Setup the stack for an init task

Bootstrap an init task which holds at least the initial stack that is to
be used when setting up the registers (such as `sp` and `tp`).

In order to guarantee that the stack and the rest of the registers are
set up correctly, this commit also provides a raw implementation of a
`printk` function.

Moreover, this commit also adds an argument that must be passed to
`start_kernel`, which is the pointer to the embedded `fdt` blob. This
argument will be used by later work so to fetch, at least, the base
address for the initial ram disk. This was also used to test that the
stack was working as expected.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/kernel/strlen.S b/kernel/strlen.S
new file mode 100644
index 0000000..6185b94
--- /dev/null
+++ b/kernel/strlen.S
@@ -0,0 +1,22 @@
+.globl strlen
+.type strlen, @function
+
+/*
+ * Defined in include/fbos/string.h
+ *
+ *     size_t strlen(const char *str)
+ *
+ * Returns (a0): string length.
+ * Parameter (a0): string to measure.
+ * Clobbers: t0, t1.
+ */
+strlen:
+	mv t1, a0
+1:
+	lbu t0, 0(t1)
+	beqz t0, 2f
+	addi t1, t1, 1
+	j 1b
+2:
+	sub a0, t1, a0
+	ret