build bootloaders from code

This adds the infrastructure for building from source the bootloaders which
will go into the application initial program, in the boot sector.  As a
simple test case, it includes a simple bootloader which merely jumpts to
the code address specified in the AIP, with the stack address specified
there.

3 files changed, 87 insertions, 0 deletions

diff --git a/boot/bootloader.h b/boot/bootloader.h
new file mode 100644
index 0000000..e5c013d
--- /dev/null
+++ b/boot/bootloader.h
@@ -0,0 +1,55 @@
+#include "../src/bootinfo.h"
+
+/* to be provider by the bootloader including this header */
+_Noreturn void bootloader(struct systemid *boot);
+
+__asm__(
+".text \n"
+".extern __bss_start \n"
+".extern __bss_end \n"
+".global __start \n"
+".type __start, @function \n"
+"__start: \n"
+
+/* zero any bss section */
+"	mov #0, r0 \n"
+"	mov.l 1f, r1 \n"
+"	mov.l 2f, r2 \n"
+"0:	cmp/hi r1, r2 \n"
+"	bf 0f \n"
+"	bra 0b \n"
+"	 mov.l r0, @-r2 \n"
+
+/* cannibalize code before this for stack space, and pass control to C */
+"0:	mov.l 3f, r15 \n"
+"	mov.l 4f, r4 \n"
+"	bra bootloader \n"
+"	 nop \n"
+
+".align 2 \n"
+"1:	.long __bss_start \n"
+"2:	.long __bss_end \n"
+"3:	.long __start \n"
+"4:	.long 0x06002000 \n"
+);
+
+/* helper to jump to the specified code with the specified stack */
+static inline _Noreturn void jump(const void *code, long *stack)
+{
+	__asm__ __volatile__(
+		"	jmp @%0\n"
+		"	 mov %1, r15\n"
+		: : "r"(code), "r"(stack)
+	);
+	__builtin_unreachable();
+}
+
+/* helper to abort by repeatedly executing an illegal instruction */
+static inline _Noreturn void fail(void)
+{
+	__asm__ __volatile__(
+		"0:	bra 0b\n"
+		"	.word 0xfffd\n"
+	);
+	__builtin_unreachable();
+}
diff --git a/boot/ldscript b/boot/ldscript
new file mode 100644
index 0000000..53a20bf
--- /dev/null
+++ b/boot/ldscript
@@ -0,0 +1,22 @@
+OUTPUT_FORMAT(elf32-shbig-linux)
+ENTRY(__start)
+SECTIONS
+{
+	. = 0x06002f00;
+	.text :
+	{
+		*(.text);
+	}
+	.data :
+	{
+		*(.data);
+		*(.rodata);
+	}
+	.bss ALIGN(4) :
+	{
+		__bss_start = .;
+		*(.bss);
+		__bss_end = (. + 3) & ~ 3;
+	}
+}
+
diff --git a/boot/simple.c b/boot/simple.c
new file mode 100644
index 0000000..ca3e796
--- /dev/null
+++ b/boot/simple.c
@@ -0,0 +1,10 @@
+#include "bootloader.h"
+
+_Noreturn void bootloader(struct systemid *boot)
+{
+	if (!boot->load_addr) fail();
+
+	long *stack = (long*) (boot->stack_master ? boot->stack_master : 0x06002000);
+	jump((void*) boot->load_addr, stack);
+}
+