[TUHS] pdp11 UNIX memory allocation.

[random832 at fastmail.us]

Apparently the message I was replying to was off-list, but it seems like
a waste to have typed all this out (including answering my own question)
and have it not go to the list.

On Tue, Jan 6, 2015, at 17:35, random832 at fastmail.us wrote:
> http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/factor.s
> wrchar:
> 	mov     r0,ch
> 	mov     $1,r0
> 	sys     write; ch; 1
> 	rts     r5
> 
> Though it looks like the C wrappers use the "indirect" system call which
> reads a "fake" trap instruction from the data segment. Looking at the
> implementation of that, my question is answered:
> 
> http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/sys/sys/trap.c
> 		if (callp == sysent) { /* indirect */
> 			a = (int *)fuiword((caddr_t)(a));
> 			pc++;
> 			i = fuword((caddr_t)a);
> 			a++;
> 			if ((i & ~077) != SYS)
> 				i = 077;        /* illegal */
> 			callp = &sysent[i&077];
> 			fetch = fuword;
> 		} else {
> 			pc += callp->sy_narg - callp->sy_nrarg;
> 			fetch = fuiword;
> 		}
> 
> http://minnie.tuhs.org/TUHS/Archive/PDP-11/Trees/V7/usr/man/man2/indir.2
> The main purpose of indir is to allow a program to
> store arguments in system calls and execute them
> out of line in the data segment.
> This preserves the purity of the text segment.
> 
> Note also the difference between V2 and V5 libc - clearly support for
> split I/D machines was added some time in this interval.
> http://minnie.tuhs.org/cgi-bin/utree.pl?file=V2/lib/write.s
> http://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s4/write.s