Eprom Modules for the HP 75

[Paul Berger (Canada)]

(08-02-2016 01:21 PM)Michael Fehlhammer Wrote:  Meanwhile Dominique went on to produce a modern version of the ROM simulator, in order to better understand all the intricacies of the ROM addressing and bank switching schemes, and before attempting to pack the logic into the small housing of an EPROM module, which is the final goal, of course.
He has got the NoMas listings of the HP 75 operating system, but they are huge, of course, and he is not (yet) familiar with them.
Can someone (Paul?) help him to locate the part (subroutine) where the bank switching (ROM addressing) is managed; literally Dominique says:

"My objective is to locate in the source file the sub's managing the paging process depending of the number of 8k pages used by external modules. The explanation in the Simulator Manual explains only the management of one (A + B double x 8ko" banks)."

Please help us; I am convinced that, with some help, Dominique finally will succeed in producing extension modules for the HP 75, and this would be a great achievement and would lift the usefulness of this calculator to a new level, wouldn't it?

By the way: Yes, Dominique has found a solution to the connector problem, but he hasn't revealed it yet. :-)

On page 2-45 of the service guide it states that up to 144K of extra ROM may be added, however on page 2-47 where there is a map of the I/O addresses which included the addresses used by the bank switching scheme there are 16 pairs enable/disable addresses listed and two of those are occupied by internal ROMs ALT an MEL which contain part of the operating system. Which does not add up to 144K. It would seem to me that a ROM emulator could use any of these other than the ones used by the two system ROMs, however you should keep in mind that the expansion POD also has a couple ROMs built into it.

The mechanism to do the bank switching seems to be that you turn off the active ROM by writing to its disable address and enable the desired ROM by writing to its enable address. The ROMs are obliged to monitor these addresses to decide when to turn on and off. There are a couple signal lines on the front slots that allow the plugin ROMs to know what slot they are in and from there it knows what address it should monitor.

There is better documentation in the NOMAS, specifically "Joey's Big Book of ROMs" if the NOMAS that Dominique is working from is the scanned copy on the MoHPC documentation set, I wish him luck, the four up page scans are not of great quality, and it has been OCRed however because the scan is not really good the OCR results are even worse you need to be really creative when searching. This documentation seems to be more complete and up to date than the service guide. It also mention the use of addresses FF50 to FF5F for bank selection, with the exception of FF55 which is used for testing. It also mentions that there was a second version of the ROMs that uses a slightly different and improved scheme for bank selecting ROMs when the ROM is turned on as usual by writing to even enable address, but the ROM is automatically deselected when any other address in the FF30-FF5F range is written to, however The ROM routines may still use the original method of using even enable addresses and odd disable addresses for ROM that are in the FF40-FF4F range that the original version of ROM supported. I just looked at the board for a production level 75C and all the on board ROMs are the original part number, two of which would be the bank selected ALT and MEL ROMs, and the enable/disable addresses for these ROMs is the FF40-FF4F range.