home *** CD-ROM | disk | FTP | other *** search
- ======================================
- DOS & Don'ts -- Part 37
-
- by James Gregory Weiler
- ======================================
-
- Part E: Files
-
- ======================================
-
- Section 1: seq & prg & usr files
-
-
- These three file types are very
-
- much alike, though they can be used to
-
- store very different types of data.
-
- Their similarity lies in the way they
-
- are laid out on a disk. See map 6,
-
- quite a ways below.
-
- Each block of these files begins
-
- with a two-byte track/block link to
-
- the next block of the files, just like
-
- the links we saw in the directory.
-
- The other 254 bytes in each block
-
- just contain the file data, whatever
-
- that may be... we're really not
-
- interested in what that data is right
-
- now, but here's the kind of stuff you
-
- might find in each kind of file.
-
- SEQ - usually ASCII text.
- USR - usually ASCII text.
- PRG - usually a BASIC program.
- PRG - often a machine code program.
- PRG - sometimes ASCII text.
-
- Now, before we wrap up with relative
-
- files, let's look at all the steps DOS
-
- could have to go through to read a
-
- five-block program file.
-
- a) Read BAM
- b) Establish directory link (18,1).
- c) Read directory (18,1).
- d) Look for file name.
- -- not found --
- e) Establish directory link (18,4).
- f) Read directory (18,4).
- g) Look for file name.
- -- found it --
- h) Establish file link (21,6).
- i) Read file block one (21,6).
- j) Establish file link (21,9).
- k) Read file block two (21,9).
- l) Establish file link (21,12).
- m) Read file block three (21,12).
- n) Establish file link (21,15).
- o) Read file block four (21,15).
- p) Establish file link (21,18).
- q) Read file block five (21,18).
- r) End of file found. All done.
-
- ======================================
- Map 6: Data block structure for all
- file types: SEQ, USR, PRG & REL
- --------------------------------------
-
- byte contents
- ---- --------
- 0-1 File link: Track & block
- number of next data block.
-
- 2-256 File data
-
-
- =======< continued in Part 38 >=======
-