ARM Club 3/TheARMClub_PDCD3.iso/programs/filing/shrink/!Help

Text File | 1998-01-29 | 8.8 KB | 439 lines

                            ShrinkWrap
                            ==========


*************************************************************************

                            Disclaimer
                            ----------
David Pilling accepts no responsibility for any loss of data
which occurs as a result of using this program, whether as a
result of correct or incorrect operation. You use ShrinkWrap
at your own risk.

*************************************************************************



This is a module which intercepts filing system vectors in such a way that
compressed files appear to applications to act like uncompressed files. The
compression is transparent in other words.


Particular filetypes are recognised as being compressed and of interest to
the module, within the compressed files are the compressed data and the file
attributes of the file which has been compressed.


The main ShrinkWrap module uses GnuZip files. An extra tag defines RISC OS file
attributes. Each GZip file maps to a single  RISC OS file. Only GZip files
with the correct filetype and which contain the special RISC OS tag are
handled by the module. Other GZips are ignored.


Sub modules can register with the ShrinkWrap module to handle other
filetypes. Currently two modules are available to support Acorn Squash files
and Computer Concepts CFS files.


The module only pays attention to files which are in certain places called
'zones' (given to the module via SWI and  CLI calls) where the left section
of a full file path name matches one of the given places. Places can include
a whole file system (e.g. one might say "SCSI" and any file held on any SCSI
disc would be a possibility for compression/decompression) or a part of a
file system (e.g. one might say  "ADFS::HardDisc4.Scrap").


Compressed files are created by just saving in the normal way.

However the following special process allows a compressed file to be created
(subject to the filename restriction) for output:

- create a file with the compressed type (maybe of zero length, maybe not)

- change the file's type to any type at any later time

- open the file for update or writing to enable writing data which is
compressed

- complete file write (OS_File) to enable writing compressed data.




*Commands
=========

*ShrinkWrapAddZone Adds an area to the ShrinkWrap zone
Syntax: *ShrinkWrapAddZone <path> [all|none|some]

If the 'all' parameter is added, all files will be compressed.
i.e. the file type control will not be used.
If the 'none' parameter is added, no files will be compressed,
other than those which already are compressed.
If the 'some' parameter is added, then file types are used.
The default setting i.e. if there is no final parameter is 'some'.


*ShrinkWrapRemZone Removes an area from the ShrinkWrap zone
Syntax: *ShrinkWrapRemZone <path>

*ShrinkWrapShowZone List ShrinkWrap zones
Syntax: *ShrinkWrapShowZone

*ShrinkWrapAddType Adds a filetype for ShrinkWrap compression
Syntax: *ShrinkWrapAddType <filetype>

*ShrinkWrapRemType Removes a filetype for ShrinkWrap compression
Syntax: *ShrinkWrapRemType <filetype>

*ShrinkWrapShowType List ShrinkWrap compression filetypes
Syntax: *ShrinkWrapShowType

*ShrinkWrapMemory sets memory slot sizes.
Syntax: *ShrinkWrapMemory <min>[K] <max>[K]

Always uses at least <min> and never more than <max>

*ShrinkWrapStrength sets how hard ShrinkWrap tries to compress files.
Syntax: *ShrinkWrapStrength <1..9>

Default 9 - max compression.

*ShrinkWrapZipCompress makes a GZip file.
Syntax: *ShrinkWrapZipCompress <source> <destination>

*ShrinkWrapZipUncompress expands a GZip file.
Syntax: *ShrinkWrapZipUncompress <source> <destination>


*ShrinkWrapOn enables shrink wrapping.
Syntax: *ShrinkWrapOn

*ShrinkWrapOff disables shrink wrapping.
Syntax: *ShrinkWrapOff

These commands remove and add the module to the file switch vectors.


*ShrinkWrapShowLink display linked ShrinkWrap filetypes.
Syntax: *ShrinkWrapShowLink

Sample output

ShrinkWrapShowLink
GZip             (&f89) *
Squash           (&fca)
CFS              (&d96)

The asterisk indicates which filetype is currently being used for compressing
files.

*ShrinkWrapSetLink set filetype to be used for compression.
Syntax: *ShrinkWrapSetLink <type name>




System Variables
================

ShrinkWrap$Scrap
Sets where scrap files are put. Defaults to <Wimp$ScrapDir>


Allocations
===========
Module name - ShrinkWrap
SWI prefix  - ShrinkWrap
SWI chunk     &4EA00
Error block   &00811400
Filetype      &F89 - GnuZip


Swis
====

ShrinkWrap_Memory (&4EA00 + 0)
------------------------------

r0=0 control memory allocations

Entry r0=0
      r1=min
      r2=max

Exit  r1=min
      r2=max
      r3=current
      r4=total memory in machine


ShrinkWrap_Zip    (&4EA00 + 1)
------------------------------

r0=0  Make a GZip file

Entry r0=0
      r1->source      filename
      r2->destination filename

Exit  -


r0=1  Expand a GZip file

Entry r0=0
      r1->source      filename
      r2->destination filename

Exit  -


ShrinkWrap_Control (&4EA00 + 2)
-------------------------------

r0=0  Add zones

Entry r0=0
      r1->path to add
      r2=0 zone uses file types, r2=1 all files are compressed
                                 r2=2 no files are compressed
      r3=0

Exit  -


r0=1  Rem zones

Entry r0=1
      r1->path to add

Exit  -


r0=2  Add type

Entry r0=2
      r1=type

Exit  -


r0=3  Rem type

Entry r0=3
      r1=type

Exit  -


r0=4  Set strength

Entry r0=4
      r1=strength

Exit  -


r0=5  On/off

Entry r0=5
      r1=0 off, r1=1 on, r1=2 no effect

Exit  r1=state



ShrinkWrap_Link (&4EA00 + 3)
----------------------------
This swi allows modules to link to ShrinkWrap


r0=0  Add link

Entry r0=0
      r1->link block
      r2=linked modules private word
      r3=compatability level

Exit  -


r0=1  Remove link

Entry r0=0
      r1->link block

Exit  -





ShrinkWrap_Util (&4EA00 + 4)
----------------------------
This swi makes available ShrinkWrap resources to the outside world.


r0=0  Create a scrap file

Entry r0=0
      r1->scrap handle

Exit  -



r0=1  Remove a scrap file


Entry r0=1
      r1==scrap handle
      r2==previous error pointer

Exit  -



r0=2  Return an error block pointer


Entry r0=2
      r1==error number


Exit  -



r0=3  Fill source buffer


Entry r0=3
      r1->buffer


Exit  -



r0=4  Dump destination buffer


Entry r0=4
      r1->buffer


Exit  -




r0=5  CRC32 block


Entry r0=5
      r1->crc
      r2->data
      r3=length


Exit  -







Service Calls
=============

Service_ShrinkWrapStarting 0x80181

Issued when ShrinkWrap starts up - chance for modules to register.






GZip &F89 file format
=====================

The GNUZip file format - see below.

With an extension

Word 0 ='A'+(((int)'C')<<8)+((4*7)<<16);
Word 1 =load address
Word 2 =exec address
Word 3 =file access bits
Word 4 =file length
Word 5 =0
Word 6 =0
Word 7 =0

GZip file format

The pkzip format imposes a lot of overhead in various headers, which are
useful for an archiver but not  necessary when only one file is compressed.
gzip uses a much simpler structure. Numbers are in little  endian format, and
bit 0 is the least significant bit.

A gzip file is a sequence of compressed members. Each member has the
following structure:

2 bytes magic header 0x1f, 0x8b (\037 \213)
1 byte compression method (0..7 reserved, 8 = deflate)
1 byte flags
bit 0 set: file probably ascii text
bit 1 set: continuation of multi-part gzip file
bit 2 set: extra field present
bit 3 set: original file name present
bit 4 set: file comment present
bit 5 set: file is encrypted
bit 6,7: reserved
4 bytes file modification time in Unix format
1 byte extra flags (depend on compression method)
1 byte operating system on which compression took place

2 bytes optional part number (second part=1)
2 bytes optional extra field length
? bytes optional extra field
? bytes optional original file name, zero terminated
? bytes optional file comment, zero terminated
12 bytes optional encryption header
? bytes compressed data
4 bytes crc32
4 bytes uncompressed input size modulo 2^32

The extra field, if present, must consist of one or more subfields, each with
the following format:

subfield id : 2 bytes
subfield size : 2 bytes (little-endian format)
subfield data

The subfield id can consist of two letters with some mnemonic value.

Ids with a zero second byte are reserved for future use. The following ids
are defined: Ap (0x41, 0x70) : Apollo file type information The subfield
size is the size of the subfield data and does not include the id and the
size itself. The field  'extra field length' is the total size of the extra
field, including subfield ids and sizes.




(c) David Pilling 1997

--
email: [e-mail]
  web: http://www.netlink.co.uk/users/pilling
  web: http://www.pilling.demon.co.uk
 post: David Pilling, P.O. Box 22, Thornton Cleveleys, Blackpool. FY5 1LR. UK.