Add file not found err

src/file.S

@@ -2,7 +2,7 @@
 cat_buf:    .space 4096
 
 .section .text
-.globl cat_file
+.globl cat_file, open_fd
 
 # Fetch argument count
 # @param[in] a0: ptr to null-terminated file str.
@@ -10,12 +10,17 @@ cat_buf:    .space 4096
 cat_file:
     mv a1, a0 # since the function takes in a0, move it to a1 for syscall
     
-    li a0, -100 # relative/absolute path
-    li a2, 0 # readonly
-    li a7, 56 # open
-    ecall
+    addi sp, sp, -8
+    sd ra, 0(sp)
+    call open_fd
+    ld ra, 0(sp)
+    addi sp, sp, 8
+
+    mv t0, a1 # put fd in t0
+    beqz a0, 1f # if we don't have an error, jump to reading
+    li a0, 1
+    j 2f
 
-    mv t0, a0 # put fd in t0
 
     1:
     mv a0, t0
@@ -23,7 +28,7 @@ cat_file:
     li a2, 4096
     li a7, 63
     ecall
-    blez a0, 2f
+    blez a0, 3f
     mv a2, a0 # move bytes read into bytes needed to write
 
     li a0, 1
@@ -32,7 +37,33 @@ cat_file:
     ecall
     j 1b
     
+    3:
+    mv a0, t0
+    li a7, 57 # close fd
+    ecall
+
     2:
     ret
 
 
+# Fetch argument count
+# @param[in] a0: ptr to null-terminated file str.
+# @param[out] a0: result code. (0-exists 1-doesn't exist)
+# @param[out] a1: fd.
+open_fd:
+    mv a1, a0 # since the function takes in a0, move it to a1 for syscall
+    
+    li a0, -100 # relative/absolute path
+    li a2, 0 # readonly
+    li a7, 56 # open
+    ecall
+    bltz a0, 1f # file doesn't exist
+    mv a1, a0 # move fd to a1
+    li a0, 0
+    j 2f
+
+    1:
+    li a0, 1
+
+    2:
+    ret