V9958

Some time ago we introduced a tiny tool called ppmview.prg in our collection of tools and progs for our Steckschwein. The ppmview.prg, as the name already tells, is able to load an image in ppm (Portable BitMap) format with a maximum size of 256x212px and displays it on the screen.

The first version of ppmview was released in 2018 and since then we just use the SCREEN 8 (MSX) also known as graphic mode 7 (RGB).
Mode 7 is a bitmap mode where we have 256 colors available and each pixel requires 1 Byte for their color information. The 1 Byte color information must be stored in VRAM encoded as GRB 3:3:2 (green, red, blue), which means 3 Bit green, 3 Bit red and 2 Bit blue.
On the other side we have the PPM color information - which is 24Bit RGB - so we have to adapt or map the 24Bit to our 8 Bit GRB color value in some way.
It becomes apparent that this comes with a price, namely the loss of color information. Especially the blue part of the pixel is just 2 Bit, hence we have 4 levels of blue per pixel available only. The following screenshot from our Emulator shows the loss of color information.

… on the way back to munich, we had some time to do a little code review of our gfx library. thinking about the cpu to video chip timings and again read the well known datasheets of the V9938/V9958. suddenly i got an enlightenment and we came to the following conclusion.

as described in the datasheet (V9958-Technical-manual_v1.0.pdf) of the V9958 there are different timings given for different kind of writes. so as far as we understand there are the following timings