Or perhaps...

In my example, the substitutional arguments for printf are pushed onto the
> stack in reverse order, then the address of the string, and then printf is
> called. After this, 6 is added to the stack pointer. I assume that the
> printf routine pops the address of the string off the stack, but leaves the
> other values on the stack, hence the need to add 2x3=6 to the stack after
> calling printf in my example.
>

... just adding 2 for every decrement that was done on sp...

Paul

*Paul Riley*

Mo: +61 (0)411 781 394
Email: paul@rileyriot.com



On Mon, 28 Jun 2021 at 12:34, Paul Riley <paul@rileyriot.com> wrote:

> Hi,
>
> I want to use printf from an assembly language program, in V6. It seems
> that the Unix Programmer's Manual doesn't show how to use it from assembly,
> so I wrote a short C program and captured the assembler output, for some
> clues. Listings below.
>
> In my example, the substitutional arguments for printf are pushed onto the
> stack in reverse order, then the address of the string, and then printf is
> called. After this, 6 is added to the stack pointer. I assume that the
> printf routine pops the address of the string off the stack, but leaves the
> other values on the stack, hence the need to add 2x3=6 to the stack after
> calling printf in my example.
>
> What troubles me is that the stack pointer is not decremented before the
> first mov, in the example below. Is this some C convention? I would assume
> that the first push in my example would overwrite the top of the stack.
> Perhaps I'm not used to PDP-11 stack conventions.
>
> I understand db only works on files like a.out or core dumps. If I wanted
> to break the assembly language program to examine values, how can I force a
> termination and core dump elegantly, so I can examine some register values?
>
> Paul
>
>
> *Paul Riley*
>
> Email: paul@rileyriot.com
>
> int a, b, c;
> int main(){
>   printf("printf: start\n");
>   a = 1;
>   b = 2;
>   c = 3;
>   printf("A = %d, B = %d, C = %d", a, b, c);
>   printf("printf: end\n");
>
> }
>
> .comm   _a,2
> .comm   _b,2
> .comm   _c,2
> .globl  _main
> .text
> _main:
> ~~main:
> jsr     r5,csv
> jbr     L1
> L2:mov  $L4,(sp)
> jsr     pc,*$_printf
> mov     $1,_a
> mov     $2,_b
> mov     $3,_c
> mov     _c,(sp)
> mov     _b,-(sp)
> mov     _a,-(sp)
> mov     $L5,-(sp)
> jsr     pc,*$_printf
> add     $6,sp
> mov     $L6,(sp)
> jsr     pc,*$_printf
> L3:jmp  cret
> L1:jbr  L2
> .globl
> .data
> L4:.byte 160,162,151,156,164,146,72,40,163,164,141,162,164,12,0
> L5:.byte
> 101,40,75,40,45,144,54,40,102,40,75,40,45,144,54,40,103,40,75,40,45,144,0
> L6:.byte 160,162,151,156,164,146,72,40,145,156,144,12,0
> #
>
>