Changed as7 to allow sys open statements, rewrite write_test.s to

use this syntax, and fixed up a bug in sys_open in a7out.

tools/write_test.s

@@ -7,16 +7,16 @@ main:
    lac in
    dac out
 
-   " The assembler doesn't know sys write, so iot 5 for now
+   " Write hello to fd1 i.e stdout
    lac d1
-   iot 5; hello; 7
+   sys write; hello; 7
 
    " open file fred
-   iot 3; fred; 0; 0
+   sys open; fred; 0; 0
 
    " read 5 words into the buffer from stdin: type in 10 or more characters!
    lac d0
-   iot 4; buf; 5
+   sys read; buf; 5
 
    " Stop and dump memory, so you can see five words at location 0400
    " Comment out the hlt instruction to test close and exit
@@ -24,10 +24,10 @@ main:
 
    " close stdin
    lac d0
-   iot 9
+   sys close
 
    " exit
-   iot 14
+   sys exit
 
    " We should not get to the halt instruction
    hlt
@@ -41,8 +41,6 @@ in: 023
 out: 0
 
 " Hello, world\n, two ASCII chars per word
-d0: 0
-d1: 1
 hello: 0110145; 0154154; 0157054; 040; 0167157; 0162154; 0144012
 
 " fred as a string, NUL terminated
@@ -51,3 +49,6 @@ fred: 0146162; 0145144; 0
 " Input buffer for read
 . = 0400
 buf: 0
+
+d0: 0
+d1: 1