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Thread: If the Genesis had been designed around the low-resolution mode...

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    Outrunner roundwars's Avatar
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    Genesis If the Genesis had been designed around the low-resolution mode...

    ...would more features have been possible?

    Maybe it's just a coincidence, but I noticed that the Genesis's BG layers add up to the same amount of data per line as the SNES's layers. That is, Genesis has two 4bpp planes at 320, which is 25% more data than the SNES's 4bpp planes at 256, but the SNES gets that 25% back with the single 2bpp layer. Does the 25% more resolution use up the same "resource" as the 25% more bitplanes, or are there other parameters involved?

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    Outrunner Stef's Avatar
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    Of course it does, actually if you compare video chip on a bandwidth eye you will see they are very close even if system are separated by 2 years.
    I believe the Megadrive even require a bit more as it does fetch extra informations from the VRAM compared to the SNES PPU (H scroll, sprites data).
    Still i'm quite happy the Megadrive supported H40 resolution, definitely better than H32...
    In the MegaDrive collected work book we can read an interview from one of the hardware designer of the Sega Megadrive, he said the trickiest part was to optimize memory bandwidth usage with the VDP. Toshiba just released some dual port memory short time ago and they designed the VDP to take advantage of this memory.
    He said also that Sega asked to increase the VRAM size late in the development process and he had to do many changes for that but it ended to not be very useful except adding more tiles in VRAM. I believe it's why the VDP support 128 KB memory configuration. Also he explain they spent a bunch of time and probably die space with the interlaced high resolution support (with 8x16 tiles). I believe that is one of the feature which is really useless on the VDP, they better go adding more CRAM
    Last edited by Stef; 12-21-2014 at 03:06 PM.

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    Mastering your Systems Shining Hero TmEE's Avatar
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    The 128KB mode has one huge advantage though : twice the speed DMA !
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    Road Rasher Bibin's Avatar
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    Going to second that another layer, even 2bpp, or more CRAM, would have been a very nice addition. Color-juggling stops being fun after a while...

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    Outrunner Stef's Avatar
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    The 128KB mode has one huge advantage though : twice the speed DMA !
    Why does it changes this ? 2 extra memory chips means extra bus line with VDP ?

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    Is this where Sega-16 admits 256 res is the better resolution?

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    ESWAT Veteran Chilly Willy's Avatar
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    Quote Originally Posted by Stef View Post
    Why does it changes this ? 2 extra memory chips means extra bus line with VDP ?
    The extra vram used it's own bus (IIRC), which means you can DMA words at a time instead of bytes.

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    Mastering your Systems Shining Hero TmEE's Avatar
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    Yes, the second VRAM bank is on its own 8 bits, and it has been verified already that DMA turns twice as fast is 128KB mode for VRAM access (CRAM and VSRAM are 16bit already, no faster with them)
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    Quote Originally Posted by TmEE View Post
    Yes, the second VRAM bank is on its own 8 bits, and it has been verified already that DMA turns twice as fast is 128KB mode for VRAM access (CRAM and VSRAM are 16bit already, no faster with them)
    Semi-related question.

    Why does the genesis VDP have a special Vertical scroll ram? The Horizontal scroll data is just a small section of VRAM determined VDP register $0D, I believe. Why can't vertical scroll data also be included in the normal VRAM?
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    Mastering your Systems Shining Hero TmEE's Avatar
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    Because as with any DRAM, random access induces a massive speed penalty, and sticking new accesses to already crowded list would have meant some cutbacks are needed. Hscroll needs just one access per line, but VSCROLL whole lot more, and each access will mean some 150ns gets wasted.
    DRAMs are fast only when accessed serially, even modern DRAMs are as slow as 80's DRAMs for random access, only serial access speed as increased tremendously.
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    Outrunner Stef's Avatar
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    Quote Originally Posted by TmEE View Post
    Yes, the second VRAM bank is on its own 8 bits, and it has been verified already that DMA turns twice as fast is 128KB mode for VRAM access (CRAM and VSRAM are 16bit already, no faster with them)
    Oh in this case, why not having used smaller capacity memory chips to allow full 16 bits bus line for 64 KB ? It's crazy to think the VDP had the capability to double the VRAM bandwidth but it was not used... Probably the too slow ROM was an argument against it.

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    Smaller size DRAMs would have been 16KBytes in size, no 32KByte option. That's a manufacturing quirk of DRAM, you get steps of 4 instead of 2 : 1, 4, 16, 64, 256....
    ROM speed isn't a factor, there will not be double accesses, just every access uses both bytes not just one of them.
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    Outrunner Stef's Avatar
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    I though that first ROM always allowed VRAM DMA but not CRAM nor VSRAM DMA because of the bandwidth difference.

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    So how do you use the whole 128 kB of VRAM then? Addind that to the cartridge?

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    Quote Originally Posted by Tom M. View Post
    So how do you use the whole 128 kB of VRAM then? Addind that to the cartridge?
    You don't unless you're programming for System C. Support for 128KB of VRAM and external CRAM appears to only be intended for use in arcade boards based on the Genesis/Megadrive VDP which seems kind of crazy to me. Why burn transistors on mass production parts for features that will never be used outside of some very low volume arcade machines? On modern chips it makes a certain amoutn of sense as you can improve yields by binning based on the parts that actually work, but I doubt that's what happened here.

    Quote Originally Posted by Stef View Post
    I though that first ROM always allowed VRAM DMA but not CRAM nor VSRAM DMA because of the bandwidth difference.
    The overall rate of access to the cartridge is lower with VRAM DMA, but the length of each individual request is the same. Additionally, it operates at full speed for the first four words as 68K -> VDP DMA goes through the FIFO.

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