MSX Assembly Page

VDP programming guide

Latest updates:

2005-10-05
Update r#18 Q: it only happens with sprites disabled.

2004-01-19
Applied gamma correction to the screen 8 palette to make it sRGB (thanks to Maarten ter Huurne).

2004-01-18
Wrote a little explanation about what the proper values for Blue colours are in screen 8, and replaced the palette download.

This article aims to document most VDP programming loopholes you may encounter. Below is a list of tips and answers to questions, which may be useful either before programming a certain VDP-related routine, or afterwards to hunt down bugs (should’ve read it beforehand after all, eh ;)). The screensplit related tips are located in a separate article. If you have any suggestions, please submit them to us!

Q: r#18 in combination with VDP commands corrupts VRAM?

A: Yes. This issue only occurs when you have the sprites (or screen) disabled. If you use the screen position adjust register while executing a copy command and with sprites disabled, it will corrupt the byte the VDP command is currently processing. So, if that is the case, you should wait with setting r#18 until the VDP’s CE bit indicates the current command is finished.

Q: What about screensplits with the help of r#18?

A: Once again, this register behaves a bit strangely when you disable the sprites. During the display cycle, if you move the screen to the left using r#18, the entire current line is drawn in the background color. This only happens when the sprites are disabled. So if you’re creating a screensplit with the help of r#18, watch out for this.

Q: VDP does lots of random strange things, how come?

A: Do you DI during the register OUTs? Is there a poll loop during which the ints are continuously disabled, hence messing up your screensplits? It has probably something to do with the interrupts.

Q: Where did screen 9 / r#24 go?

A: ASCII’s official MSX Magazine says about this: SCREEN9は、韓国のMSX2にのみ搭載されたハングル表示専用モードで、国内のMSXと同様にMSXPLAYerでは搭載していません。. For the ones among us who can’t read Japanese *cough*, it translates to something like this: “SCREEN 9 is not available, it is a Hangul (Korean character set) display mode only equipped on Korean MSX2 computers, while the MSXPLAYer is like the domestic (Japanese) MSX.” So, the screenmode is supposed to be a textmode only available in the v9948 videochip, a Korean version of the v9938. In the v9958, this mode was excluded, so screen 9 isn’t available there, nor is r#24 which is used to control the mode (although this is not for sure). Bit 2 of r#8 also seems to be used by the v9948 (unused in the v9938 and v9958). The v9958 continues with modes 10-12 and registers 25-27.

What is there to say about screen 8’s colours?

A: Screen 8’s colours are composed of Red and Green, with ranges of 0-7, and Blue, with a range of 0-3. Red and Green’s colours are divided over a gliding scale. So, if you were to map these 3-bit colours to 8-bit colour information, you would get steps of (255/7), or in other words: 0, 36, 73, 109, 146, 182, 219, 255. In Assembly, this can by the way easiest be calculated by shifting and OR-ing bits 0-2 as follows: xxxxx210 --> 21021021 (although a table may be faster :)). Anyways, the real subject of this Q&A is the colour Blue. This one does NOT operate on a gliding scale from 0-3 (so no 255/3 stuff), but, just like the other two colours, also from 0-7. It then maps its four colours as follows to that range: 0->0, 1->2, 2->4, 3->7. There is a small test program below for you to experiment with, if you want. Anyways, because of this, gray colours can be really gray and not blue-ish or yellow-ish teints.

Another issue is that although the voltages the VDP outputs increase linearly, the actual intensity of the colours displayed has an exponential curve (thanks to Maarten ter Huurne, btw). To cope for this, you have to apply gamma correction to the colours. A television or an MSX monitor generally has a gamma of 2.5, while on a PC the usual colourspace is sRGB nowadays, with a gamma of 2.2. So, in order to get the correct colors displayed on your PC, you’ll have to apply a gamma correction of 2.2 / 2.5 = 0.88 to the palette. Paint Shop Pro can do this fairly easily.

Many who designed or converted graphics on the PC have probably not considered these two ‘complications’, and instead used a gliding blue scale and no gamma correction, which would yield somewhat inaccurate results. I have however put a proper palette online for your convenience. It used to be a ‘wrong’ one too, so if you downloaded it before, do it again.

10 SCREEN 8:SETPAGE 1,1:LINE (0,0)-(255,211),&B11111111,BF
20 SCREEN 1:KEY OFF:COLOR 15,4,0:COLOR=(4,5,5,5)  'change to (4,4,4,4), etc
30 VDP(0)=0:VDP(2)=6
40 IF INKEY$="" GOTO 40
50 VDP(0)=14:VDP(2)=63
60 IF INKEY$="" GOTO 60
70 GOTO 30


As you can see, this article is still being worked on. If you have any ideas for good Q&A’s, please let us know and we will add them.

~Grauw