[TUHS] Query on PDP-11 assembly

Carl Lowenstein carl.lowenstein at gmail.com
Thu May 1 01:08:51 AEST 2008

On Wed, Apr 30, 2008 at 4:56 AM, Warren Toomey <wkt at tuhs.org> wrote:
> All, I'm trying to write a PDP-11 disassembler for a.out files. I'm having
>  trouble dealing with jsrs. Take, for example, the code here:
>  http://minnie.tuhs.org/UnixTree/1972_stuff/s1/frag19.html
>  I can happily deal with the   jsr pc,do   type of jsr, but the ones
>  involving r5 have me stumped, e.g.:
>         jsr     r5,questf; < nonexistent\n\0>; .even
>  It appears that data is being inserted into the executable directly
>  after the jsr instruction. How does the rts which returns from the jsr
>  know how much data to skip, and what is the involvement of r5 here?

Standard subroutine calling sequence.

The called routine must know how many parameters it is called with.
It retrieves them by MOV (R5)+, <somewhere>.
This advances R5 so that eventually it points to the return address,
and the return is done as RTS R5.

A more advanced calling sequence could insert the number of parameters
as the first value after the JSR, and the called routine would then
retrieve that number and use it to tell when it had fetched the right
amount of data.

    carl lowenstein         marine physical lab     u.c. san diego
                                                 clowenstein at ucsd.edu

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