30 Apr
2008
30 Apr
'08
9:08 p.m.
On Wed, Apr 30, 2008 at 4:56 AM, Warren Toomey <wkt(a)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
--
carl lowenstein marine physical lab u.c. san diego
clowenstein(a)ucsd.edu