20 Feb
2018
20 Feb
'18
5:18 p.m.
It was thus said that the Great Eric Christopherson via cctalk once stated:
On Tue, Feb 20, 2018 at 5:30 PM, dwight via cctalk
<cctalk at classiccmp.org>
wrote:
It depends upon how cycle accurate you want. My own MC6809 emulator [1]
keeps track of cycles on a per-instruction basis, so it's easy to figure out
how many cycles have passed. Hardware emulation can be done between
instructions by updating per the number of cycles passed (if required). I
don't have the code for the MC6840 (a timer chip) in my MC6809 emulator
repository (it's still somewhat under construction) but I do update the
timer based upon the elapsed cycle count of the previous instruction [2].
-spc (Code available upon request if you want to look at it)
[1] https://github.com/spc476/mc6809
[2] It's not emulating any existing machine. Rather, I'm emulating a
system with a few serial ports (MC6850) and a few timer chips
(MC6840). I have plans on adding a few floppy controllers (MC6843)
and a DMA chip (6844).
In order to connect to the outside world, you
need a way to queue event
based on cycle counts, execution of particular address or particular
instructions. This allows you to connect to the outside world. Other than
that it is just looking up instructions in an instruction table.
Dwight
What I've always wondered about was how the heck cycle-accurate emulation
is done. In the past I've always felt overwhelmed looking in the sources of
emulators like that to see how they do it, but maybe it's time I tried
again.