Hi, This technique was a very common one in assembler sources for PDP-11 versions of UNIX. For example, m40.s of Version 6 UNIX shows a line "jsr r0,call1; _trap" in its trap routine. I feel very funny to tell this to Warren, the author of apout! Anyway, it could be a source of the null-terminated expression of character strings of the C language, I guess. Naoki Hamada nao(a)tom-yam.or.jp

A subprogram using this calling convention would look something like this: questf: mov (r5)+,r0 / play with string pointed to by r0. rts r5 On Apr 30, 2008, at 8:55 AM, Brantley Coile wrote: The rts doesn't know anything about how much data to skip. In this snippet r5 is a linkage register that's doing double duty: it's both an argument pointer to the location immediately following the jsr and once r5 has been adjusted to point to the location after the argument list it becomes a return address. This programming technique is dependent on the subprogram and its callers agreeing on the number of arguments though it's possible to do a vararg style as well. It's necessary for the subprogram to pick up the arguments and adjust the linkage register accordingly AND to return to the caller with the same register named in the rts instruction as was used in the calling jsr. / variable argument list with linkage register jsr r5,some_fn; $argc; arg1; arg2; ... argn / in the function it's necessary to pick up all the args some_fn: mov (r5)+,r0 / pick up argc beq 2f / if no arguments, process simplest case. 1: / pick up an argument: mov (r5)+, somewhere / or mov *(r5)+, somewhere ... dec r0 bne 1b / process remaining arguments. 2: ... rts r5 I've seen other ways of processing arguments passed with this method that involved using indexing and adding the argument count to the linkage register. Tastes and mileage may vary. Regards, Milo -- Milo Velimirović, Unix Computer Network Administrator 608.785.6618 Office - 608.386.2817 Cell University of Wisconsin - La Crosse La Crosse, Wisconsin 54601 USA 43 48 48 N 91 13 53 W

Larry McVoy scripsit: Indeed. PDP-8 and PDP-11 assembly are the only assembly languages I've ever used, and I liked both of them a lot. -- John Cowan <cowan(a)ccil.org> http://www.ccil.org/~cowan One time I called in to the central system and started working on a big thick 'sed' and 'awk' heavy duty data bashing script. One of the geologists came by, looked over my shoulder and said 'Oh, that happens to me too. Try hanging up and phoning in again.' --Beverly Erlebacher

On Wed, Apr 30, 2008 at 11:41:17PM +0900, Naoki Hamada wrote: Well, I borrowed the PDP-11 instruction simulaton code from Eric Edwards, and spent most of my time on emulating the syscalls, so there are still things I don't know! I've got the disassembler to a point where it works OK on the instructions, I haven't worked on the data side yet. I have put it aside for a bit, due to increasing frustration levels and other things to do. If someone wants a copy, e-mail me. Thanks for all your responses, they were very useful. Cheers, Warren