cfg macro example

cortex-m-macro.rs

#[unsafe(naked)]
extern "C" fn Reset() {
    core::arch::naked_asm!(
        // debug info
        ".cfi_sections .debug_frame",
        ".cfi_startproc",
        // If enabled, initialise the SP. This is normally initialised by the CPU itself or by a
        // bootloader, but some debuggers fail to set it when resetting the target, leading to
        // stack corruptions.
        #[cfg(feature = "set-sp")]
        concat!("ldr r0, =_stack_start", "msr msp, r0",),
        // If enabled, initialise VTOR to the start of the vector table. This is normally initialised
        // by a bootloader when the non-reset value is required, but some bootloaders do not set it,
        // leading to frustrating issues where everything seems to work but interrupts are never
        // handled. The VTOR register is optional on ARMv6-M, but when not present is RAZ,WI and
        // therefore safe to write to.
        #[cfg(feature = "set-vtor")]
        concat!(
            "ldr r0, =0xe000ed08",
            "ldr r1, =__vector_table",
            "str r1, [r0]",
        ),
        // If enabled, set the Main Stack Pointer Limit (MSPLIM) to the end of the stack.
        // This feature is only available on ARMv8-M Mainline, where it helps enforce stack limits
        // by defining the lowest valid stack address.
        #[cfg(all(armv8m_main, feature = "set-msplim"))]
        concat!("ldr r0, =_stack_end", "msr MSPLIM, r0",),
        // Run user pre-init code which must be executed immediately after startup, before the
        // potentially time-consuming memory initialisation takes place.
        // Example use cases include disabling default watchdogs or enabling RAM.
        "bl __pre_init",
        // If enabled, initialize RAM with zeros. This is not usually required, but might be necessary
        // to properly initialize checksum-based memory integrity measures on safety-critical hardware.
        //
        // Otherwise, initialise .bss memory. `__sbss` and `__ebss` come from the linker script.
        cfg_match! {
            feature = "zero-init-ram" => {
                concat!(
                    "ldr r0, =_ram_start",
                    "ldr r1, =_ram_end",
                )
            }
            _ => {
                concat!(
                    "ldr r0, =__sbss",
                    "ldr r1, =__ebss",
                )
            }
        },
        // zero out r0..r1
        "movs r2, #0",
        "0:",
        "cmp r1, r0",
        "beq 1f",
        "stm r0!, {{r2}}",
        "b 0b",
        "1:",
        // If enabled, paint stack/heap RAM with 0xcccccccc.
        // `_stack_end` and `_stack_start` come from the linker script.
        #[cfg(feature = "paint-stack")]
        concat!(
            "ldr r0, =_stack_end",
            "ldr r1, =_stack_start",
            "ldr r2, =0xcccccccc", // This must match STACK_PAINT_VALUE
            "0:",
            "cmp r1, r0",
            "beq 1f",
            "stm r0!, {{r2}}",
            "b 0b",
            "1:",
        ),
        // Initialise .data memory. `__sdata`, `__sidata`, and `__edata` come from the linker script.
        "ldr r0, =__sdata",
        "ldr r1, =__edata",
        "ldr r2, =__sidata",
        "0:",
        "cmp r1, r0",
        "beq 1f",
        "ldm r2!, {{r3}}",
        "stm r0!, {{r3}}",
        "b 0b",
        "1:",
        // Potentially enable an FPU.
        // SCB.CPACR is 0xE000_ED88.
        // We enable access to CP10 and CP11 from priviliged and unprivileged mode.
        #[cfg(has_fpu)]
        concat!(
            "ldr r0, =0xE000ED88",
            "ldr r1, =(0b1111 << 20)",
            "ldr r2, [r0]",
            "orr r2, r2, r1",
            "str r2, [r0]",
            "dsb",
            "isb",
        ),
        // Jump to user main function.
        // `bl` is used for the extended range, but the user main function should not return,
        // so trap on any unexpected return.
        "bl main",
        "udf #0",
        ".cfi_endproc"
    )
}