basics: Add some examples on RISC-V assembly

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

1 files changed, 28 insertions, 0 deletions

diff --git a/basics/factorial.S b/basics/factorial.S
new file mode 100644
index 0000000..7e3df12
--- /dev/null
+++ b/basics/factorial.S
@@ -0,0 +1,28 @@
+.text
+.globl factorial
+.type factorial, @function
+
+factorial:
+	// If input <= 1, just return the given thing.
+	addi t0, zero, 1
+	bgt a0, t0, else
+	jr ra
+else:
+	// Preserve the argument and the return address before the call.
+	addi sp, sp, -16
+	sd a0, 0(sp)
+	sd ra, 8(sp)
+
+	// Call the same function but with a value of argument-1.
+	addi a0, a0, -1
+	call factorial
+
+	// Restore back the original argument and the return address. Then multiply
+	// the original argument with the returned one from the previous call.
+	ld t1, 0(sp)
+	ld ra, 8(sp)
+	addi sp, sp, 16
+	mul a0, t1, a0
+
+	// And return to the caller.
+	jr ra