basics: Add an example on floating instructions

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

1 files changed, 40 insertions, 0 deletions

diff --git a/basics/float.S b/basics/float.S
new file mode 100644
index 0000000..f481f3e
--- /dev/null
+++ b/basics/float.S
@@ -0,0 +1,40 @@
+.text
+.globl greater_than_ten
+.type greater_than_ten, @function
+
+// bool greater_than_ten(double a, uint64_t b);
+greater_than_ten:
+    // Upon entry a -> fa0; and b -> a0. That is, the ABI increments the
+    //(f)a<n> register per argument type.
+
+    // Change the rounding to "Rounds Towards Zero" (bits: 0b001).
+    li t0, 0b001
+    fsrm t0
+
+    // Convert the second parameter to double.
+    fcvt.d.lu ft0, a0
+
+    // ret = a + b
+    fadd.d fa0, fa0, ft0
+
+    // ret = (ret * b) + b
+    fmv.d ft1, ft0
+    fmadd.d fa0, fa0, ft0, ft1
+
+    // Now we need to compare the number with 10.0. There are multiple ways to
+    // load a floating point immediate:
+    //   1. Loading an integer immediate and then convert it into a double. This
+    //      is discouraged on the RISC-V assembly manual.
+    //   2. Load a double with `fld` from a memory location on the data section.
+    //   3. Load a floating point immediate with `fli` (requires the `Zfa`
+    //      extension).
+    // I don't have the `Zfa` extension on my board, so I'm opting for the
+    // second way.
+    fld ft1, .TEN, t0
+    fle.d a0, ft1, fa0
+
+    ret
+
+.data
+.TEN:
+    .double 10.0