Suggestion: Turbo support
sdsnatcher opened this issue · comments
I added this suggestion here on its own issue, to avoid cluttering on the MSX.org news thread.
In fact, the circuit for the turbo is very simple. For the MSX1 and MSX2, the turbo will be disabled on all I/O operations, but it still can provide a very welcome boost in performance. An MSX1 at 5.37MHz with this turbo can run ZX-Spectrum ports at their correct speed, i.e.
As mentioned before, for the MSX2+ it's possible to speed-up the access to the VRAM, thanks to the V9958 /WAIT feature. For this, Belavenuto and myself have designed a GAL logic that:
- Turns the turbo ON/OFF at the switched I/O port 41h (you can change it to another port if needed)
- Disables the turbo for a selection of I/O ports, but not the VRAM I/O port.
It only can't read the status of the turbo, as it was designed to complement implement the turbo on Sanyo2+ machines, and those already provide the turbo status feature internally. You probably would be able to easily add the turbo status feature
If you're interested, I can gladly send you the .EQN file for the turbo GAL as an open-source contribution.
frankly, the benefit won't be that great.
I beg to disagree. Once you have used a turbo MSX machine, you won’t want to go back and will start wondering why they haven’t done this earlier. 😉
There are plenty of heavy games and apps that will run much better when a turbo is present. Just to name a few:
- Games: Metal Gear 2, Space Manbow, King’s Valley 2, Penguin Adventure, F1 Spirit, Undeadline, Snatcher, SD Snatcher, Valis 2, Ys 3, any ZX-Spectrum port, any PC-88 port, Aleste, Aleste 2, 1942, Gunship, Sim City, Red Zone, Chibi’s Akuma, Tales of Popolon, Playball 3, Xak1/2/3, Fray, Illusion City, etc
- Apps: Nextor/MSX-DOS2, SymbOS, Graph Saurus 2.0, Philips Video Graphics, Print Shop II, InterNestor, MUE, MuPlay, etc
To replicate turbo functionality I would
need to implement:
Glitch-less clock switching circuit, an I/O port to turn on/off turbo,
As you have seen, the SuperTurbo design is very simple and compact
a wait state generator to generate additional wait states when accessing PSG
SuperTurbo takes a very simple approach to this. It just lowers the clock when needed
and perhaps the cartridge slots.
Not really necessary, for speeds up to 10.74MHz. But the approach I use when I want to be conservative is to connect the SLTSL pin of the slot-1 to one of the SuperTurbo break inputs. This allows a flexible configuration where the slot-2 is the “turbo slot” and the slot-1 is the “slow slot”. On a new motherboard like this, configuration switches could be added to select if a slot will be turbo or not.
As far as circuit protection and turbo goes, I simply don't have the real estate on the board to implement these. Turbo is on my wish list for a CPLD implementation.
Ok. For the current PCB, maybe it could be added later as a daughterboard piggybacked to the U12 position (+RD signal). It seems to be possible since you took great care on the mobo design. Just please add simple jumpers to each slot /CPUCLK signal, to make it easier to inject the turbo clock there for a specific slot if needed.
Due to being very simplistic, the SuperTurbo 3.0 is a good design to add turbo for older/existing machines, or when space/cost/DIP-requirement are of concern. If you're going to use a CPLD, then it's much better to take the wait-state generator approach.
The VDP requires a custom waitstate generator. Please let me know at this thread when you decide to implement it, so I can come back to describe the specifications. A generic waitstate generator will have a horrible impact on the CPU->VDP access, as happened on the MSX Turbo-R.