diff --git a/lang/src/int_to_str.asm b/lang/src/int_to_str.asm
index 1e1f71c..ad86d71 100644
--- a/lang/src/int_to_str.asm
+++ b/lang/src/int_to_str.asm
@@ -1,5 +1,72 @@
 section .text
 global int_to_str
+global int_to_str2
+
+;; rdi: input integer
+;; rsi: pointer to output buffer (at least 21 bytes)
+;; rdx: length of buffer
+;; fn int_to_str2(value: i64, buffer: *mut u8, len: usize) -> (*mut u8, usize)
+int_to_str2:
+    push rbp
+    mov rbp, rsp
+    push r12
+    push r13
+    xor r12, r12                ; r12 = index
+    mov r13, rdx                ; r13 = buffer length
+
+    test r13, r13
+    jz .epilogue                ; if length is 0, return
+
+    mov rcx, rdi
+    sar rcx, 63                 ; check sign
+    cmp rcx, 0
+    je .convert
+.negative:
+    mov byte [rsi], '-'         ; write minus sign
+    inc r12
+
+.convert:
+    test rdi, rdi
+    jnz .convert_loop
+    mov byte [rsi + r12], '0'
+    jmp .epilogue
+
+.convert_loop:
+    cmp r12, r13
+    jge .epilogue               ; prevent buffer overflow
+
+    mov rax, rdi
+    xor rdx, rdx
+    mov rcx, 10                 ; base 10
+    div rcx                     ; rax = rdi / 10, rdx = rdi % 10
+    add rdx, '0'                ; convert digit to ASCII
+    mov byte [rsi + r12], dl
+    inc r12
+    mov rdi, rax
+    test rdi, rdi
+    jnz .convert_loop
+
+    ; Reverse the digits
+    xor r13, r13
+    mov rdx, r12
+    dec r12                     ; last digit index
+.reverse_loop:
+    cmp r13, r12
+    jae .done_reverse
+    mov al, [rsi + r13]
+    mov bl, [rsi + r12]
+    mov [rsi + r13], bl
+    mov [rsi + r12], al
+    inc r13
+    dec r12
+    jmp .reverse_loop
+.done_reverse:
+    mov rax, rsi
+.epilogue:
+    pop r13
+    pop r12
+    pop rbp
+    ret
 
 ;; Converts integer in rcx to string at rdx
 ;; rcx: input integer
diff --git a/lang/tests/int_to_str.rs b/lang/tests/int_to_str.rs
new file mode 100644
index 0000000..0183ad0
--- /dev/null
+++ b/lang/tests/int_to_str.rs
@@ -0,0 +1,34 @@
+#[unsafe(no_mangle)]
+extern "C" fn panic() -> ! {
+    panic!("Called panic from external code.");
+}
+
+#[repr(C)]
+struct FFISlice {
+    ptr: *const u8,
+    len: usize,
+}
+
+impl FFISlice {
+    fn as_slice(&self) -> &[u8] {
+        unsafe { core::slice::from_raw_parts(self.ptr, self.len) }
+    }
+    fn as_str(&self) -> &str {
+        unsafe { core::str::from_utf8_unchecked(self.as_slice()) }
+    }
+}
+
+unsafe extern "C" {
+    unsafe fn int_to_str2(value: usize, buffer: *mut u8, buffer_len: usize) -> FFISlice;
+}
+
+fn main() {
+    let value = 1234567890usize;
+    let mut buffer = [0u8; 32];
+    unsafe {
+        let slice = int_to_str2(value, buffer.as_mut_ptr(), buffer.len());
+        let s = slice.as_str();
+        println!("Integer: {}, String: {}", value, s);
+        assert_eq!(s, value.to_string());
+    }
+}