Offset Name Description/contents
------------------------------------------------------------------------------
00-03 Volume Label ID and Number Identifies this sector as Volume
Label; contains the text 'VOL1'
04-09 Volume Identifier Name of the disk; standard 'VOLID '
0A Accessibility If non-blank access to the diskette is
restricted
0B-0F (reserved)
10-17 (reserved)
18-24 System Identification Identity of the system that initialized
the diskette, e.g. 'IBM5100' on an
IBM 5110/5120.
25-32 Owner Identification optional name of the owner;
standard 'OWNERID '
33-3F (reserved)
40 Extended Label Area Contains the additional number of tracks
for the directory on a Diskette 2D
(Blank or 0 to 9)
41-46 (reserved)
47 Volume Surface Indicator Blank=Diskette 1, '2'=Diskette 2,
'M'=Diskette 2D
48 Extended Arrangement Ind. unused, must be blank
49 Special Requirements Ind. unused, must be blank
4A (reserved)
4B Physical Sector Length Blank=128, '1'=256, '2'=512, '3'=1024
4C-4D Phys. Record Sequence Code Specifies an optional interleave factor;
Blank or '01'=Interleave 1:1
4E (reserved)
4F Label Standard Version must be 'W'
50-7F (unused)
Offset Name Description/contents
------------------------------------------------------------------------------
00-03 Header Label ID and Number Contains 'HDR1' with used entries; unused
ones have a 'D' in the first position
04 (reserved)
05-15 Data Set Identifier File name; must be unique on a disk
16-1A Block Length Size of a logica file block; e.g.
' 128', '00256' etc.
1B Record Attribute must be blank
1C-20 Beginning of Extent (BOE)1 First sector of a file given as CCHSS
CC=cylinder, H=head, SS=sector
21 Physical Record Length Sector size, must be equal to the entry in the
Volume Label
22-26 End of Extent (EOE)1 Last sector that has been marked for
the file
27 Record Block Format Indicates FM or MFM data recording
28 Bypass Indicator unused, must be blank
29 Data Set Security must be blank
2A Write Protect Indicator Blank=Writing allowed;
'P'=File is write protected
2B Exchange Type Indicator Depends on the disk format;
usually blank, 'E' or 'H'
2C Multivolume Data Set Ind. unused
2D-2E Volume Sequence Indicator unused
2F-34 Creation Date unused; format 'YYMMDD'
35-38 Record Length Size of a record within the file;
depends from the file type
39-3D Offset to Next Record Space unused; must contain blanks
3E-41 (reserviert)
42-47 Expiration Date unused; must contain blanks
48 Verify/Copy Indicator unused
49 Data Set Organization usually blank
4A-4E End of Data (EOD)1 Address of the first sector following
data
4F-5E (reserviert)
5F-6B System Code Identity string of the system that created
the file, e.g. 'IBM5100 '
6C-6D File Application Type File type, always two numbers
6E-7F (unbenutzt)
1 Relationship of BOE, EOE and EOD in graphical form:
/--------- marked space on diskette ------------\
/ \
...-----------------------------------------------------...
| | |
| used space by file | (free) |
...-----------------------------------------------------...
^ ^ ^
| | |
BOE EOD EOE
BOE contains the first sector of a marked file, EOE the last sector. Within
this boundary a file can be of any size (a multiple of the sector size). The
first sector of the free area (or the first sector after the data area) is
stored in EOD. An empty file like one created with the MARK command has
EOD equal to BOE. If the file space is completely used EOD points to the sector
after EOE. You have to pay attention to this when doing file handling.
Offset Size Description/Contents ------------------------------------------------------------------------------ 00 2 Track (Bits 0-7), Side (Bit 8), Sector (Bits 9-15) 02 2 Size of the used space in number of sectors (EOD-BOE) 04 2 Size of the maked file in number of sectors (EOE-BOE) 06 2 File number after Find operation 08 1 Physical record size (0=128 bytes, 1=256 bytes, ...) 09 1 [last I/O function number]
There is no real "Open" command like in other systems. Instead you have to find the location of a file on disk before using the file. To do this the diskette must be known by the system by issuing a Sense command. Next the directory entry for the desired file must be found with the Find command. Two methods are possible for doing a find. The first is to give the file number that is equal to the number of the directory entry to the find command. The other one is to tell Find to look for a file with a given file name.
In either case the first operation must always be a Sense. It is done by calling the I/O supervisor as explained in IOCB. The IOCB for a Sense operation must have the following fields defined:
Important: During all operations concerning the same file IOCB_WA has to point to the same buffer all the time. The I/O routines store information about the current file like file offsets etc.
If the file has been found IOCB_Ret will contain the value 0. Otherwise
(and no other error has occured) $F4F1 = '41' is returned as error code.
On successful operation the directory entry can be found in the specified
buffer. IOCB_CI2 contains a pointer to it. The values are the same as
explained above with two exceptions:
Offset Name Description/contents
------------------------------------------------------------------------------
25-26 End of Extent This is now a 16 bit integer value for
the size of the marked area in number of
sectors.
4D-4E End of Data 16 bit integer value for the size of
the used file area in number of sectors.
This way you don't have to worry about the diskette layout to get the
size of the file.
Reading or writing can only be done in a multiple of the sector size, also called block size. The IOCB:
With an offset each location of a file can be reached, there is no explicit file seek operation.
To create a new file one has to find an unused directory entry. The simplest
way of doing this is to do a Find over all file numbers until either an error
'41' or '36' is returned. A '41' indicates an unused directory entry that can
be used to create a new file. When '36' is returned the end of the directory
has been reached so there are no free entries left.
With a free entry just found the Mark command is called. The IOCB looks
like this:
This command has the same effect as the MARK command in BASIC or )MARK in APL.
To be able to use the new file a Find command has to be issued a second time,
now with the file number of the new file. Then data can be written or read.
You must not forget to update the file size (EOD) in the directory entry, eventually
you also want to change the file name from 'SYSxxxx' (xxxx=file number) which
is the default name to something else. The directory entry is loaded into
a buffer with the Find command and the desired fields are changed. The
entry is updated with a Write Header command.
Freeing a file means to make all the marked space available for new data. This is done by changing the EOD field of the directory entry to 0 (zero) with a Find / Write Header command.
Deleting a file is different in that it alters the directory entry to the unused state ('DDR1' instead of 'HDR1'). This is accomplished by a Mark command and a file size of 0 kB (in IOCB_CI1).
Reading or writing an absolute sector bypasses the file structure to gain access to any sector on a diskette. After the obligatorical Sense command no Find command is needed. The IOCB for direct access to a sector looks like this:
Track changes are done automatically.
Bit 0 = MSB, bit 15 = LSB!
When the bit for absolute sector access is set only the following commands are allowed: Read, Write, Find ID and Initialize Head, all others will be rejected with error code 02.