http://delivery.acm.org/10.1145/1000...FTOKEN=6184618

Aha... That paper was wonderful... Thanks a lot... Now I understand the real reason behind that classification... But even then, what dictates the fact that "CP/M can support only disks as large as a floppy?" The paper just discusses taking into account a disk with 102,400 bytes divided into 800 128-byte CP/M sectors.

Ok the disk allocation map, as the paper says, contains 16 1-byte address pointers. The following is a paragraph from the paper itself:

The paper clearly says that there are 16 1-byte address pointers and each represents one group => there are 16 groups and a group includes 8 sectors => 128 sectors in total. But there are 800 sectors... I know I'm missing something but I don't understand what I'm missing....

clusters!

the list of "things" that are referenced in each directory entry are clusters, not sectors.

A single disk can contain a maximum of 255 clusters, where a cluster contains 2^N sectors, and a sector is 128 bytes.

If a single byte is used to indicate how many 128byte sectors are used in a cluster then depending on the algorithm you have either a max of 128 or 256 sectors per cluster, giving an absolute max disk size of ether 4Mb or 8Mb.