Re: What exactly is HiROM?

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This message was posted by neviksti, posted on December 03, 2001 at 12:26:21 coming from 144.54.3
This message is a reply to Re: What exactly is HiROM? posted from Utopium posted at December 03, 2001 at 09:53:34
> The SNES ran at two speeds, one for slower memory and one for faster.

Yes, it could do that.

>Essentially there are two seperate memory areas in the SNES, HiROM and LoROM.
>Where the game is actually adressed in memory depends on what speed it needs to use.

Oh my gosh no!
HiRom and LoRom have nothing to do with memory speed and they aren't "separate memory areas in the SNES". HiRom and LoRom put ROM data into the same memory locations... its how the ROM is mapped into this area that changes. They have to do with the memory mapping of the cartridge.
Both HiRom and LoRom can take advantage of the faster cycle time ... the program just needs to request it (set a bit in a particular hardware register) ... and then any memory accessed in banks $80 and above are acceessed with the faster cycles time.

Without going into technical details, let's just put it this way: for cartridges up to 32Mbits there are two easy ways to connect the address lines of the SNES to the address lines of the ROM chip and have the whole ROM chip accessible to the SNES.

Now, for a little better explanation:
LoRom is much more common than HiRom ... one is not 'harder' to implement in hardware than the other, its just that LoRom is more common. It however IS 'harder' to design a circuit that can change its memory mapping between LoRom and HiRom. From a design standpoint, its actually easier to just put a whole 64Mbits (one chip) or 48Mbits (two chips) in so that a SNES address (ignoring A23 line) corresponds to a unique data byte so no switching between HiRom and LoRom is necessary... although this require alot more than the minimum 32Mbits that is required to play all HiROm and LoRom games ... so it's almost as if the designers 'wasted' memory to make it easier and more versatile.

Therefore, some of the earlier (or cheaper) copiers may not have the circuitry to switch between the two addressing schemes or the extra memory to waste for the easier way.

So the 'decoder' you're referring to is a memory decoder ... if it can't map the cartridge memory into the SNES like HiRom should ... then any HiRom game will not be playable. Sorry.

I kinda glossed over a lot of points, so if you want to know more about HiRom or LoRom I can fill in some more for you.

I hope I answered your question,
-neviksti





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