14 Jun
2005
14 Jun
'05
9:47 a.m.
From: "Dave Dunfield" <dave04a at
dunfield.com>
---snip---
If you do need RAM at 0000, then you will need to add a bit of circutry to
allow your rom to "MOVE". One simple way, assuming you want to put your ROM
at an address >= 8000 is to upon RESET, double-map your ROM to it's correct
address (>= 8000) AND at 0000. While doing this, you need to pull the
phantom signal to disable any RAM at 0000 - Begin your bootstrap code with
a JMP to it's correct address (+3) ... As soon as you see an instruction
fetch with the high bit set, release the double-map at 0000 and phantom,
and your system will be operating normally from the ROM at the higher
address.
Hi
An alternate method, since this it to be your tinker system,
would be to put a switch so that the EPROM board always
is seen at 0000 when the switch is held. When released,
it takes on the address that is out of the RAM's way.
If you make it so that it is double mapped while the switch
is pushed, making the transition to the normal address is
trivial.
One could even include a one-shot to trigger the reset.
Dwight