This is a basic extension original written by the company Print-Technik for the Commodore PET in 1981, and later converted to VIC20, and C64/C128. Information about this company you can find at: https://www.c64-wiki.de/wiki/Print-Technik
The latest known Version is a module for the C128/C64, and its called "Help PC-128 Plus C-64".
A special thanks goes to Johann Klasek, who has support me to get the C64/C128 version, and for his first examination on it, and a special thanks goes to Diddl, for providing V4
You can found more information about the C64/C128 Modul version on Johann Klasek's webpage: https://klasek.at/c64/helpplus/
So far, there are 5 different version for the C64 known by me.
This should be the initial version for the C64, the screen message shows "*** HELP C-64 PLUS ***".
So far i can identify one error at address $8182, there is a "BCS $810C", and should be "BCS $810E"
The screen message is the same with version 1, the error in $8182 is corrected, a new command is added, and some other changes are applied.
Added command #B = "RENEW".
There is a change in the directory command, it checks now additional for device No.4.
Additional one, in my opinion not necessary, JSR command was changed at address $89E3.
The screen message is changed to " PRINT-TECHNIK-HELP-PLUS", additonal compare to version 2 a change in the dump command is done.
A space is added to the output.
Before the output was: "A =12".
now the output is like: "A = 12".
There are also other changes in other places, on which i did not do any further investigation.
The screen message is changed a little bit to "PRINT-TECHNIK-HELP PLUS ",
There are some smal code changes applied in differnt are. This should be some improvements only.
Change a branch in the DUMP area.
Add some code to the RENUMBER part, a check for 'qoutes' is added.
In the 'ASSEMBLER' area, a branch instruction is changed from 'bne' to 'beq'
Additional some changes in the 'COMPACTOR' area are applied.
The verion 1 to 4 can be easy used as a modul, by programming it on an EPROM, and connect it to the C64
This is as far as i know the last version.
In the Manual it's named "Help PC-128, Plus C-64"
This version has a new hardware design for the modul. It will also mirror a part of the modul to address $DE00.
At this location the modul has some switching commands, which allows to switch off the modul, and switch between RAM and ROM by changing the GAME and/or EXROM lines. Btw, this is also a nice copy protection.
The code is rearranged, and optimezed in several places. So far i have not seen some functional changes in the different program parts, except the jumpings to the addresses in $DExx, for switching between RAM and ROM.