2. XXDP Disk Structure
paramucho
2. XXDP DISK STRUCTURE
NOTE
This article forms part of an attempt to document XXDP. The
information is based on an examination of XXDP disks and the
scant documentation available on the web. Thus omissions and
errors are guaranteed. Ian Hammond/2008 alt.sys.pdp11
This section describes the structure of an XXDP disk volume along with
disk initialization, bootstrap and monitor initialization.
The XXDP disk structure is largely compatible with the DOS11/DOSBATCH
structure.
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2.1 DISK OVERVIEW
The disk structure basics are illustrated by the layout of the sample
XXDPV1 disk:
block size area flags area
000000/0 1 BOO FB C Bootstrap
000001/1 1 HOM FB C Home block
000002/2 12. MAP HB L Block allocation bitmap
000030/24. 146. MFD HB L Directory
000252/170. 30. MON HB C Monitor image
000310/200. vary DAT HBDLC File storage area
Where: Mapping F = Fixed disk area
H = Mapped by home block
B = Mapped by bitmap
D = Mapped by directory
Storage L = Linked file
C = Contiguous file
There are six major areas identified above:
o BOOTSTRAP: A bog-standard PDP-11 bootstrap located in block zero.
See below for a description of the operation of the bootstrap.
o HOME BLOCK: The XXDP home block contains information about the
disk volume. See section 2.3.
o BLOCK ALLOCATION BITMAP: This linked file maps each disk block with
a single bit flag. See section 2.4.
o MONITOR IMAGE: The monitor image is converted and stored in this
area as a contiguous file (V1 SAVM, V2 CREATE). The first block of
the monitor image is copied to block zero of the device, creating
the bootstrap. See sections 2.6 through 2.9.
o DIRECTORY: A single-level directory, implemented as a linked file.
See section 2.5
o DATA STORAGE: The remainder of the disk is allocated to file
storage.
There are four forms of mapping:
o FIXED DISK AREA: The boot block and home block are fixed at blocks
zero and one on all XXDP system disk volumes (the same holds all
the PDP-11 systems I know off).
o HOME BLOCK MAPPING: the home block contains parameters that map
the bitmap, system image, directory and data area.
o BITMAP BLOCK MAPPING: indicates which blocks are in use or free.
o DIRECTORY MAPPING: the directory maps the linked and contiguous
files in the data area.
There are two basic kinds of file storage:
o LINKED FILES: Linked files reserve the first word of each block for
the block number of the succeeding block. See section 2.2.
o CONTIGUOUS FILES: Data is stored in a single, continuous disk area.
See section 2.2.
Two features are notable for their absence:
o CLUSTERING: The simplicity of the disk structure would make it easy
to add clustering. However, XXDP had neither the design discipline,
the available space or the requirement for a disk clusters.
o SUB-DIRECTORIES: A single master file directory is present. Again,
it would be reasonably easy to add a sub-directory structure since
the MFD itself is just a file.
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2.2 FILE STORAGE
o LINKED FILES: The map, directory and most files are linked file
structures where the first word of each block contains the block
number of the succeeding block. The first word of the last block in
a chain is zero.
Here is an example of a four block file
block link data
350 351 .... First block
351 370 ....
370 353 ....
353 0 .... Last block
Linked files must be interpreted, during reading and writing, to
remove or insert the block links. This imposes block-at-a-time I/O.
Random access I/O requires a sequential search through a file for a
given block. Linked files can be extended (and possibly truncated).
Note: The linked file structure handled the early papertape-based
file structures easily: these were, by their heritage, inherently
sequential.
Note: The bitmap and directory are both "linked files", but these
two files exist outside the directory structure.
Note: I have a source MACRO program that converts an "XXDP" loader
program to a task or save image. The program "skips" the first two
bytes of the file: what it skips is the block linkage. Of coures,
the conversion utility would fail with a "contiguous" file.
o CONTIGUOUS FILES: These files are stored in a contiguous disk area
without a linkage structure. Contiguous files may be read and
written without interpretation. They were called "Fast" for that
reason: they supported multi-block and random-access I/O.
The directory provides all the information that is required to read
and write any or all of the file: the start block and the length of
the file.
Contiguous files were probably not extendable. The last block entry
in the directory was probably maintained for compatibility.
Two files on XXDPV1 are marked contiguous. XXDPV2 has none. Some
utilities refuse to work with contiguous files (indicating that
they are deblocking linked files internally!).
Note: The block-at-a-time I/O required for linked files, was a real
performance killer on early floppy disks. The sudden arrival and
departure of contiguous files may have coincided with the floppy-
based deployment of XXDP.
Note: the bootstrap, homeblock and monitor are stored as contiguous
files, but these exist outside the directory system.
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2.3 HOME BLOCK
The XXDP home block contains information about the disk volume. It has
little similarity with the later "standard" DEC home block structures.
Here is an interpretation of a sample XXDPV1 home block. XXDPV2 values
are shown in parentheses where the differ from XXDPV1.
address contents usage
001000 000000 0 Link to next block (zero)
001002 000030 24. First block of directory.
001004 000222 146. Directory length, in blocks.
001006 000002 2 First block of bitmap.
001010 000026 22. Bitmap length, in blocks.
001012 000001 1 ??
001014 000000 0 ??
001016 047754 20460. Disk size in blocks (10MB = 20480*512)
001020 000310 200. Start block of data area.
(000313 203.) Start block of data area on V2.
001022 000001 1 ??
001024 000000 0 ??
001026 000252 170. B Start block of monitor system image.
001030 000000 0. ??
001032 000100 64. No idea what this is, but 1032/1034
(001000 512.) parallels 1036/1040, along with V4
001034 000377 255. changes.
(000632 410.)
001036 000100 64.
(001000 512.)
001040 000777 511.
(001456 814.)
The only value referenced by the bootstrap is location 1026: the start
block of the monitor system image.
Note: An examination of the ZERO and LOAD utilities, which initialize
a new volume, might provide more information on the remaining values
in the home block.
Note: While the location of the bootstrap and homeblock is fixed, the
position of the monitor, directory and data areas is defined by the
home block and could, theoretically, be ordered in other ways.
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2.4 BITMAP
Blocks 2 through 25. contain the block allocation bitmap, where a bit
value of one indicates that a block is in use. Each bitmap block has
the following header:
000 .word successor Block number of next block in bitmap
002 .word predecessor Block number of predecessor block in bitmap
004 .word units Number of words in this map segment
012 .word 2? ?
014 .word flags... First word of bitmap
...
176 .word flags Last word of bitmap
Each bitmap block maps 960 blocks on the disk. There are 22 bit map
blocks, mapping potentially a total of 21120. blocks. However the disk
has only 20480. blocks. In fact location 016 of the home defines the
maximum disk block available as 20460.
001016 047754 20460. Disk size in blocks (10MB = 20480*512)
002000 000003 3 Successor block
002002 000001 1 Predecessor block (1 is a bit strange)
002004 000074 60. Bitmap length in words
002006 000002 2 ?
002010 177777 60. bitmap words mapping 960. blocks.
002012 177777
...
002174 177777
002176 177777
002200 000000 Remaining 75% of block is unused.
002202 000000
The final block in the bitmap does not have a successor link.
027000 000000
027002 000025
027004 000074
etc
The bitmap flags for the boot block, home block, bitmap, directory and
system image are presumably set during disk initialization.
Directory operations that create or delete file blocks must search the
bitmap for free blocks. Since only the first 64 words of a block are
used, the system could, theoretically, perform bit map operations
using a single 64 word buffer.
Note: The double link (successor and predecessor), permit a backward
walk through the bitmap, which might have been aimed at optimising the
use of DECtape.
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2.5 MASTER FILE DIRECTORY
The start and length of the directory are specified in the home block.
Each directory segment consists of a link word, 29 directory entries
and three unused words.
The link word specifies the next directory segment (i.e. block). The
link of the final segment is zero.
The nine word directory entry has the following format:
000 .rad50 /FILNAMTYP/ 6 character filename, 3 character type.
006 .word Date File date and Contiguous flag
010 .word Unknown Unknown
012 .word Start First physical block of file
014 .word Length Total length of file in blocks
010 .word Last Last physical block of file
020 .word Flags Flags
Filename: A standard PDP-11 six character name and three character
filetype (aka "file extension"). Valid characters are A-Z
and 0-9. Wildcards are represented by '*' and '?'.
Deleted files are flagged with a null file name (all three
words).
Date: The year, less 1970, is multiplied by 1000 and the day-of-
year is added. 15-bits are available but only 11 are used
by DOS. The maximum year for 15 bits is 2002. The display
maximum is 1999.
Contiguous: The high-order bit of date word is set to mark contiguous
files. Contiguous files are marked with a 'C' in listings.
Note: the SETUP program does not support contiguous files.
Next: DOS: Next free byte in the last file block (see below).
Start: The physical block number of the first block of the file.
Length: The length of the file, in blocks.
Last: The physical block number of the last block of the file.
Used when adding blocks to a file.
Flags: DOS: [Lock|Usage count | Protection ]
15 14 8 7 0
Note: XXDP does not support DOS sub-directories (aka UFDs, User File
Directories).
Note: .SYS version information, displayed by the V2 directory utility,
is extracted from file images and not from directory entries.
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2.6 DISK INITIALISATION
Creating an XXDP disk volume is fairly straight-forward:
o Obtain the base initialization parameters: the size of of the disk,
bitmap, monitor and directory. These parameters seem to be fixed.
o Create the home block.
o Create the block bitmap.
o Create the initial empty directory segment(s).
o Optionally, create a dummy bootstrap block
The following V1 UPD1/UPD2 commands initialize DL1:
ZERO DL1: ; initialize basic structures
LOAD DL0:RLDP.BIN ; load the monitor
SAVM DL1: ; save monitor to block 26.
DUMP DL1:RLDP.BIN ; copy monitor
LOAD DL0:UPD1.BIN ; copy upd1
DUMP DL1:UPD1.BIN ;
LOAD DL0:UPD2.BIN ; copy upd2
DUMP DL1:UPD2.BIN ;
The V2 CREATE command combines some of the commands above.
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2.7 SYSTEM DISK INITIALISATION
A separate operation copies a target monitor to the system. HMDLD0.SYS
is used as an example.
o HMDLD0.SYS contains the DL: monitor executable stored in absolute
loader format. This is converted to a memory image.
o The monitor is stored as a contiguous image starting at the block
specified in location 026 of the Home Block.
o The first block of the memory image is copied to block zero of the
device, where it becomes the bootstrap.
o Required system files are then manually copied to the volume.
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2.8 BOOTSTRAP OPERATION
The XXDP+ boot sequence is as follows:
o Read the home block into a buffer at 1000.
o Extract required system parameters from home block.
o Read blocks 1 to 15 of the XXDP monitor (starting with block 0253)
into memory starting at location 1000. Block zero of the monitor is
the boot block which has already been loaded (and modified).
o Pass control to the XXDP monitor.
Here is an edited trace of the bootstrap disk activity:
Read Unit=0 Block=1|1 Buffer=1000 Count=512.
Read Unit=0 Block=253|171 Buffer=1000 Count=512.
Read Unit=0 Block=254|172 Buffer=2000 Count=512.
... more of the same ...
Read Unit=0 Block=270|184 Buffer=16000 Count=512.
Read Unit=0 Block=271|185 Buffer=17000 Count=512.
The XXDP V2 bootstrap is considerably more ornate.
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2.9 MONITOR STARTUP (V1)
The monitor loads into low memory at 01000. The initiation code of the
monitor relocates the monitor to high memory, leaving it in the 15xxxx
(26kw) region.
+----------+
| i/o page |
+----------+ 160000 PDP-11 4kw I/O Page
| monitor |
+----------+ 15xxxx XXDP Monitor
| hooks | Relocated components
| |
+----------+
| program |
+----------+ 000200 Application Programs
| vectors |
+----------+ 000000 Interrupt Vectors
The V1 Monitor then prompts for the date:
CHMDLD0 XXDP+ DL MONITOR
BOOTED VIA UNIT 0
28K NON-UNIBUS SYSTEM
ENTER DATE (DD-MMM-YY):
The more elaborate V2 startup has not been studied in detail.
paramucho
with further info from a previous posting. I'll send it when I get
time to clean it up.