Initial micro:bit support

Add support for the micro:bit 5x5 led display.

Linker Script Generation

Every firmware needs a linker script that places stuff where it belongs in memory. When porting microzig to a new target you must face the challenge of dealing with a linker script. But fear not as microzig has your back (in most cases). Let’s checkout the linker_script field in Target.

linker_script: LinkerScript = .{},

pub const LinkerScript = struct {
    /// Will anything be auto-generated for this linker script?
    generate: GenerateOptions = .{ .memory_regions_and_sections = .{} },
    /// Linker script path. Will be appended after what is auto-generated if it's not null.
    file: ?LazyPath = null,

    pub const GenerateOptions = union(enum) {
        /// Only generates a comment with target info.
        none,
        /// Only generates memory regions.
        memory_regions,
        /// Generates memory regions and default sections based on the provided options.
        memory_regions_and_sections: struct {
            /// Where should rodata go?
            rodata_location: enum {
                /// Place rodata in the first region tagged as flash.
                flash,
                /// Place rodata in the first region tagged as ram.
                ram,
            } = .flash,
        },
    };
};

For an example let’s look at the target definition of rp2040. In this case we need a linker script that should also place the bootrom at the beginning of flash. Fortunately, we can still mostly auto-generate one and just patch it up a bit.

// port/raspberrypi/rp2xxx/build.zig

const chip_rp2040: microzig.Target = .{
    // ...
    .chip = .{
        // ...
        .memory_regions = &.{
            .{ .tag = .flash, .offset = 0x10000000, .length = 2048 * 1024, .access = .rx },
            .{ .tag = .ram, .offset = 0x20000000, .length = 256 * 1024, .access = .rwx },
        },
    },
    .linker_script = .{
        // The `generate` field defaults to `.memory_regions_and_sections`.

        // This will be appended at the end of the auto-generated linker
        // script.
        .file = b.path("ld/rp2040/sections.ld"),
    },
};

/* port/raspberrypi/rp2xxx/ld/rp2040/sections.ld */

SECTIONS
{
  .boot2 : {
    __boot2_start__ = .;
    KEEP (*(.boot2))
    __boot2_end__ = .;
  } > flash0

  ASSERT(__boot2_end__ - __boot2_start__ == 256,
    "ERROR: Pico second stage bootloader must be 256 bytes in size")
}
INSERT BEFORE .flash_start;

This is the generated linker script:

/*
 * Target CPU:  cortex_m0plus
 * Target Chip: RP2040
 */

/*
 * This section was auto-generated by microzig.
 */
MEMORY
{
  flash0 (rx!w) : ORIGIN = 0x10000000, LENGTH = 0x00200000
  ram0 (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00040000
}
SECTIONS
{
  .flash_start :
  {
    KEEP(*(microzig_flash_start))
  } > flash0

  .text :
  {
    *(.text*)
    *(.srodata*)
    *(.rodata*)
  } > flash0

  .ARM.extab : {
    *(.ARM.extab* .gnu.linkonce.armextab.*)
  } > flash0

  .ARM.exidx : {
    *(.ARM.exidx* .gnu.linkonce.armexidx.*)
  } > flash0

  .data :
  {
    microzig_data_start = .;
    *(.sdata*)
    *(.data*)
    KEEP(*(.ram_text))
    microzig_data_end = .;
  } > ram0 AT> flash0

  .bss (NOLOAD) :
  {
    microzig_bss_start = .;
    *(.sbss*)
    *(.bss*)
    microzig_bss_end = .;
  } > ram0

  .flash_end :
  {
    microzig_flash_end = .;
  } > flash0

  microzig_data_load_start = LOADADDR(.data);
}
/*
 * End of auto-generated section.
 */
SECTIONS
{
  .boot2 : {
    __boot2_start__ = .;
    KEEP (*(.boot2))
    __boot2_end__ = .;
  } > flash0

  ASSERT(__boot2_end__ - __boot2_start__ == 256,
    "ERROR: Pico second stage bootloader must be 256 bytes in size")
}
INSERT BEFORE .flash_start;

FYI

• If the ram memory region used by the linker script generator is executable, a .ram_text section will be included for code that should be placed in ram. This applies to the rp2040 target where the section tagged as ram is executable.

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