Tim Allen My CV

January 29, 2015

Compile environment for BrailleBuzz (STM32F103)

Filed under: BrailleBuzz,embedded — tallen @ 5:02 pm

Okay three big topics here: crt0 (also known as c0), crt1 (also known as c1), and the linking scripts. crt0 is the code that needs to run before your main, it sets up things like the stack. crt1 is the low level stuff that would normally be OS calls. I sorta simulate an OS because we don’t have one, so I simulate a heap and malloc, etc. The linking scripts tell the linker where to put things in the address space.

crt0

This is the code that executes before your main function gets called. Execution starts at 0000h (actually it may be a pointer on this implementation). But before one can actually run any C one has to do some set-up stuff. Traditionally one writes crt0 in assembly and indeed my original was in assembly. If your careful and don’t use certain C constructs you can write crt0 in c itself. There is heavy interaction with the linker script as we need to prepare things like the BSS, EBSS, stack etc. Remember C is gonna need to know where in the address space (which changes from implementation to implementation) things like RAM(rw), flash(ro) and heap are.

crt0.c

crt1

This is the low level operating system type stuff. Well, we don’t have one, but we’d still like things like printf and malloc to work or at least not crash. Remember after you call printf, somewhere down the pike some ascii (or unicode) has to go somewhere. In our case we’re going to output it to one of the UARTs. This is very convenient when debugging! This code took a long time to sort out and is incomplete. I got some of the stubs from the manufacturer. Recall that in a unix OS we would be opening files (in dev), reading from and writing to them but here we don’t have an OS or a file-system so we make some assumptions, lots of assumptions.

crt1.h

crt1.c

linker script

We need to tell the linker where to put things. What addresses correspond to RAM? What addresses correspond to ROM(flash)? Where’s the “heap”, how big is it.

stm32.ld

IRQ handlers

Lastly we have the irq handlers. We mostly just go into an infinite loop. Naturally we really should do better than this but I never got round to it.

irq.h

irq.c

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