You got my curiosity up and found the V5 and V6 source code (I did not have V4 easy to get, where I am today) ;-)
A big clue of it being C will be having crt0.s (below) in the first few bytes of the disassembled code. We see the a.out header (i.e. start at offset 20 for the code) and look what's there. I'm going to guess that is at 046 is the address of _main, from the call instruction at address 034. The Trap 1 is a sys exit @ address 044.
But .. the V6 crt0.s source has a call to _exit, which is lacking in the binary below. So it means that the binary was not created with the C runtime and probably not the v6 C compiler in the sources. So I took a peak at the V6 crt0.s and guess what -- it matches!
So, I'm going to guess the binary was compiled and linked with an earlier compiler. Ao ... if I had to guess, the programs are similar, but possibly different.
% more wump.das
;
; pdp11dasm version 0.0.3
; disassembly of wump
;
000000: 000407 br 20 ; ..
;
000002: 005334 dec @(r4)+ ; \.
000004: 004524 jsr r5,(r4)+ ; T.
000006: 002312 bge 37777777634 ; J.
000010: 000000 halt ; ..
000012: 000000 halt ; ..
000014: 000000 halt ; ..
;
000016: 000001 wait ; ..
000020: 170011 setd ; .p
000022: 010600 mov r6,r0 ; ..
000024: 011046 mov (r0),-(r6) ; &.
000026: 005720 tst (r0)+ ; P.
000030: 010066 000002 mov r0,2(r6) ; 6...
000034: 004767 000006 call 46 ; w...
000040: 022626 cmp (r6)+,(r6)+ ; .%
000042: 005000 clr r0 ; ..
000044: 104401 trap 1 ; ..
000046: 004567 005174 jsr r5,5246 ; w.|.
000052: 005746 tst -(r6) ; f.
000054: 012716 011230 mov #11230,(r6) ; N...
000060: 004737 002776 call @#2776 ; _.~.
000064: 004767 002262 call 2352 ; w.2.
000070: 022700 000171 cmp #171,r0 ; @%y.
000074: 001027 bne 154 ; ..
000076: 005004 clr r4 ; ..
000100: 010400 mov r4,r0 ; ..
000102: 006300 asl r0 ; @.
000104: 005760 005334 tst 5334(r0) ; p.\.
000110: 001421 beq 154 ; ..
000112: 032704 000001 bit #1,r4 ; D5..
000116: 001403 beq 126 ; ..
000120: 012716 000024 mov #24,(r6) ; N...
000124: 000402 br 132 ; ..
;
000126: 012716 000003 mov #3,(r6) ; N...
000132: 010400 mov r4,r0 ; ..
000134: 006300 asl r0 ; @.
000136: 016046 005334 mov 5334(r0),-(r6) ; &.\.
000142: 004737 002776 call @#2776 ; _.~.
V6: s4/crt0.s:
/ C runtime startoff
.globl savr5
.globl _exit
.globl _main
start:
setd
mov sp,r0
mov (r0),-(sp)
tst (r0)+
mov r0,2(sp)
jsr pc,_main
mov r0,(sp)
jsr pc,*$_exit
sys exit
.bss
savr5: .=.+2
V5: s4/crt0.s:
/ C runtime startoff
.globl savr5
.globl _main
start:
setd
mov sp,r0
mov (r0),-(sp)
tst (r0)+
mov r0,2(sp)
jsr pc,_main
cmp (sp)+,(sp)+
clr r0
sys exit
.bss
savr5: .=.+2