From 8e968078e6aee470a02bed7728100b2df1877e90 Mon Sep 17 00:00:00 2001
From: Miquel Sabaté Solà <mikisabate@gmail.com>
Date: Mon, 12 Aug 2024 14:50:07 +0000
Subject: basics: Add some examples on RISC-V assembly
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Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
---
 basics/string.S | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)
 create mode 100644 basics/string.S

(limited to 'basics/string.S')

diff --git a/basics/string.S b/basics/string.S
new file mode 100644
index 0000000..e04e4de
--- /dev/null
+++ b/basics/string.S
@@ -0,0 +1,93 @@
+.text
+
+.globl reverse_string
+.type reverse_string, @function
+
+// char * reverse_string(char *str);
+reverse_string:
+    // Return early on null pointer.
+    beq a0, zero, end
+
+    // Preserve the original pointer.
+    addi sp, sp, -8
+    sd a0, 0(sp)
+
+    // Set `t0` to point to the end of the string.
+    add t0, a0, zero
+set_end_ptr:
+    lbu t2, 0(t0)
+    beq t2, zero, end_ptr_done
+    addi t0, t0, 1
+    j set_end_ptr
+
+    // We are done iterating, if `a0` and `t0` are equal, then there's nothing
+    // to be done and we can return early. Otherwise decrement `t0` so it points
+    // to the byte right before the null termination.
+end_ptr_done:
+    beq a0, t0, reverse_done
+    addi t0, t0, -1
+
+reverse_loop:
+    // Swap values between the two pointers.
+    lb t1, 0(a0)
+    lb t2, 0(t0)
+    sb t1, 0(t0)
+    sb t2, 0(a0)
+
+    // Move pointers and check whether the pointers have already crossed. If
+    // they have not crossed yet there is still looping to be done. Otherwise
+    // we are done.
+    addi a0, a0, 1
+    addi t0, t0, -1
+    bltu a0, t0, reverse_loop
+
+reverse_done:
+    // Restore things back and return to the caller.
+    ld a0, 0(sp)
+    addi sp, sp, 8
+end:
+    jr ra
+
+.globl is_palyndrome
+.type is_palyndrome, @function
+
+// bool is_palyndrome(char *str);
+is_palyndrome:
+    // Return early on null pointer.
+    beq a0, zero, palyndrome_no
+
+    // Set `t0` to point to the end of the string.
+    add t0, a0, zero
+pal_set_end_ptr:
+    lbu t2, 0(t0)
+    beq t2, zero, pal_end_ptr_done
+    addi t0, t0, 1
+    j pal_set_end_ptr
+
+    // We are done iterating, if `a0` and `t0` are equal, then there's nothing
+    // to be done and we can return early. Otherwise decrement `t0` so it points
+    // to the byte right before the null termination.
+pal_end_ptr_done:
+    beq a0, t0, palyndrome_no
+    addi t0, t0, -1
+
+pal_loop:
+    // Swap values between the two pointers.
+    lb t1, 0(a0)
+    lb t2, 0(t0)
+    bne t1, t2, palyndrome_no
+
+    // Move pointers and check whether the pointers have already crossed. If
+    // they have not crossed yet there is still looping to be done. Otherwise
+    // we are done.
+    addi a0, a0, 1
+    addi t0, t0, -1
+    bleu a0, t0, pal_loop
+
+palyndrome_yes:
+    li a0, 1
+    jr ra
+
+palyndrome_no:
+    li a0, 0
+    jr ra
-- 
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