env: fix strlen and add more comments

env.s

@@ -21,19 +21,22 @@ _start:
     addi s1, sp, 16 # add 8 for argc, 8 for the NULL at the end of argv
 
     # compute start of envp
-    # we could use a sh3add instruction
-    # but the tools i'm using don't support the bitmanip extension
+    # we could use a sh3add instruction (from the bitmanip extension)
+    # but it's not supported by the tools i'm using
     #sh3add s1, sp, s0
     sll t0, s0, 3 # argc*8
     add s1, s1, t0
 
-    # allocate space for an iovec
+    # allocate space for two iovecs on the stack
+    # an iovec is just a pointer and a length
     addi sp, sp, -(8+8)*2
 
+    # initialize the second iovec with "\n", 1
     la t0, newline
     sd t0, 16(sp)
     li t1, 1
     sd t1, 24(sp)
+
 loop:
     # check for null pointer, which signals the end of envp
     ld a1, 0(s1)
@@ -46,28 +49,14 @@ loop:
     lbu t0, 0(t1)
     addi t1, t1, 1
     bnez t0, strlen
+    addi t1, t1, -1 # NUL
     sub a2, t1, a1
     sd a2, 8(sp)
 
-    /*
-    # print string
-    li a7, 64
-    li a0, 1
-    # a1 already set to start of string
-    # a2 already set to string length
-    ecall
-
-    # print newline
-    li a7, 64
-    li a0, 1
-    la a1, newline
-    li a2, 1
-    ecall
-    */
     li a7, 66 # sys_writev
-    li a0, 1
-    mv a1, sp
-    li a2, 2
+    li a0, 1  # stdout
+    mv a1, sp # pointer to iovec
+    li a2, 2  # number of iovecs
     ecall
     bltz a0, write_error