jhs@MITRE-BEDFORD.ARPA (03/17/86)
I just noticed that "Inside Atari DOS" apparently has an error in describing
the sequence of the control bytes at the end of DOS sectors. I would
appreciate confirmation by anyone who knows.
Inside Atari DOS (IAD) says that a sector consists of 128 bytes, numbered 0
through 127, and that in DOS files only 125 bytes (0 - 124) are used for data.
(This much is true.) They go on to say that byte numbers 125, 126, and 127
are numbered as follows:
125 126 127
IAD | # of bytes actually 6 hi bits=file number low 8 bits of next-
Version| used in the sector 2 lo bits=hi bits of sector number (link)
next-sector number
Documentation supplied with David Young's OMNIMONXL resident monitor/debugger
says that the numbering is
125 126 127
The | 6 hi bits=file number low 8 bits of next- number of bytes
Truth!| 2 lo bits=hi bits of sector nr (link) actually used in this
| next sector nr (link) sector.
Based on what I can see in the actual sectors I have looked at, both with
OMNIMONXL and with another independently developed debugger, David Young's
information appears to be correct and Inside Atari DOS appears to have
incorrect information.
I would appreciate confirmation of my conclusion by others who have noticed
the error in IAD. (Also if they have any other errors I would like to be
made aware of them!)
John Sangster
jhs at mitre-bedford.arpa
<anypath to ihnp4>!ihnp4!linus!mbunix!jhsdyer@atari.UUcp (Landon Dyer) (03/18/86)
Here is the REAL story of an Atari DOS data sector. This is taken verbatim from the Atari OS manual. ---------------- The format of a sector in a user's data file is shown below: 7 0 +-+-+-+-+-+-+-+-+ | data | +0 +-+-+-+-+-+-+-+-+ | file # |hi | +125 +-+-+-+-+-+-+-+-+ |forward pointer| +126 +-+-+-+-+-+-+-+-+ |S| byte count | +127 +-+-+-+-+-+-+-+-+ The file # is a redundant piece of information which is used to verify file integrity; the file number field contains the value of the directory position of that file. If there is ever a mismatch between the file's position in the directory and the file number as contained in each sector, the Disk File Manager will generate the error $A4. The forward pointer field contains the ten bit value of the disk sector number of the next sector of the file. The pointer will equal zero for the last sector of the file. The S bit indicates whether or not the sector is a "short" sector (one containing fewer than 125 data bytes). S is equal to one when the sector is short. The byte count field contains the number of data bytes in the sector. ---------------- -Landon
DYOUNG@USC-ISID.ARPA (C. David Young) (03/20/86)
I don't know where you got your information but the "S bit" you describe is definately not correct. All 8 bits of the last byte of the sector are use for byte count. Of course the MSB (your "S bit") can be a 1 only in double density. David Young -------
dlm@ihelp.UUCP (Daryl Monge) (03/22/86)
> I don't know where you got your information but the "S bit" you describe > is definately not correct. All 8 bits of the last byte of the sector are > use for byte count. Of course the MSB (your "S bit") can be a 1 only in > double density. > > David Young > ------- He got it from the ATARI Personal Computer System OPERATING SYSTEM Users Manual, section 5, page 84; which I have sitting in my lap this minute. Are you saying the manual is wrong? This manual came directly from ATARI. Daryl Monge AT&T Bell Labs, Naperville, Ill ...!ihnp4!ihelp!dlm