HP97 The journey begins

[teenix]

Hi all,

While fiddling with the Continuous Memory for the 67, I thought I would try and implement more memory as well. This is not an easy task if you delve into the Microcode, as all sorts of problems and gremlins come into play, so (for me) the best way is to leave that code as is.

I was experimenting with this idea using the 67 emulator as a guide, and I noticed that there appears to be 3 RAM registers that are not used by the 67 - addresses 58, 59 and 60 in the last RAM bank. They never seem to be written to no matter what function is used by the 67. I imagine that all ROM/RAM chips are basically the same except for the ROM code in each of them. Maybe there was not enough ROM available to make use of these registers.

Anyway, using the emulator and without modifying the original Microcode, I found that I could expand the RAM 4 fold, increasing the storage from 26 to 104 registers.

The trick is that this RAM is not contiguous and sort of mimics the way the P<>S key works. That is, you only have access to a selected bank.

There are some unused programming codes available, so I used one to create a new code which is the [h] key followed by the [E] key. This is a "RAM Swap" instruction. It can be used in RUN mode, entered into a program, and executed during a running program. It can also be stored and read from magnetic cards.

To select one of the 4 banks, you key in 0, 1, 2 or 3 so that this value is in the C register, and then press [h][E] and the bank swap takes place. The actual 67 memory still works exactly the same, so you still have STO n, RCL n, P<>S, WDATA, (i), etc.

The downside is that you have to manage the RAM usage.

This should easily work in the new 67 CPU board but only when Continuous Memory is selected. Unfortunately, the original CPU board does not have enough RAM to implement this.

The next stage is using the available resources to bank swap the Program Memory, giving 896 steps but there are a different set of gremlins involved, one of which is you cannot bank swap during a subroutine :-)

cheers

Tony