Question about Step 21. C code output to UART

[pongsagon]

In assembly, we can set the memory address to point to UART to output to it.
When we write in C using putchar, how does it know our custom defined UART memory mapped address?
Thank you very much
Matt

[yrabbit]

Because they are clearly stated in the putchar sources?

https://github.com/BrunoLevy/learn-fpga/blob/master/FemtoRV/TUTORIALS/FROM_BLINKER_TO_RISCV/FIRMWARE/putchar.S

[pongsagon]

Yes, the putchar asm code seem to be there as you have mentioned.
I am not quite understand about the makefile and compile process.
Does it mean that when we compile C, we set the compiler flag so that it uses the custom putchar.s and not the stdlib?

Thanks

[yrabbit]

When linking, functions from explicitly specified object files take precedence over library functions. The object files listed in this variable will take precedence.

learn-fpga/FemtoRV/TUTORIALS/FROM_BLINKER_TO_RISCV/FIRMWARE/Makefile

Line 7 in 5bb5c5e

LIBOBJECTS=putchar.o wait.o print.o memcpy.o errno.o perf.o

[pongsagon]

Got it.
Thank you very much for your reply.

[BrunoLevy]

In fact, the standard library is not linked at all, thanks to the -nostdlib flag, so the only version of putchar() and printf() are the ones that are explicitly linked (I would not trust too much the rules of precedence for that).

[pongsagon]

I see.
Thanks a lot for this great tutorial.