Alright, I have manged to get a little charplasma going now (that's the democoders equivalent of Hello World! 
)
I'm still a bit confused on the VDP though. I read somewhere that you could only access VDP during the vertical blank period, otherwise you would get lockups...?
I did some tests with a loop that constantly wrote to VRAM using the VDP, and it seemed to work fine, have I misunderstood something?
Speaking of VBL, I also wanted to find a way to wait for VBL to time my effects. There seems to be a standard interrupt that kicks once per frame, so I did like this
ei halt di
(Enable interrupts, wait for it, then disable again)
Is that a sensible way to make a VBL wait?
Then I do have some issues with setting gfx modes. The BIOS way works fine to set Screen 1 (32x24 text mode). However when I set Screen 0, I get 40x24 as expected in emulator, but on my real MSX (NMS8245) instead I get some kind of MSX2 mode with 80 characters wide screen.
I also tried setting modes with the VDP directly, using the code hitchhiker posted, but then I only get garbage on screen no matter what mode I try. Strange...
Alright, I have manged to get a little charplasma going now (that's the democoders equivalent of Hello World! 
)
I'm still a bit confused on the VDP though. I read somewhere that you could only access VDP during the vertical blank period, otherwise you would get lockups...?
I did some tests with a loop that constantly wrote to VRAM using the VDP, and it seemed to work fine, have I misunderstood something?
when working on a msx2 @3.5Mhz (V9938 VDP) you can output data to vdp at full z80 speed, even outside VBLANK, no problem at all.
when working on a msx1, (TMS VDP), the maximum speed you can achieve depends if you are in vblank or not.
outside vblank, every byte write or read should wait almost 26 z80 T-States (including the instruction that write data to VDP port 98h)
in vblank, no limit or delay.
For waiting for interrupts the tecnique below is OK, however i do not know why you disable ints
ei halt
Then I do have some issues with setting gfx modes. The BIOS way works fine to set Screen 1 (32x24 text mode). However when I set Screen 0, I get 40x24 as expected in emulator, but on my real MSX (NMS8245) instead I get some kind of MSX2 mode with 80 characters wide screen.
Umh, may be the real machine remember the width of screen so calling the bios to set screen 0, remember the 80 char width. the width is stored on the CMOS RAM. maybe if you do in basic (on the real machine):
Screen 0 Width 40 SET SCREEN : REM SAVE ON CMOS BATTERY THE WIDTH of 40
I also tried setting modes with the VDP directly, using the code hitchhiker posted, but then I only get garbage on screen no matter what mode I try. Strange...
to get a good setup of the screen mode, you need also to intialize the VRAM....