Sphinx I/O is performed by device drivers that reside entirely in the Propeller's cogs, taking up almost no hub memory. For example, the video driver runs in a cog and maintains its screen buffer (13 lines of 40 characters) in its cog memory. It communicates with the rest of Sphinx through a single long memory location (known as a rendezvous, to use terminology borrowed from fsrw) at the top of hub RAM space.
Similarly, the keyboard driver occupies no hub memory except for a rendezvous location.
The SD card driver is somewhat more complicated. It takes up three cogs. One is a low-level SD SPI driver (fsrw's sdspiqasm, lightly modified). The other two cogs implement a very barebones FAT16 file system that provides basic read, write, and execute functionality. Programs can open multiple files but must allocate some hub memory for each file. Communication with the file system is through several rendezvous locations. The file system requires a 512-byte buffer for metadata.
There are simple interface objects that hide the details of using the drivers.
Sphinx memory and cog usage
Sphinx.bin installs the drivers when it first boots up. From that point on, the drivers remain resident and running even as different programs execute in hub memory (as long as they are Sphinx-aware programs). For example, if you run link.bin by typing "link" at the command prompt, what happens is this:
Sphinx-aware programs do not have to carry device driver code inside themselves; they can rely on Sphinx drivers. Sphinx-aware programs can execute other programs, typically sphinx.bin, but not necessarily (for example, lex.bin executes codegen.bin so that the two parts of the compiler run seamlessly). Running sphinx.bin puts up a command prompt immediately after a program ends, without the long delay that a reset would cause. This, coupled with the fact that the display accumulates the output from each program (rather than clearing between each program), gives the convincing illusion of a traditional command-line shell.
Sphinx can also execute regular (that is, non-Sphinx-aware) programs by performing the equivalent of a reset before running the program. This shuts down the driver cogs. The program must perform its own I/O. When the program finishes, a reboot will presumably occur, either programmatically or manually. At that time, Sphinx will load in from EEPROM and install the Sphinx drivers.