Ion is a library of functions that abstract interacting with the hardware. For example, Ion exposes calls such as
LED::setColor(). Code in Epsilon always uses Ion functions to perform operations that deal directly with the hardware — be it reading a key, setting a pixel on the screen or setting the color of the LED.
By providing multiple implementations of the Ion functions, we therefore can get Epsilon to run on multiple platforms. For example, we have an implementation of
Display::pushRect() that knows how to drive the LCD panel of an actual calculator, and another implementation of the same
Display::pushRect() function that knows how do display content in a web browser. This way, the rest of the Epsilon code can run unmodified either on a calculator or a web browser.
This is the reference platform corresponding to the actual device. To really understand what the code is doing, you'll need to refer to our Electrical Engineering pages. Among other thing, Ion is responsible for handling the boot process and the memory layout of the code on the device:
On boot, the Cortex core interprets the beginning of Flash memory as an array of addresses, each having a specific meaning. For example, the first value gives the address of the stack and the second one the address the processor jumps to right after reset. This list of addresses is called the ISR table.
Like we saw in the previous paragraph, the MCU has a specific memory layout (for example, Flash starts at address 0x08000000) and expects certain values at certain addresses. To ensure the firmware is laid out in memory exactly how the processor expects it, we use a custom linker script.
The simulator platform implements Ion calls using the FLTK library. The result is a native GUI program that can run under a variety of operating systems such as Windows, macOS or most Linux distributions.
It's very practical to code using the simulator, but one has to pay attention to the differences from an actual device: it'll be significantly faster, and will have a lot more memory. So code written using the simulator should always be thoroughly tested on an actual device.
Building on Emscripten takes quite a lot of time so you will most likely not want to use it for development purposes. But obviously it's a very neat feature for end users who can give the calculator a spin straight from their browser.
The blackbox platform may seem odd at first sight because it implements most Ion calls doing nothing. In practice, a blackbox build results in an executable that runs on a regular PC but does virtually no I/O. It is in fact very useful for measuring and instrumenting the code. We use it for fuzzing and running the test suite.