[sebhc] Dave Dunfield's H8 emulator

[Dave Dunfield]

>Usually, H8 comes up with a constant beep on the sx. It responds to a
>reset but then goes back to beeping as soon as a key is pressed. It does
>seem to be attempting to synch, but it's not succeeding. I'll try giving
>it several-to-many seconds between keypresses to see if that helps, but
>clearly this is not a big problem.

Hi Jack,

This is "normal" on a slow machine - it will (well - should) eventually
sort itself out, but it takes a LONG time on a machine less than 386/33
- if you want closely, you will (after while) see the display segments
start to update - one at a time.

This is because the emulator is inserting virtual interrupts between
intervals of 8080 instructions, and on a slow machine they are inserted
very infrequently - the VM self-adjusts to bring them to a 500hz rate,
however this adjustment is very slow, so that it does not get fooled by
momentary changes in system performance (interrupts, multitasking etc.)
- It's more of a "slow drift".

Basically what is happening is that the H8 turns on the buzzer, and is
now waiting for a timeout - on a slow machine, the interrupts are
happening so infrequently that this takes a LONG time.

I chose this method rather than programming a PC timer interrupt at a
500hz rate because it makes the emulator very much more portable and
tolerant of non-DOS environments... I may still change it to a better
approach, but at this stage it works well enough - on my todo list.

Regards,
Dave

-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.
                http://www.parse.com/~ddunfield/museum/index.html

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