Antigrav Toolkit Notebook
by Randy Winchester

File Transfers with GEOS

GEOS files are a new type of construction - much different than other types of files that existed prior to GEOS. Special handling is required to prepare GEOS files before shipping them off along the phone lines. The tools to prepare GEOS files are well developed, making file transfers highly reliable.

One of the first things I tried to do with GEOS was to write some utilities that would work with it. I finally settled on a project - read a standard SEQuential text file and output a GEOS Text Scrap. Then I would be able to take files I created with SpeedScript, convert them to Text Scraps, then paste them into geoWrite or geoPaint documents. I had a lot of good information on SEQuential files, but no information on Text Scraps. Examining GEOS disks with a disk editing utility revealed a lot of information about GEOS files.(1)

The directory entry for a GEOS file has more data than a standard file entry. The GEOS entry has been expanded to provide room for time, date, and file structure data, and a pointer to a block called the file header. The file header contains, among other things, the file's icon that you see while using the deskTop.

So, after a week or so of hacking, I had a program ready.(2) I wanted to upload this program to a local BBS, but before I tried, I was already aware I had some problems to solve first.

GEOS File Structure

Before GEOS, uploading and downloading a file was a clearly defined process. You got the telecommunications software ready on the sending and receiving ends.(3) You told the receiving end to expect a file named "OLDFILE" and you ready your software to start sending "OLDFILE." Commodore DOS finds "OLDFILE" in the directory and sends the file.

Take a look at the simplified diagram of the structure of a GEOS VLIR file. VLIR stands for Variable Length Indexed Record. It consists of multiple segments, called records, which can be of any length. One block is called an index. The index is a table of pointers to the records. There is also the info block and the time/date data following the directory entry. The only part of this file Commodore DOS is able to locate from the directory entry is the index block. If you use the same telecommunications software to send this file, you're in for a big disappointment. It'll only transmit one block, the index block.

GEOS CONVERT

A simple solution I devised was to write a short program that would convert disk blocks to BASIC DATA statements.(4) Searching through the disk, I made notes to include the blocks containing the directory entry (which would also include the time, date, and structure data, and the pointer to the file header), the file header, the program, and if a VLIR file, the index and all of the records. I then fed this data to the utility to produce the DATA statements. The resulting DATA statements were then appended to a companion program that would write everything back where it should be on another disk. This approach produced a file which could safely be uploaded and downloaded as a BASIC program and reproduce a complicated GEOS file on the receiving end. The disadvantages were that the process wasn't automatic enough, and that DATA statements can require more than twice as much storage as raw disk blocks.

These days, the utility of choice is CONVERT.(5) It runs in the GEOS environment as an application. It converts GEOS files into SEQuential files by changing the pointers so that the directory entry points to the file header, the file points to the index block, the index block points to the first record, first record to the second record, etc. The time/date/structure data from the directory gets written to the file and the file type gets changed to sEQ. When converting a SEQuential file back to GEOS, time/date/structure data gets written to the directory entry, the files gets it's pointers changed back to what they should be, and the original file type is restored. Since CONVERT doesn't actually create a whole new file, it operates very fast.

ARC and SDA

Don't start thinking that CONVERT is the last word. While it does what it is supposed to do, there is a problem with the files it creates that will sometimes cause difficulties. The glitch is in the way some telecommunications software handles SEQuential files. Usually executable code is stored in PRoGram files using bytes that are eight bits long. SEQuential files are typically used for text files. Since only seven of the eight bits in a byte are needed to represent the entire ASCII character set, the eighth bit sometimes gets altered. If you transmit executable code in a SEQutential file and your software assumes that it is text and does something funny to the eighth bit, the receiver gets a mutilated file.

The solution is to change the SEQuential file into a PRoGram file for it's trip over the phone lines, then change it back to SEQutential on the receiving end. Although this is a trivial operation, it is one more step. Using ARC can make it worth your while in more ways than one.(6)

ARC is short for ARChiver. It does some very nice things other than change SEQ files to PRG. ARC can combine multiple files into a single file. Say you have 3 files - an application, a data file, and a documentation file. ARC will read those files and output a single file which contains the three original files. ARC also compresses the output file. With some files I've ARCed, the output file was only half the size of the input file. A real benefit of using ARC is that you'll spend less time uploading files. And you'll also spend less time downloading files that other people have ARCed. A recent development in the ARC world is the Self-Dissolving ARChive, or SDA. It creates an ARC file that doesn't require ARC to dissolve. Just LOAD and RUN any file with .SDA at the end of it's file name, and the original files get written back to disk in their original form. How nice!

SUMMARY

One of the tradeoffs of user-friendly systems is usually added complexity at some level. With GEOS, that complexity shows up in the file structure. Until a user-friendly GEOS terminal program is available to handle user-nasty files, we'll have to rely on good utilities and a little knowledge of how things work.

I would like to include your GEOS hints & tips and bug reports in future columns. If you have any brief GEOS tricks or bugs that you've discovered, please send them to me.

Randy Winchester
P.O. Box 426074
Cambridge, MA 02142

In exchange for material submitted on disk, I'll return a disk of public domain GEOS software.

1. I use the public domain DISK PATCH V2.0 by Kevin Pickell, L. Phillips and J. May.

2. SEQ->Text Scrap. Versions 1.0 - 1.3 are freely available to the public. Versions 2.0 and higher are a commercial product.

3. Recommended terminal software: 64 mode - the public domain CBTERM by CHRISDOS. 128 mode - ULTRATERM, shareware by Steve Boerner.

4. SECTORS->DATA, DATA->SECTORS, public domain. A related utility SECTORDUMP, creates a formatted file suitable for printing.

5. CONVERT by T Bone 22. Version 1.4 is current. Freely available to the public.

6. ARC is available for both 64 and 128 modes. It is a shareware program by Chris Smeets.

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