Initial Cortex-M4F bootstrap code

.cargo/config.toml

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+[build]
+target = "thumbv7em-none-eabihf"
+
+[target.thumbv7em-none-eabihf]
+rustflags = ["-C", "link-arg=-Tlink.x"]

.gitignore

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+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "wocchi"
+version = "0.1.0"

Cargo.toml

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+[package]
+name = "wocchi"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[[bin]]
+name = "wocchi"
+test = false
+bench = false

link.x

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+/* SoC Memory Layout */
+MEMORY
+{
+	FLASH : ORIGIN = 0x00000000, LENGTH = 512K
+	RAM : ORIGIN = 0x20000000, LENGTH = 64K
+}
+
+/* Declare the program entrypoint, each sections and variable called from it will not be
+   discarded by the linker. */
+ENTRY(reset_handler);
+
+PROVIDE(reset_handler = default_exception_handler);
+PROVIDE(nmi_handler = default_exception_handler);
+PROVIDE(hardfault_handler = default_exception_handler);
+PROVIDE(memmanager_handler = default_exception_handler);
+PROVIDE(busfault_handler = default_exception_handler);
+PROVIDE(usagefault_handler = default_exception_handler);
+PROVIDE(svcall_handler = default_exception_handler);
+PROVIDE(debugmonitor_handler = default_exception_handler);
+PROVIDE(pendsv_handler = default_exception_handler);
+PROVIDE(systick_handler = default_exception_handler);
+
+SECTIONS
+{
+	_sp_init = ORIGIN(RAM) + LENGTH(RAM);
+
+	.vector_table ORIGIN(FLASH) :
+	{
+		KEEP(*(.vector_table .vector_table));
+	} > FLASH
+
+	.text :
+	{
+		*(.text .text.*);
+	} > FLASH
+
+	.rodata :
+	{
+		*(.rodata .rodata.*);
+	} > FLASH
+
+	.bss :
+	{
+		. = ALIGN(4);
+		_bss_section_start = .;
+		*(.bss .bss.*);
+		. = ALIGN(4);
+		_bss_section_end = .;
+	} > RAM
+
+	.data : AT(ADDR(.rodata) + SIZEOF(.rodata))
+	{
+		. = ALIGN(4);
+		_data_section_start = .;
+		*(.data .data.*);
+		. = ALIGN(4);
+		_data_section_end = .;
+	} > RAM
+	_data_section_lma = LOADADDR(.data);
+
+	/DISCARD/ :
+	{
+		*(.ARM.exidx .ARM.exidx.*);
+	}
+
+}

shell.nix

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+let
+  pkgs = import <nixpkgs> {
+    crossSystem = (import <nixpkgs/lib>).systems.examples.armv7l-hf-multiplatform;
+    overlays = [(import (builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz"))];
+  };
+in
+  pkgs.mkShell {
+    packages = with pkgs.buildPackages; [
+      rust-analyzer      
+    ];
+
+    nativeBuildInputs = with pkgs.buildPackages; [
+      (rust-bin.stable.latest.default.override {
+        extensions = [ "rust-src" ];
+        targets = [ "thumbv7em-none-eabihf" ];
+      })
+    ];
+
+    env = {
+      CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = "${pkgs.stdenv.cc.targetPrefix}cc";
+    };
+  }

src/bootstrap.s

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+/* Use the UAL syntax */
+        .syntax unified
+
+/* Symbols declared in the linker script */
+        .global _sp_init
+        .global _bss_section_start
+        .global _bss_section_end
+        .global _data_section_start
+        .global _data_section_end
+        .global _data_section_lma
+
+/* Main function of the program to which this script is linked */
+        .global main
+
+/* ARM vector table (aka isr_vector) */
+        .section .vector_table
+        .align 2
+        .word _sp_init /* Initialisation value of the stack pointer register */
+/* Exceptions handlers */
+        .word reset_handler /* Reset Exception */
+        .word nmi_handler /* NMI Exception */
+        .word hardfault_handler /* HardFault Exception */
+        .word memmanager_handler /* MemManage Exception */
+        .word busfault_handler /* BusFault Exception */
+        .word usagefault_handler /* UsageFault Exception */
+        .word 0 /* Reserved */
+        .word 0 /* Reserved */
+        .word 0 /* Reserved */
+        .word 0 /* Reserved */
+        .word svcall_handler /* SVCall Exception */
+        .word debugmonitor_handler /* DebugMonitor Exception */
+        .word 0 /* Reserved */
+        .word pendsv_handler /* PendSV Exception */
+        .word systick_handler /* SysTick Exception */
+
+        .text
+        .align
+
+        .global default_exception_handler
+        .thumb_func
+default_exception_handler:
+        b .
+
+/* The CPU Reset Exception Handler, initializing register and data section, then call the main function */
+        .global reset_handler
+        .thumb_func
+reset_handler:
+/* Initialize the CPU general purpose registers */
+        mov r0, 0
+        mov r1, 0
+        mov r2, 0
+        mov r3, 0
+        mov r4, 0
+        mov r5, 0
+        mov r6, 0
+        mov r7, 0
+        mov r8, 0
+        mov r9, 0
+        mov r10, 0
+        mov r11, 0
+        mov r12, 0
+
+/* .bss section initialization (see linker script) */
+bss_init:
+        mov r0, 0
+        ldr r1, =_bss_section_start
+        ldr r2, =_bss_section_end
+
+bss_init_loop:
+/* Fill the section word-by-word with zeros */
+        cmp r1, r2
+        beq data_init
+        str r0, [r1], 4
+        b bss_init_loop
+
+/* Data section initialization */
+data_init:
+        ldr r1, =_data_section_start
+        ldr r2, =_data_section_end
+        ldr r3, =_data_section_lma
+
+/* Load the initial values of the variables from the section LMA */
+data_init_loop:
+        cmp r1, r2
+        beq bootstrap_end
+        ldr r0, [r3], 4
+        str r0, [r1], 4
+        b data_init_loop
+
+/* Bootstrap completed, branch to Rust code now ! */
+bootstrap_end:
+        bl main

src/main.rs

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+#![no_main]
+#![no_std]
+
+use core::{arch::global_asm, panic::PanicInfo};
+
+global_asm!(include_str!("bootstrap.s"));
+
+#[panic_handler]
+fn panic(_: &PanicInfo<'_>) -> ! {
+    loop {}
+}
+
+#[no_mangle]
+fn main() -> ! {
+    loop {}
+}