So,

in v6, it was possible to use the mesg function from the system library with:

        ed hello.s
	/ hello world using external mesg routine

	.globl  mesg

        mov     sp,r5
        jsr     r5,mesg; <Hello, World!\n\0>; .even
        sys     exit


	as hello.s
	ld -s a.out -l
	a.out
	Hello, World!

This was because v6 included mesg in the library, in v7, it doesn't look like mesg is included, so doing the same thing as above requires that code to write the message out be included and in addition system call names are not predefined, so exit and write have to be looked up in /usr/include/sys.s, resulting in the v7 equivalent file:

ed hello2.s
/ hello world using internal mesg routine

        mov     sp,r5
        jsr     r5,mesg; <Hello, World!\n\0>; .even
        sys     1

mesg:
        mov     r0,-(sp)
        mov     r5,r0
        mov     r5,0f
1:
        tstb    (r5)+
        bne     1b
        sub     r5,r0
        com     r0
        mov     r0,0f+2
        mov     $1,r0
        sys     0; 9f
.data
9:
        sys     4; 0:..; ..
.text
        inc     r5
        bic     $1,r5
        mov     (sp)+,r0
        rts     r5

as hello2.s
a.out
Hello, World!

My questions are:

1. Is mesg or an equivalent available in v7?
2. If not, what was the v7 way of putting strings out?
3. Why aren't the system call names defined?
4. What was the v7 way of naming system calls?

Will