Search found 256 matches

by tomaitheous
Wed Nov 28, 2007 3:53 pm
Forum: Mega/SegaCD
Topic: 3d software rendering on megacd
Replies: 20
Views: 17249

Is it possible to switch between H32 and H40 mode midscreen and back again during vblank(right before active display)?
by tomaitheous
Wed Nov 28, 2007 3:45 pm
Forum: Megadrive/Genesis
Topic: Genesis Address Bus and Max RAM
Replies: 35
Views: 25177

Ohh? It was microcoded? Maybe that would explain the slow memory access (4cycles). That's strange they did that considering the 6800/6809 series wasn't microcoded. Anyway, I always considered it RISC in that it had a small library of instructions, had a large array of registers(compared to other CPU...
by tomaitheous
Wed Nov 28, 2007 12:25 am
Forum: Megadrive/Genesis
Topic: Genesis Address Bus and Max RAM
Replies: 35
Views: 25177

The 68000 implements a 24-bit address bus, allowing it to address up to 16 MB of physical memory. Address storage and computation used 32 bits, however, with the high-order byte ignored due to the physical lack of pins. This allowed it to run software written for a flat 32-bit address space. By mod...
by tomaitheous
Tue Nov 27, 2007 1:33 pm
Forum: Mega/SegaCD
Topic: 3d software rendering on megacd
Replies: 20
Views: 17249

I have a few of the bulletins, I shift through them. How old is the bulletin. I'd consider that bit of information fairly important if I were a developer.
by tomaitheous
Tue Nov 27, 2007 1:29 pm
Forum: Megadrive/Genesis
Topic: Genesis Address Bus and Max RAM
Replies: 35
Views: 25177

ED: It's a 16bit DATA BUS and a 24bit ADDRESS BUS.

Sega stated it as a 16bit console because the ALU's are 16 bit. It has multiple 16bit ALUs that can be used for handling 32bit operand (which take longer to execute), but it's not a 32bit processor with a 32bit ALU like the 68020.
by tomaitheous
Sun Nov 25, 2007 11:21 pm
Forum: Mega/SegaCD
Topic: 3d software rendering on megacd
Replies: 20
Views: 17249

There is also the "write behind" or "write on top" or "write if !=0" feature, even more powerful than the 32x writes mode. I suppose they are still available in "software" writing... That's through the ASIC? For overlaying multiple object on the frame buffer? I figured it had some sort of overlayin...
by tomaitheous
Fri Nov 23, 2007 11:34 pm
Forum: Mega/SegaCD
Topic: 3d software rendering on megacd
Replies: 20
Views: 17249

Stef wrote: You forgot something by the way : the font data converter.
It's a sort of 1 bit to 4 bits color converter.
You can use it for fast mask or font decompression :)
Is that a BIOS routine or something that the ASIC can do(didn't see it listed in the specs)?
by tomaitheous
Mon Oct 22, 2007 1:18 am
Forum: Megadrive/Genesis
Topic: Data compression
Replies: 8
Views: 7130

Nice comparison list Shiru. I converted pucrunch decompressor for another platform - it's a pretty decent compressor. I found quite a few LZSS compression schemes that use a circular buffer for mem->I/O (one that did I/O->I/O), avoiding the need to decompress large files before copying them to VRAM,...
by tomaitheous
Thu Oct 11, 2007 1:26 pm
Forum: Video Display Processor
Topic: TV safe area
Replies: 123
Views: 57270

You're thinking about it wrong: there are two fields that make up each frame, an even and an odd field. We say the field rate is 60Hz because every 60th of a second, ONE of the two fields is displayed. Since a single field is not the entire frame, the field rate is not the frame rate. It takes two ...
by tomaitheous
Thu Oct 11, 2007 3:21 am
Forum: Video Display Processor
Topic: TV safe area
Replies: 123
Views: 57270

It's not technically a legal NTSC signal (out of all the NTSC books and docs I have anyway), but almost all (99.9999999%) TVs support it. One of most popular DIY ZX Spectrum clones in Russia had so simplified video system so it had whole 320 lines instead of 312.5 per frame (so framerate was 48.82H...
by tomaitheous
Thu Oct 11, 2007 2:24 am
Forum: Video Display Processor
Topic: TV safe area
Replies: 123
Views: 57270

The FRAME rate is 30Hz. The FIELD rate is 60Hz. Are you sure it's not the other way around? On an interlaced TV, if I play a video at both 30fps and 60fps, there is a difference, so 60fps can be seen. However, each field is being displayed only 30 times a second. (for example, even fields 30, odd f...
by tomaitheous
Thu Oct 04, 2007 2:02 am
Forum: Video Display Processor
Topic: TV safe area
Replies: 123
Views: 57270

1/ how do you get that the pixel clock is the main clock (5.3693175 Mhz) divided by 8 in H40 ? The x10 factor in H32 mode seems fine as it is compatible with Charles Mc Donald's tests on the SMS. The main crystal is 53.69mhz. Divide by 10 to get 5.37mhz and divide by 8 to get 6.7125mhz. 2/ how do y...
by tomaitheous
Tue Oct 02, 2007 9:15 pm
Forum: Video Display Processor
Topic: TV safe area
Replies: 123
Views: 57270

Did you miss the formula that I posted? 40 CELL mode uses a 6.7125Mhz dot clock and 32 CELL mode uses a 5.37mhz dot clock. That's what defines the "width" of a pixel for determining correct pixel aspect ratio and overscan area. 6.7125mhz/ 15735hz = 426 dot clock(pixel) length scanline / 1.186 (not d...
by tomaitheous
Fri Sep 28, 2007 1:54 am
Forum: Video Display Processor
Topic: TV safe area
Replies: 123
Views: 57270

Before you find out the safe area of 40 cell mode, you need to figure out the dot clock for the horizontal line. 320 pixels isn't edge to edge. If I remember correctly, the mast clock for the NTSC Genesis is 53.7mhz. 40 cell mode should be 53.7mhz/8 = 6.7125mhz / 15735 hz = 426 pixels per scanline /...
by tomaitheous
Fri Sep 14, 2007 1:06 am
Forum: Mega/SegaCD
Topic: How hard would be to code a NES/SMS emulator
Replies: 31
Views: 26764

I don't think you could use the square-wave channels to do the NES's pulse channels for two reasons: one, the NES has adjustable pulse width where the Genesis's chip does not, and two, the Genesis's chip, in the grand Yamaha tradition, uses logarithmic volume levels where everybody else, NES includ...