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Hello World

Building embedded applications is tricky in part because of the huge number of configuration settings necessary to get something that works. This example shows how to use the installed version of picolibc, and uses 'make' instead of 'meson' to try and make the operations as clear as possible.

Selecting picolibc headers and C library

Picolibc provides a GCC '.specs' file (generated from picolibc.specs.in) which sets the search path for header files and picolibc libraries.

gcc --specs=picolibc.specs

Semihosting

Our example program wants to display a string to stdout; because there aren't drivers for the serial ports emulated by qemu provided, the example uses Picolibc's semihosting support

gcc --specs=picolibc.specs --semihost

Target processor

For RISC-V, QEMU offers the same emulation as the "spike" simulator, which looks like a SiFive E31 chip (sifive-e31). That's a 32-bit processor with the 'imac' options (integer, multiply, atomics, compressed) and uses the 'ilp32' ABI (32-bit integer, long and pointer):

riscv64-unknown-elf-gcc --specs=picolibc.specs --semihost -march=rv32imac -mabi=ilp32

For ARM, QEMU emulates a "mps2-an385" board which has a Cortex-M3 processor:

arm-none-eabi-gcc --specs=picolibc.specs --semihost -mcpu=cortex-m3

Target Memory Layout

The application needs to be linked at addresses which correspond to where the target memories are addressed. The default linker script provided with picolibc, picolibc.ld, assumes that the target device will have two kinds of memory, one for code and read-only data and another kind for read-write data. However, the linker script has no idea where those memories are placed in the address space. The example specifies those by setting a few values before including picolibc.ld.

For 'spike', you can have as much memory as you like, but execution starts at 0x80000000 so the first instruction in the application needs to land there. Picolibc on RISC-V puts _start at the first location in read-only memory, so we set things up like this (this is hello-world-riscv.ld):

__flash = 0x80000000;
__flash_size = 0x00080000;
__ram = 0x80080000;
__ram_size = 0x40000;
__stack_size = 1k;

INCLUDE picolibc.ld

The mps2-an385 has at least 16kB of flash starting at 0. Picolibc places a small interrupt vector there which points at the first instruction of _start. The mps2-an385 also has 64kB of RAM starting at 0x20000000, so hello-world-arm.ld looks like this:

__flash =      0x00000000;
__flash_size = 0x00004000;
__ram =        0x20000000;
__ram_size   = 0x00010000;
__stack_size = 1k;

INCLUDE picolibc.ld

The -T flag is used to specify the linker script in the compile line:

riscv64-unknown-elf-gcc --specs=picolibc.specs --semihost -march=rv32imac -mabi=ilp32 -Thello-world-riscv.ld

arm-none-eabi-gcc --specs=picolibc.specs --semihost -mcpu=cortex-m3 -Thello-world-arm.ld

Final Commands

The rest of the command line tells GCC what file to compile (hello-world.c) and where to put the output (hello-world-riscv.elf and hello-world-arm.elf):

riscv64-unknown-elf-gcc --specs=picolibc.specs --semihost
-march=rv32imac -mabi=ilp32 -Thello-world-riscv.ld -o
hello-world-riscv.elf hello-world.c

arm-none-eabi-gcc --specs=picolibc.specs --semihost
-mcpu=cortex-m3 -Thello-world-arm.ld -o hello-world-arm.elf
hello-world.c
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