MANUAL FOR UNZIP (BBC BASIC+ARM/6502 ASSEMBLER)

This file describes the program UNZIP which opens and extracts the contents
of a ZIP file into the currently selected directory.
The manual and software are (C)1999 SPROW

INSTRUCTIONS-
At the BASIC prompt, type CHAIN"UNZIP"

The program will then ask for the source filename of the ZIP file. Enter this.

After some checks have been made on the ZIP file then the stored files will be
unpacked to the currently selected directory, telling you also the date they 
were stored and their size.

Files which weren't stored (ie. in some way compressed) or encrypted will be 
skipped over, and files can be optionally skipped by answering 'N' when 
prompted.

If the locally computed checksum doesn't match the one read from the file
then the file will be extracted anyway and a warning raised.

KNOWN PROBLEMS/FUTURE ENHANCEMENTS-
No known problems
No name or directory truncation occurs, thus names and directories in the
ZIP must be short enough to be supported by the target filing system.
Assumes ADFS *CDIR command is available to create directories, this might
not be the case.

EXIT MESSAGES-
File not found - the specified source file could not be opened
Not a ZIP file - the specified start file did not start with the ZIP id
Corrupt ZIP file - an invalid header was read from the ZIP file
Not enough memory for extras field - the extras field exceeded 128 bytes
Name longer than buffer - the filename and path exceeded 128 bytes
Ignored attributes - a warning that the attributes were missing or not 
                     written on an Acorn, so will be set to &00001900 &FFFF8023
File may be corrupt! - the stored CRC did not match the one calculated when the
                       data was extracted
I cannot run this code - an attempt was made to run UNZIP on a non 6502/ARM
Object in the way - when trying to create a directory, a file was found with
                    the same name as the directory which was being created

HISTORY-
V1.06 Can optionally stop unzip extracting a file. Several other cosmetic 
      changes. Files are extracted much faster due to use of OSGBPB instead of
      OSBGET
V1.11 Added CRC calculator, recoded BGET for 32 bit words
V1.12 Slightly altered the method for discovering which machine it's running
      on, improved error messages, made some (former) errors just warnings now.
      Full stops in filenames will be mapped to "/" as "." is a dir separator
      on Acorn machines
V1.13 Bugfix to CRC code when the block to be CRC'd is an exact multiple of
      256 bytes. Thanks to Ben Avison for that.
V1.14 Added sketchy directory support, assumes ADFS *CDIR command exists.
      Enlarged name buffer to 128 and range check it too.
V1.19 Can now choose to extract all files in a ZIP without prompting.
      ZIPs where the entire contents are within directories inside the ZIP do 
      now have those top level directories created first.