Hermit
Registered: May 2008
Posts: 208 |
Proposal for 2SID hardware-compatibility
I'm fiddling with 2SID hardware solution right now, and got an idea for a most suitable address for the 2nd SID, which could be compatible with almost all hardware solutions (except $de00, $df00, but they're not good selection anyway due to conflicting with AR,TFR, etc. cartridges.)
In VICE it's easy to set 2nd SID base-address, but it's not the case with real C64.
There's a multiplexer circuitry, which uses one of the address lines between A5..A9 ($20..$200) beside the original $d400..$d7ff chip-select signal. The address-line is hardwired in most of the cases, and not easily selectable/switchable in the real machines.
(Btw. I could solve it by hardware to see 2nd SID both in $d420 and $d500 locations, by "OR"-ing A5 and A8 lines...)
I think using/preferring only base-address $D7E0 for 2nd SID in 2SID programs & music releases would ensure that any of the A5..A9 address lines could select the 2nd SID in the machines, similarly to an "OR" function...
(the address came from this simple calculation: $d400+$20+$40+$80+$100+$200 = $D7E0)
It's easier by software than by hardware.
And VICE's 2nd SID address can be set to $D7E0. If VICE emulates the hardware multiplexor, the $d420, $d500, etc. settings could be fine too in VICE, just as on real C64, all would call 2nd SID at $d7E0
I guess it's not hard to configure in SID-players too...
Just, because 2SIDs seem to get more popular and real machine is always better to compose & listen with :)
Maybe the issue, it's not forward-compatible with 3SID, 4SID, etc solutions.
A further improvement to this 'specification' could be to distinguish mono (6 channels) vs stereo (2x3 channels) SID tunes by the A6 or A7 ($40 or $80) address lines, which seem rarely used for 2nd SID selection...
So in that case SID addresses could follow like this:
$d400..$d41f: 1st SID
$d7a0..$d7bf: 2nd SID, A6 ($40) low - line says "mono"
$d7e0..$d7ff: 2nd SID, A6 ($40) high - line says "stereo"
It was just a idea, i couldn't find this in the forum after some search, maybe it was mentioned before, maybe not... |
