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Where the IOS might be stored depends on the actual supervisor (and possibly how it's being used). Most should have one or more internal flash memory banks on the supervisor card. Some supervisor cards also provide an external memory slot (or slots), often one that supports compact flash cards.
For example a 6500 sup720b has an internal flash for the L2 engine, an internal flash for the L3 engine, and two CF card slots. The internal L2 flash was used when running the sup720b in hybrid IOS mode, or using Catalyst IOS, but isn't used when running in native IOS mode.
To confuse things, on the 6500, the same flash bank might be accessible by more than one logical device name, and the one internal flash name might also have a different name depending whether the internal flash provides more than 512 MB (flash: or (bootflash: or. bootdisk:)).
You can also have an IOS image on different flash devices, and space permitting, different IOS images on the same flash. You can specify multiply device:IOSName in the bootstring, where the system will boot into the first sucessful match.
Also, in devices that support multiple supervisors, the secondary device's flash banks will have device prefix names to distinguish them from the primary. For example, on a 6500's active supervisor, it's first CF slot will be disk0: while the secondary's will be slavedisk0:. Also confusing, this naming is based on hot vs. standby supervisor, not actual slot the supervisor is installed in. I.e., if slot 5 is the active and slot 6 is the standby, the slot 6 memory devices have the slave prefix but if slot 6 becomes the active supervisor, then slot 5 gets the slave prefix.
