feat(kernel): linker and source code

kernel.ld

@@ -0,0 +1,49 @@
+/*
+ * This is the entry point
+ * `boot` should be a defined label
+ */
+ENTRY(boot)
+
+/* SECTIONS tells to linker how to arrange sections inside the memory */
+SECTIONS {
+    /*
+     * This is the base address (start address of the kernel)
+     * OpenSBI takes little ram
+     */
+    . = 0x80200000;
+
+    /* This section contains the code of the program */
+    .text :{
+        /*
+         * Take the .text.boot before everything
+         * KEEP() prevents the linker to mark it as "unused"
+         */
+        KEEP(*(.text.boot));
+        /* Include everything else inside the .text section */
+        *(.text .text.*);
+    }
+
+    /*
+     * Contains constant read-only data
+     * Align the start of the section at 4 byte
+     */
+    .rodata : ALIGN(4) {
+        *(.rodata .rodata.*);
+    }
+
+    /* Contains read/write data */
+    .data : ALIGN(4) {
+        *(.data .data.*);
+    }
+
+    /* Contains read/data with an initial value of zero */
+    .bss : ALIGN(4) {
+        __bss = .;
+        *(.bss .bss.* .sbss .sbss.*);
+        __bss_end = .;
+    }
+
+    . = ALIGN(4);
+    . += 128 * 1024; /* 128KB */
+    __stack_top = .;
+}

src/kernel.c

@@ -0,0 +1,28 @@
+typedef unsigned char u8;
+typedef unsigned int u32;
+typedef u32 size_t;
+
+extern char __bss[], __bss_end[], __stack_top[];
+
+void *memset(void *buf, char c, size_t n) {
+	u8 *p = (u8 *)buf;
+	while (n--) {
+		*p++ = c;
+	}
+
+	return buf;
+}
+
+void kernel_main(void) {
+	memset(__bss, 0, __bss_end - __bss);
+
+	for (;;)
+		;
+}
+
+__attribute__((section(".text.boot"))) __attribute__((naked)) void boot(void) {
+	__asm__ __volatile__("mv sp, %[stack_top]\n"
+						 "j kernel_main\n"
+						 :
+						 : [stack_top] "r"(__stack_top));
+}