Add stdin cat

src/file.S

@@ -2,7 +2,7 @@
 cat_buf:    .space 4096
 
 .section .text
-.globl cat_file, open_fd
+.globl cat_file, open_fd, cat_stdin
 
 # Cat out the given file to stdout
 # @param[in] a0: ptr to null-terminated file str.
@@ -17,12 +17,12 @@ cat_file:
     addi sp, sp, 8
 
     mv t0, a1 # put fd in t0
-    beqz a0, .LReadBlock # if we don't have an error, jump to reading
+    beqz a0, .LReadFileBlock # if we don't have an error, jump to reading
     li a0, 1
-    j .LRet
+    j .LCatFileRet
 
 
-    .LReadBlock:
+    .LReadFileBlock:
     mv a0, t0
     la a1, cat_buf
     li a2, 4096
@@ -35,16 +35,37 @@ cat_file:
     la a1, cat_buf
     li a7, 64
     ecall
-    j .LReadBlock
+    j .LReadFileBlock
     
     .LCloseFD:
     mv a0, t0
     li a7, 57 # close fd
     ecall
 
-    .LRet:
+    .LCatFileRet:
     ret
 
+# Cat out stdin to stdout
+# @param[in] a0: Not used.
+# @param[out] a0: result code.
+cat_stdin:
+    .LReadStdinBlock:
+    li a0, 0
+    la a1, cat_buf
+    li a2, 4096
+    li a7, 63
+    ecall
+    blez a0, .LCatStdinRet # if we haven't read any bytes, EOF, close FD
+    mv a2, a0 # move bytes read into bytes needed to write
+
+    li a0, 1
+    la a1, cat_buf
+    li a7, 64
+    ecall
+    j .LReadStdinBlock
+
+    .LCatStdinRet:
+    ret
 
 # Open fd
 # @param[in] a0: ptr to null-terminated file str.