Add pre-cat file existance check

src/file.S

@@ -2,9 +2,9 @@
 cat_buf:    .space 4096
 
 .section .text
-.globl cat_file, open_fd
+.globl cat_file, open_fd, file_exists
 
-# Fetch argument count
+# Cat out the given file to stdout
 # @param[in] a0: ptr to null-terminated file str.
 # @param[out] a0: result code.
 cat_file:
@@ -46,7 +46,7 @@ cat_file:
     ret
 
 
-# Fetch argument count
+# Open fd
 # @param[in] a0: ptr to null-terminated file str.
 # @param[out] a0: result code. (0-exists 1-doesn't exist)
 # @param[out] a1: fd.
@@ -67,3 +67,20 @@ open_fd:
 
     2:
     ret
+
+# Check if the given file exists without opening fd
+# @param[in] a0: ptr to null-terminated file str.
+# @param[out] a0: result code (0-exists 1-doesn't exist).
+file_exists:
+    mv a1, a0
+    li a0, -100 # relative/absolute path
+    li a2, 0
+    li a7, 48
+    ecall
+    beqz a0, 1f
+    li a0, 1
+    ret
+
+    1:
+    li a0, 0
+    ret