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CASIOWIN is the name of the Operating System used on the Prizm and other Casio calculators. Note that despite the name being the same across devices, CASIOWIN is actually quite different from system to system, even though the editions on newer models are usually the result of the evolution of the previous model edition. In the case of the Prizm, its CASIOWIN is the result of the port of fx-9860G's edition of the OS to a SH7305 CPU, [[Display|high-resolution color screen]] and big [[flash]] (with a new filesystem for the storage memory).

On the Prizm, the OS is located in flash from address 0x80020000 to 0x80B5FFFF (a bit over 11 MB) - that is, from after the [[bootloader]] to before flash regions that are used to store data that varies from device to device, like currently selected add-in language, user information (that is shown on power off), [[File_System#Storage_Memory|Main Memory]] backups, and of course, the [[File_System#Storage_Memory|Storage Memory]].

== Features unique to the Prizm ==

Here are some features of CASIOWIN which were introduced with the Prizm, some of which have not yet found their way to updates for older models:
* Full-color, high-resolution screen support;
** Color picture displaying in proprietary format (g3p);
** Colored graphs;
** Color-link on Statistics and Spreadsheet;
** Brackets coloring on certain expression inputs;
** Addition of a color option to lots of things.
* Status area (current angle unit, display mode, complex mode...);
** Battery meter on the status area, along with "battery type" setting;
* [[USB_Communication|USB]] mass-storage function (previously, link software was necessary);
* "Proper" [[flash]] file system, with support for sub-folders (3 levels on the native OS file browsers, theoretically unlimited in custom add-ins);
* Support for Chinese language (the OS includes Hanzi/kanji, Hiragana and Katakana characters);
* Dual screen (Graph+Graph or Graph to Table);
* Inclusion of more [[fonts]]...

== Start-up/shutdown processes ==
''Note: this is very incomplete, things may be in the wrong order, etc. More investigation is needed.''

=== Cold-boot start-up ===
This is what happens when the OS starts up for the first time after batteries having been removed, after the RESTART button is pushed, or when a [[Restart|software reset]] is performed.

After execution jumps to the OS entry point, thanks to the [[bootloader]], it proceeds to initialize various memory areas, mounts the storage memory file system using the Fugue API, loads MCS and [[setup]] backups from flash, if they exist (at this point the brightness is set to the user-chosen level) and shows a set-up wizard where users can choose for example language, display brightness and battery type. This wizard is shown even if the users have already set these settings and the OS still remembers them; it is skipped on the "fx-CG10/20 Manager" emulator. Finally, the Main Menu is shown.

=== User-requested standby ===

After the user presses Shift+AC/On, [[GetKey]] calls [[PowerOff]] with its parameter set to 1. This way, the Casio logo is shown. After a delay for showing the logo, the current VRAM, [[stack]], [[heap]], [[setup]] and [[File_System#Main_Memory|MCS]] contents, are saved to [[flash]] and the code for coming from the standby state is copied to RS memory. The screen is turned off. After this, the CPU enters R-Standby Mode, where all peripherals are turned off, clocks are halted except for RCLK and power is cut to most CPU areas except I/O areas. The contents of the RS memory and some registers are held in memory.

=== User-requested power on after standby ===

After the user presses AC/On, an interrupt is generated, the CPU is woken from R-Standby, and the interrupt handler code that was copied to RS memory on power off takes care of turning on the screen, displaying the busy-indicator, and restoring the stack, heap, VRAM and MCS contents. Possibly [[Setup]] is not restored because it is stored on RS memory which was kept, it only is backed up on shutdown in case batteries are removed. Finally, the program execution continues, and the [[PowerOff]] syscall returns - application execution continues. If the responsible for calling PowerOff was GetKey, to the running application the result is no different than any other call to GetKey.

=== User-requested power off ===

There are no known methods for safely powering off the calculator to the point where no state on the running application is kept. The recommended method is to issue a user-requested standby, which saves the important RAM areas, then take off the batteries. Since RS memory is not kept, the next boot will be a cold one, and only MCS, setup and possibly other settings will be restored from flash, but not the stack or heap.

== Add-in software and language ==
The OS supports add-in software in the form of g3a files and add-in languages in the form of g3l files. (It also supports BASIC scripts that are stored in Main Memory, but they are much less powerful than add-in software.)

The g3a files consist of a header, containing the add-in name, icons, other values and checksums, followed by the executable binary. More information can be found on [[G3A File Format]].

The g3l files consist of a header, containing the language name and other values, followed by a list of [[multi-byte strings]].

To date, Casio released five official software add-ins (usually simply mentioned as "add-ins"): Geometry, Picture Plot, Conversion, Physium and ProbSim. The first three add-ins were released on launch day, with Conversion being just a placeholder that the OS checks to enable the conversion function on the built-in apps. Physium was released with OS 1.03 and that is the lowest version it will run on. ProbSim was released with OS 2.00 but can work on earlier OS versions.

With OS 2.00, Casio also released the Russian language add-in, which seems to work with problems in earlier OS versions. More information on add-in versions can be found on [[Versions and versioning]].

For a long time (since at least April 2011) the community has been able to run custom machine code in the form of unofficial add-ins. Since then, dozens of unofficial add-ins have been developed (possibly over a hundred), with varying degrees of stability, user experience and OS integration.

Add-ins use OS functions as [[syscalls]]. Most OS functions, even insignificant ones, appear to be exposed as syscalls, but apparently certain libraries (such as zlib, included for decoding and encoding of g3p pictures) are not exposed. Most system settings and flags can (and should) also be read and set using syscalls, but there are some omissions (see for instance [[SetGetkeyToMainFunctionReturnFlag]]).

Syscalls are called by jumping to address 0x80020070 with a syscall number in r0, and after the syscall is executed, execution jumps back to the calling function. More details can be found in the article [[Syscalls]] and a list of the known syscalls can be found [[:Category:Syscalls|here]].

Shared libraries are not supported by the OS. This means each add-in has to be statically linked to any necessary libraries, and the only external code is in the form of OS-provided syscalls.

== Process and task model ==

The OS does not present any multitasking or multithreading mechanisms, other than [[Timers]], which at a stretch can be considered a very basic multithreading mechanism. A predefined number of "processes" is supported. For more information, see the article [[Processes]].

== OS versions ==

See the article [[Versions and versioning]].

== Secret key combinations ==

See the article [[Secret Key Combinations]].

== Behavior on program error ==

See the article [[Error handling]].

== Locale ==

See the article [[Locale]].

== Bitmaps ==

A notable part of the flash occupied by the OS is used to store bitmaps, from icons and screen backgrounds, to font glyphs. See [[Bitmaps]] for more information.

== Checksum ==
The OS has a checksum at address 0xA0B5FFF8, which is the sum of the bytes in the address range 0xA0020000 to 0xA0B5FEAF plus the sum of the bytes in the address range 0xA0B5FEF0 to 0xA0B5FFF7. The checksum is verified by the bootloader and possibly the OS itself, and in case of a mismatch special behavior will be triggered, like entering a emergency OS update mode.