src/cmd/cat.s: More comments added, but still more to add.

src/cmd/cat.s

@@ -1,17 +1,18 @@
-" cat
+" cat: cat [arg1 arg2 ...]
 
-   lac 017777 i			" exit if four argument words only?
-   sad d4
-   jmp nofiles
-   lac 017777
-   tad d1			" why not tad d5 (add 5)?
-   tad d4
-   dac name			" name = *(017777) + 5
+   " Load the pointer pointer in 017777 to see if we have any arguments
+   lac 017777 i
+   sad d4			" Skip if we have more than four argument words
+     jmp nofiles		" Only four argument words, so no arguments
+   lac 017777			" Move five words past the argument word count
+   tad d1			" so that AC points at the first argument
+   tad d4			" and save the pointer in name
+   dac name
 
 loop:
-   sys open; name; 0; 0		" open file, get fd back
-   spa				" exit if fd == -1
-   jmp badfile
+   sys open; name; 0; 0		" Open file, get fd back
+   spa
+     jmp badfile		" Negative fd, exit with an error message
    dac fi			" save file descriptor in fi
 
 1:
@@ -22,33 +23,35 @@ loop:
    jmp 1b
 
 1:
-   lac fi
+   lac fi			" Close the file descriptor in fi
    sys close
 
 loop1:
    -4
-   tad 017777 i
+   tad 017777 i			" Subtract 4 from the count of argument words
    dac 017777 i
-   sad d4
-   jmp done
-   lac name
-   tad d4
+   sad d4			" Is the value 4, i.e. no args left?
+     jmp done			" Yes, so exit
+
+   lac name			" Still an argument, so move up
+   tad d4			" to the next filename argument
    dac name
-   jmp loop
+   jmp loop			" and loop back to cat this file
 
 badfile:
-   lac name
-   dac 1f
+   lac name			" Get the pointer to the filename
+   dac 1f			" Store it in 1f below
+   lac d8			" Load fd 8 which is stderr
+   sys write; 1:0; 4		" Write the four words of the filename
    lac d8
-   sys write; 1:0; 4
-   lac d8
-   sys write; 1f; 2
-   jmp loop1
+   sys write; 1f; 2		" and then write " ?\n"
+   jmp loop1			" Now try doing the next argument
+
+1: 040;077012			" String literal: " ?\n"
 
-1: 040;077012
 nofiles:
    lac d8
-   sys write; 1f; 5		" write 5 words to fd 8?
+   sys write; 1f; 5		" Write "No files\n" to stderr
    sys exit			" and exit
 
 1: <no>; 040;  <fi>;<le>;<s 012
@@ -66,7 +69,7 @@ done:
    lac noc
    rcr
    dac 1f
-   lac fo			" load fd 1, stdout
+   lac fo			" Load fd 1, stdout
    sys write; iopt+1; 1:..
    sys exit
 
@@ -127,6 +130,7 @@ putc: 0
    dac opt
    dzm noc
    jmp putc i
+
 2: 0;0
 ipt: 0
 eipt: 0