Slots

The calculator operating system is stored in slots, with the flash being divided in two equal parts of 4MB (total of 8MB).

Epsilon

On the stock OS, each slot contain a kernel, an userland, an username and an exam mode storage zone. In most cases, both slots contains the same OS version. Having two copies of the OS doesn't have a big benefit in their case:

• It allows loading the official OS when resetting the calculator instead of falling back to the rescue screen.
• When enabling exam mode in an untrusted (user built) OS, the calculator will enable exam mode then reboot to the official slot, which has LED access

Except the specific case of rebooting to stock OS, the slot system is pretty useless.

Slots doesn't exist hardware-wise, it's just a software separation. In fact, if the slot A took 8MB, it would overwrite the slot B and work as-is (there would be a linker script to edit as the linker is told to link only in a 4MB zone, but it doesn't require big code changes).

KhiCAS on Epsilon

If you installed KhiCAS on Epsilon, with the NWA version, you probably saw the installation procedure require installing KhiCAS through another website, then a small NWA app.

This is required as KhiCAS is quite big, reduced to 4MB in the small Epsilon version. Space available to external apps is variable between Epsilon versions, but is roughly around 2.5MB.

In the old KhiCAS version, the storage limit was simply bypassed using Nwagra, but Nwagra broke when Epsilon stored it's exam mode state right after the applications zone, as applications were now conflicting with exam mode state, leading to the calculator showing exam mode enabled and corrupting the apps when trying to disable it.

To workaround this problem, KhiCAS binary is flashed on the slot B manually. To launch KhiCAS, a small NWA app is installed, doing a few initialization things, then starting the full app.

Custom firmwares

On custom firmwares, slots are exploited in a more useful way than on stock OS with locked bootloader: both slots can contain different OS, allowing on-the-fly switching, without computer, just by resetting the calculator. For example, the most common dual-boot config is Upsilon in the slot A (first slot) and Epsilon in the slot B (second slot).

On Upsilon and Omega, external apps archive is stored at the address 0x90200000, 2MB after the beginning of the slot A. In case more than 2MB of external apps are flashed, the slot B is overwritten to allow up to 6MB of external apps, required when using KhiCAS (5.2MB). Upsilon bootloader require flashing apps from the bootloader as the other slot is read-only.

Slot Khi

With Upsilon and Khi bootloader, there is a third slot (bound to the 2 key in Upsilon bootloader).

This slot was designed to hold a specific version of Khi (a fork of Omega), with all native applications removed to reduce memory usage, both in terms of flash and RAM. It's designed to be used alongside with KhiCAS. In this config, you would have way more RAM for KhiCAS, to do more complex computations or run big scripts. It also allowed to have KhiCAS alongside Epsilon in exam mode, but is a bit broken on recent Epsilon versions and Upsilon bootloader (but should work with Khi bootloader).

This version of Khi is called "Khi slot 2" on Khi website

Here is a schema describing some external flash layout for unlocked calculators. External apps are not stored the same way on locked calculators (NWA).