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40

It is not intentionally mirrored, it is just a side effect of making the address decoding hardware for RAM as simple and cheap as possible with a single common 74LS139 chip used for the task, when an 8k area of addresses are reserved for RAM, but only 2k of RAM is present in the 8k area. If you look at the address map, 0x0000 to 0x1FFF is reserved for the ...


14

My question is, what is the purpose of this mirroring? There is no purpose. It's simply the way the address range is decoded. The decoder looks only at the top 3 bits to decide which internal device (RAM, I/O) is to be accessed. If the two topmost bits are zero then the third (A13) selects between RAM and I/O: A15/A14/A13 Addressrange Device 0 ...


11

It was most likely a pirate cart. I don't know about Finland but here in Sweden they weren't uncommon, you could often find them at fairs and similar places where traveling salespeople put up shop. Most of them were the usual "10 000 in 1" type cartridges. I have a pirate NES console called "Good Boy" that I got for free from a friend ...


9

To my knowledge, the NES and the Famicom had the exact same CPU (a Ricoh 2A03). So, it's unlikely there were any differences in behavior between the two with regards to undocumented opcodes. As the Ricoh had a 6502 core, you could also expect it has the same undocumented opcodes as the 6502 (not your question, though) - And this document seems to confirm ...


6

What is the lowest available to the cartridge CPU address, i.e. how much down to zero the cartridge space may span, whether it would be PRG-ROM, PRG-RAM or cartridge registers? Well, everything up to 4020 is filled, or better internal chip select decoding will fire for each and every address. For example the 2 KiB internal RAM are visible 4 times on $0000, $...


5

I'll assume you only want to modify small parts of the game, rather than the entire game. (If you wanted to modify the entire game, you'd just make a new cartridge holding the entire image.) Your objective: replace some but not all of the words* of the game with a new set, leaving the majority of the game unchanged. (* as in addressable data units, not ...


5

So I was wondering what happen if battery run out. Do you lose your save games? In an unplugged/stored cardridge yes. After all, they are RAM based. Now, if it's still pluged and powered, then usualy not. Ofc. it depends how it's wired. With standard circuitry that's also the way to change batteries before they go dead.


4

Educated guess here... $07ff is 0000 0111 1111 1111 in binary. So all addresses in that 2k are represented in the 11 least significant bits (bit 0 through bit 10). Mirror #8 runs from $1800 thru $1fff. $1fff is 0001 1111 1111 1111. So you can see that bits 11 and 12 don't matter (ie: 000x x111 1111 1111) - they're ignored. Ignoring a bit on a CPU is ...


4

Many cart-based games used battery-backed SRAM for storing saved game state. The RAM was usually powered by a lithium coin cell that could keep the RAM contents powered for a number of years. Once the battery died, the cartridge lost the ability to store game save data. While the cart was plugged in, the console would power the SRAM so the game would ...


3

The two disk systems' technology were completely different. The Famicom's disk system used a magnetic-storage floppy disk, which (according to Wikipedia) had a total capacity of 112 kilobytes (56kB per side). These were also known as Super Disks. Even when launched in 1986, this was relatively small: since 1984 IBM's PC-AT could store 1.2MB on a 5 1/4" ...


3

The cartridge can map anything anywhere, however there's restrictions if it overlaps existing hardware in the console. Write-only registers could be mapped in a way that overlaps RAM in $0000-$1fff range. Read-only registers could be mapped overlapping $2000, $2001, $2003, $2005 and $2006 PPU registers (or any of their mirrors), as well as $4000-$4014 pAPU ...


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