Monday, August 22, 2016

Double-tap RESTORE to enter Hypervisor "freezer"

After a lot of little bug fixing, I finally got the keyboard based trap-to-Hypervisor function working.  While this might sound dull at first, it is actually super important, and enables a bunch of really interesting and fun features.

The best way to think of this, is as an integrated "freeze" button, that is triggered by tapping the RESTORE key twice in quick succession (between 50ms and 800ms apart).  When this occurs, it triggers a transparent trap to the Hypervisor. That is, the running program doesn't have the slightest idea that it has happened.  This allows the Hypervisor to run some arbitrary routine, before exiting back to the running program.  In other words, it really is just an integrated freeze function. Right now, that routine just toggles between slow and fast CPU speed, which is kind of fun, but not really that exciting.  Here is the current Hypervisor RESTORE trap routine:

; For now we just want to toggle the CPU speed between 48MHz and
; 1MHz

; enable 48MHz for fast mode instead of 3.5MHz
lda $D054
eor #$40
sta $D054

; enable FAST mode,
lda $D031
ora #$40
sta $D031

; bump border colour so that we know something has happened
lda $D020
and #$0f
sta $D020

; return from hypervisor
sta hypervisor_enterexit_trigger

There isn't anything really to exciting to see there: It just fiddles a couple of registers to toggle the CPU speed between normal and 48MHz, increments the border colour as a bit of a debug aid, and then exits from the hypervisor.

For the technically inclined, what you might be noticing what isn't there.  As I have talked about previously, the Hypervisor trap process is super efficient: The entire CPU state is saved to shadow registers, resulting in a 1 cycle entry and exit time from the Hypervisor, because you don't need to save or restore any CPU or memory mapping registers.  The CPU is automatically set to a known configuration on entry to the Hypervisor, and restores the running program's configuration on exit.  Thus, this entire Hypervisor trap takes only about 40 cycles, or about 850 nanoseconds.  Most modern desktop processors probably would have trouble beating that.  Indeed, as previously mentioned, a minimalistic Hypervisor trap can complete in under 200 nanoseconds.

Anyway, back to the story at hand...

Like on a freeze cartridge, we will implement a freeze menu, that will allow a number of useful operations.  The usual staples will be there, including memory monitor, the option to reset the machine, probably some poke finder type functions, the option to freeze the currently running program to disk, and so on.

However, the MEGA65 has been designed from the outset to do much more in the freeze menu.

First up, you will be able to switch tasks, by browsing through the list of tasks, complete with 80x50 pixel thumbnails that are drawn using the previously described hardware thumbnail generator, that continuously generates little screen captures of the running program.

Similarly, you will be able to delete tasks and start new ones.

So, for example, if you have a sudden need to show off your BASIC programming prowess half-way through a game of Ghosts and Goblins, you can just double-tap RESTORE, choose the menu option to create a new C64-mode task, demonstrate your elite status by typing something like:


and then when you have demonstrated your mastery over coding to whoever was doubting it, you can double-tap RESTORE again, and switch back to Ghosts and Goblins, which you can easily find from the thumbnails.

Similarly, when approaching a hard part of a game, you could freeze it, make a back up of the game where you are up to, and then go on to play that hard level, and reload the saved state until you can conquer it.  In this way, mere mortals should be able to get a score of at least 7 in Flappy Birds without too much trouble.

While these use-cases might be a bit simplistic and contrived, it is hopefully not too hard to see how the Hypervisor freeze menu will likely play a central role in the use and experience of the MEGA65 for many.  Thus it is really nice to have the hardware side of it implemented.  The next step is to start working on the menu program itself, the freeze/unfreeze routines, and getting saving to the SD card actually working, so that things can get saved.

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