You're right, it's not mask ROM. It's NROM (Nitride ROM) - whatever that's supposed to mean:I don't think the 3DS cards are mask ROM. Back in the DS days there were two types of cards: mask ROM (written using lithography) and 1T-EPROM (written by flashing the EPROM). Mask ROM was only available for cards up to 512MBit: 1Gbit cards and up had to use 1T-EPROM and were reported as having smaller transfer rates than M-ROM cards.
4Gbit MXIC NAND interface XtraROM. Here's the datasheet. And here's a press release about the technology:
Macronix announces a new nitride-based multiple bits/cell technology - NBit(TM), starting with 2bits/cell solutions. This trap-charge technology stores two bits of information in each cell which provides double the storage capacity for the same die size in comparison to conventional technologies. This technology is utilized to provide highly cost-effective NOR Flash and Mask ROM solutions across a wide range of densities.
Conventional silicon-based floating gate NOR Flash memory products can physically store only one bit of information in each cell. NBit(TM) technology physically stores 2 bits of information in each cell as shown in the diagram below. The nitride-based cell "traps" the charge on one side of the cell and the charge does not flow to the other side of the cell unlike floating gate cells. Hence another charge can be stored physically on the other side of the cell providing two bits per cell.
Since there is no floating gate, the process is simpler and easier to scale down to smaller geometries, and the process development time is shorter. The cell size is compact and the data is stored reliably in the device.
Programming of each bit is performed by hot electron injection, erase is performed by band to band tunneling, and read by the reverse read mechanism.
"As Macronix develops the NBit(TM) family of NROM-based products, the storage side of the non-volatile market opens up to them," states Alan Niebel, principal analyst at Web-Feet Research. "This expansion into the storage market validates the NROM-type of Flash offered through Spansion, Infineon and Saifun, and positions Macronix as a world-class memory manufacturer."
To store two bits of information in a cell, other solutions offer a Multi Level Cell (MLC) with voltage dividers to store multiple bits in the cell. The MLC technique requires several more layers of processing and is not as scalable as the NBit(TM) technology. NBit(TM) technology is much more cost effective and easily scalable compared to the MLC technology.
"NBit(TM) is the technology of the future for the company and we are offering our high density flash and ROM products starting with 32Mb going to 1Gb and beyond," said Dr. C. Y. Lu, Senior Vice President and CTO of Macronix.
For details on NBit(TM) technology refer to the White Paper & Technology Paper at the Macronix website, http://www.macronix.com.