diff --git a/lang/src/int_to_str.asm b/lang/src/int_to_str.asm
index a28fe73..fa0543a 100644
--- a/lang/src/int_to_str.asm
+++ b/lang/src/int_to_str.asm
@@ -1,6 +1,73 @@
 section .text
 global int_to_str
 global int_to_str2
+global str_to_int
+
+;; rdi: pointer to input string
+;; rsi: length of input string
+;; dl: radix
+;; fn str_to_int(s: *const u8, len: usize, radix: u8) -> i64
+str_to_int:
+    push rbp
+    mov rbp, rsp
+    push r12
+    push r13
+    sub rsp, 0x10
+    mov qword [rsp], 0              ; result = 0
+    mov [rsp + 8], dl          ; multiplier = radix
+
+    xor r12, r12        ; r12 = index
+.loop:
+    cmp r12, rsi
+    jge .epilogue
+    mov rax, [rsp]
+    movzx rcx, byte [rsp + 8]
+    xor rdx, rdx
+    imul rcx
+    jo .overflow             ; overflow
+    mov [rsp], rax
+
+    mov al, byte [rdi + r12]
+    cmp al, '_'
+    je .skip_underscore
+    cmp al, '9'
+    ja .check_alpha
+    cmp al, '0'
+    jb .not_a_digit
+    sub al, '0'
+    jmp .digit_parsed
+.check_alpha:
+    cmp al, 'a'
+    jb .not_a_digit
+    cmp al, 'f'
+    ja .not_a_digit
+    sub al, 'a' - 10
+    jmp .digit_parsed
+.digit_parsed:
+    cmp al, byte [rsp + 8]
+    ja .not_a_digit
+    movzx rax, al
+    add [rsp], rax
+
+.skip_underscore:
+    inc r12
+    jmp .loop
+
+.not_a_digit:
+    mov qword [rsp], 0
+    jmp .epilogue
+.overflow:
+    mov rax, -1
+    shr rax, 1
+    mov qword [rsp], rax
+
+.epilogue:
+    mov rax, [rsp]
+    add rsp, 0x10
+    pop r13
+    pop r12
+    pop rbp
+    ret
 
 ;; rdi: input integer
 ;; rsi: pointer to output buffer (at least 21 bytes)
diff --git a/lang/tests/int_to_str.rs b/lang/tests/int_to_str.rs
index 0f3b73d..87db35f 100644
--- a/lang/tests/int_to_str.rs
+++ b/lang/tests/int_to_str.rs
@@ -20,6 +20,7 @@ impl FFISlice {
 
 unsafe extern "C" {
     unsafe fn int_to_str2(value: isize, buffer: *mut u8, buffer_len: usize, radix: u8) -> FFISlice;
+    unsafe fn str_to_int(s: *const u8, len: usize, radix: u8) -> isize;
 }
 
 fn main() {
@@ -51,5 +52,29 @@ fn main() {
         let s = slice.as_str();
         println!("Integer: {:b}, String: {}", value, s);
         assert_eq!(s, format!("{value:b}"));
+
+        let s = "1234567890";
+        let parsed = str_to_int(s.as_ptr(), s.len(), 10);
+        println!("String: {}, Integer: {}", s, parsed);
+        assert_eq!(parsed, 1234567890isize);
+
+        let s = "499602d2";
+        let parsed = str_to_int(s.as_ptr(), s.len(), 16);
+        println!("String: {}, Integer: {}", s, parsed);
+        assert_eq!(parsed, 1234567890isize);
+
+        let s = "11145401322";
+        let parsed = str_to_int(s.as_ptr(), s.len(), 8);
+        println!("String: {}, Integer: {}", s, parsed);
+        assert_eq!(parsed, 1234567890isize);
+        let s = "11101011";
+        let parsed = str_to_int(s.as_ptr(), s.len(), 2);
+        println!("String: {}, Integer: {}", s, parsed);
+        assert_eq!(parsed, 235isize);
+
+        let s = "9999999999999999999999999999999999999999";
+        let parsed = str_to_int(s.as_ptr(), s.len(), 10);
+        println!("String: {}, Integer: {}", s, parsed);
+        assert_eq!(parsed, isize::MAX);
     }
 }