MSX Z80 replacement / new IC

Page 1/2
| 2

Par Leo

Paragon (1236)

Portrait de Leo

27-10-2012, 15:37

Hi All,
It is been a while i have not came to the site, i discover the new interface. Smile
This post is an enquiry for a Z80 replacement a new ASIC: about price and features.
I think i'd like to do one , technically it is something i know how to do, but the
problem is having the good features for the price.
I think keeping the price below 150 euro per part is not possible , 150 being the target.
Even for that price the chip will be quite small but enough to contain Z80 core and some
extra's : little cache memory, dedicated transfer unit to vdp/psg to enable playback while
cpu can work on cache, little fpu (this one will hardly fit ),

So any idea of feature that would make this appealing ?
Cheers,
-Leo

!login ou Inscrivez-vous pour poster

Par PingPong

Prophet (3459)

Portrait de PingPong

28-10-2012, 11:16

Leo wrote:

extra's : little cache memory, dedicated transfer unit to vdp/psg to enable playback while
cpu can work on cache
, little fpu (this one will hardly fit ),

-Leo

Can you describe more in depth those features, please?

Par Leo

Paragon (1236)

Portrait de Leo

28-10-2012, 21:13

the simplest way to see it could be : a Z80 plus a few kb of ram on chip , Z80 can work fast on the internal ram
and slower with external ram.

Par hit9918

Prophet (2868)

Portrait de hit9918

29-10-2012, 01:13

I dont want to spoil hardware makers feature fun.
But I got to tell the software mans side.

If I were finished with my MSX1 game, I would not switch to the chip with extensions, but the MSX2. I write an app for that, and many apps exist for that, and userbase exists for that.

Better than 9918 with additional funky mode is 9938.
Better than z80 with DMA is R800.
I.e. chips of existing models.

There are two MSX2, 9938 and 9958. The latter one does not waste the blitter on scrolling.
In case of 9958, there are again two models, the second one with R800.
In case of R800 the 9958 speed cries for 9990 VDP.

split split split, all these versions are waiting for more games, dont need more split.

In case R800 is asked too much (cycle exact is much prefered), still better than Z80 with DMA is z80 with ability to switch to higher Mhz.
The z80 with DMA is an excellent idea, but a problem when considering the software side.

The 3.57Mhz mode still must exist for timed software.
I just imagine this extension:
a delay register that is active in high Mhz mode. Delaying every IN/OUT instruction by that amount of cycles.
On power-up it should initialize to e.g. 29 (another VDP delay for a 7Mhz mode).
It should not change on reset, so one can change config and then reset.

This is an example where there actually is a new extension,
but not asking for new software for a small userbase,
instead with the aim of getting existing software to run.

Par PingPong

Prophet (3459)

Portrait de PingPong

29-10-2012, 20:17

Leo wrote:

the simplest way to see it could be : a Z80 plus a few kb of ram on chip , Z80 can work fast on the internal ram
and slower with external ram.

yes, but what are you meaning about the psg/vdp features you mentioned earlier?

Par hit9918

Prophet (2868)

Portrait de hit9918

30-10-2012, 01:06

@PingPong,
a DMA unit sending RAM to IO port. just a separate unit doing "an outi". can e.g. turn PSG into a sample player!
Also DMA unit could "copy screen to vram while z80 does something else".
Boy the TurboR should have had this.

copy to vram, 3 of 29 cycles used for RAM read, DMA unit slows the z80 down to 90%. Very aceptable.
Need a delay register to not transfer beyond VDP speed limit. At same time the delay register is frequency register for PSG samples. Give it 16 bits.

To play a sample at different loudness, need this:
A register that is the byte that gets ADDed to every byte transfered.
This ADD also could be useful for nametable transfers to VDP.

For mixers to play digi without cracks, DMA unit would need this:
interrupt when finished (generaly wanted feature)
when poking the startaddress and endaddress while the unit is running, these pokes get delayed till the transfer is finished. When finished, chip continues with these register settings, i.e. "run a new job without cracks".

Par hit9918

Prophet (2868)

Portrait de hit9918

30-10-2012, 01:17

@Leo, how about a Z80 DX2 Smile double Mhz core speed.

If you can separate memory refresh in R800 style, an opcode fetch goes in 3 cycles.
Then make waitstates for the core so memory acess does not suceed 3 cycles.

speed:
add b in 3 instead 5 cycles (figures in 3.57Mhz scale)
ld a,(hl) in 6 instead 8 cycles
inc hl in 3 instead 7 cycles (more than 2x faster, the idea is: 6 cycles without M1 wait, /2 = 3 cycles in 3.57Mhz scale)

But there should be a cycle exact original speed mode.

Par Leo

Paragon (1236)

Portrait de Leo

30-10-2012, 18:44

so what i understand is that a r800 upgrade for a regular msx-Z80 is more interesting.
i was thinking something way more faster than r800, for instance allowing new sw like mp3 decoding or
msx emulator inside a msx

Par Leo

Paragon (1236)

Portrait de Leo

30-10-2012, 18:56

by the way R800 has dma but for some reason it is unsuable , i think i read somewhere the r800 can address 512kb directly but we use only first 64kb and this makes the dma channels unusable, i tried to dig some information about it but without success. i think some parts of the z280 (alu) is reused for r800 while some parts of the 64180 (dma) is reused inside the R800.

Par hit9918

Prophet (2868)

Portrait de hit9918

31-10-2012, 01:41

"mp3 instructions", it is the same issue as with the DMA idea. I can't resist mentioning the software situation Smile When one is glad when there are new productions at all.

Example "z80 with no speedup feature, but mp3 and DMA":
User unpacks it and asks "so where are the mp3 music apps", "where are the DMA games" Wink
If you got the coding power at hands to do THAT, I would please you "forget the chip and release those games for normal MSX" Wink

Example "z80, the same old instructions, but fast":
Games timing their speed with vblank interrupt, they go better framerate on fast cpu, all these apps got the chance to make a difference on your chip, even when the coder never heard of it. Said game could be written on a machine with your chip, and also run on TurboR, and maybe it would not have been written if TurboR users would not exist.

This is why I think a lot value got "same old z80 instructions running faster".

Par hit9918

Prophet (2868)

Portrait de hit9918

31-10-2012, 01:54

@Leo, in search of some special gimmick to put in the commercial, I think I got one:

Faster vram acess than TurboR! Smile

A fast z80 + cycles configurable cpu brake for IN/OUT instructions.
The brake of TurboR in turbo mode does brake too much.

Page 1/2
| 2