raspi3-tutorial/02_multicorec/README.md

Tutorial 02 - Multicore C

Now let's try something more complex, shall we? By complex I mean stop the CPU cores just like in the first tutorial, but this time stop one of them from C!

Start

Now we have to distinguish the cores. For that, we read the mpidr_el1 system register. If it's not zero, we'll do the former infinite loop. If it's zero, then we'll call a C function. But for that, we need a proper stack, and a zeroed out bss segment in memory before the call instruction. I've added some more code to the Assembly to do all of that. In case the C code returns (shouldn't), we also jump to the same infinite loop the other CPU cores running.

NOTE: depending on your firmware version, it is possible that application cores are stopped. If that's the case then our code only runs on Core 0, but there's no harm in checking the core number. To start the other cores, you have to write an address of a function to execute at 0xE0, 0xE8, 0xF0 (one address for each core, in order). See armstub8.S.

Makefile

It became a bit trickier. I've added commands for compiling C sources, but in a comform way. From now on, we can use the same Makefile for every tutorial, regardless of the number of C sources, and I won't discuss it any further.

Linker script

Likewise, the linker script became more complex too, as C needs data and bss sections. I've also added a calculation for the bss size, so that we can refer to it from the Assembly without any further hassle.

It is important to start the text segment with the Assembly code, because we set the stack right before it, hence the KEEP(). This way our load address is 0x80000, the same as _start label and stack top.

Main

Finally, our first C code. Just an empty loop, but still! :-)