Timer IRQ reload value low ($4020) 7 bit 0 FDS file format (.FDS) - an archival file format for storing and emulating FDS disks.FDS disk format - the disk data format and file structure.The FDS disk is a modified version of the Mitsumi Quick Disk. The disc drive only contains basic electronics, there is no "intelligence" in it therefore, the serial interface almost directly represents what is stored on the disc. A complete cycle through the entire disc takes about 7 seconds. Data can only be accessed by spinning the disc, waiting for the head to reach the inner edge of the disc, then waiting again until the desired data file is reached. Because of this limitation, random access to the disc is impossible, making FDS disk drive data access behave similarly to a reel of tape (but much faster). There is a mechanism that detects when the head reaches the end of the disc and makes it return to the start (making an audible click). By comparison, the FDS only contains a single motor that does both at once, so it instead stores the data in a single spiral-shaped track. Most disk drives contain two motors: a spindle motor that spins the disk at a specific speed, and a stepper motor which moves the read/write head between each circular data track. The Disk Drive has to be powered separately and is only connected to the Famicom/NES via a serial cable to the RAM adapter. The RAM adapter is a special shaped cartridge that contains the RAM chips and an ASIC with DRAM controller, IRQ hardware, sound generation hardware, serial interface for the disk drive, and parallel port. The Famicom disk system comes in two parts : The disk drive and the RAM adapter. The FDS BIOS is used to load data from disks to PRG RAM or VRAM, and games can execute from there. Games are stored on one or multiple disk sides.
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