MCODE: XKD another bug fixed

[hth]

The page 4 trick is unfortunately still needed to maintain the changed keyboard and display behaviour in Ladybug, which is unrelated to semi-merged prompts and XKD.

The changed keyboard and display trick is really done by the I/O poll vector (and partial key sequence parsing), but unfortunately the HP-41 firmware skips calling the I/O poll vector when it is in the hurry, so the changed behaviour needs page 4 to make it reliable.

What this correction does is that if you press a semi-merged prompting function on the keyboard, the prompt now always come up, even if you keep the key pressed. Try the ordinary RCL key, press and hold. The prompt comes up immediately, no "NULL" or cancellation.

This bug causes the corresponding prompting semi-merged LDI instruction to only have that behavior in program mode. In run mode it will show "LDI" followed by "NULL". This bug fix makes it work in a consistent way, always immediate prompting no matter what mode the calculator is in.

I find it interesting that they thought of this, provided functionality for it and even commented on it, but unfortunately managed to screw it up slightly and no one ever seem to have noticed. I mean, this functionality can be used for other things, it just happened to be the perfect match for my stuff.

XKD is used by the built-in mode shifting keys, ordinary SHIFT, BST, SST and R/S instructions. This feature means that other special behaviours on key down can be implemented by XROMs, at least that was the thinking when the firmware was originally created.