[9fans] ARM question

[9fans] ARM question

[Ronald G. Minnich]

here's the code in c:
#include <u.h>
#include <libc.h>
y(){
	unsigned long *x;
	x = (unsigned long *) 0x4000;
	*x = (unsigned long) 0x40e;
}
main(){
	unsigned long *x;
	static unsigned long y[64];
	int i;

	x = (unsigned long *) 0x4000;
	*x = (unsigned long) 0x40e;
	x++;
	*x = (unsigned long)0x100000 + 0x40e;
	x++;
	*x = (unsigned long)0x200000 + 0x40e;
	x++;
	*x = (unsigned long)0x300000 + 0x40e;
	x++;
	*x = (unsigned long)0x400000 + 0x40e;
	x++;
	*x = (unsigned long)0x500000 + 0x40e;
	x++;
	*x = (unsigned long)0x600000 + 0x40e;
	x++;
	*x = (unsigned long)0x700000 + 0x40e;

}

And the .s
	TEXT	y+0(SB),0,$4
	MOVW	$1038,R4
	MOVW	$16384,R1
	MOVW	R4,0(R1)
	RET	,
	TEXT	main+0(SB),0,$8
	MOVW	$1049614,R10
	MOVW	$2098190,R9
	MOVW	$3146766,R8
	MOVW	$4195342,R7
	MOVW	$5243918,R6
	MOVW	$6292494,R5
	MOVW	$7341070,R4
	MOVW	$16384,R3
	MOVW	$1038,R2
	MOVW.P	R2,4(R3)
	MOVW.P	R10,4(R3)
	MOVW.P	R9,4(R3)
	MOVW.P	R8,4(R3)
	MOVW.P	R7,4(R3)
	MOVW.P	R6,4(R3)
	MOVW	R5,0(R3)
	MOVW	R4,4(R3)
	RET	,
	GLOBL	y$2<>+0(SB),R0,$256
	END	,

The code for y() I can understand.

but this:

	MOVW	$16384,R3
	MOVW	$1038,R2
	MOVW.P	R2,4(R3)

I don't get. How can R2 get stored to 16384, given the 4(R3)?

Are we pretty much agreed that the MOVW.P is a post-increment mode?

ron

Re: [9fans] ARM question

[Russ Cox]

perhaps the argument to MOVW.P is the final
(post-increment) value instead of the value used
during the write.


On Thu, 10 Mar 2005 14:55:37 -0700 (MST), Ronald G. Minnich
<rminnich@lanl.gov> wrote:
>
> here's the code in c:
> #include <u.h>
> #include <libc.h>
> y(){
>         unsigned long *x;
>         x = (unsigned long *) 0x4000;
>         *x = (unsigned long) 0x40e;
> }
> main(){
>         unsigned long *x;
>         static unsigned long y[64];
>         int i;
>
>         x = (unsigned long *) 0x4000;
>         *x = (unsigned long) 0x40e;
>         x++;
>         *x = (unsigned long)0x100000 + 0x40e;
>         x++;
>         *x = (unsigned long)0x200000 + 0x40e;
>         x++;
>         *x = (unsigned long)0x300000 + 0x40e;
>         x++;
>         *x = (unsigned long)0x400000 + 0x40e;
>         x++;
>         *x = (unsigned long)0x500000 + 0x40e;
>         x++;
>         *x = (unsigned long)0x600000 + 0x40e;
>         x++;
>         *x = (unsigned long)0x700000 + 0x40e;
>
> }
>
> And the .s
>         TEXT    y+0(SB),0,$4
>         MOVW    $1038,R4
>         MOVW    $16384,R1
>         MOVW    R4,0(R1)
>         RET     ,
>         TEXT    main+0(SB),0,$8
>         MOVW    $1049614,R10
>         MOVW    $2098190,R9
>         MOVW    $3146766,R8
>         MOVW    $4195342,R7
>         MOVW    $5243918,R6
>         MOVW    $6292494,R5
>         MOVW    $7341070,R4
>         MOVW    $16384,R3
>         MOVW    $1038,R2
>         MOVW.P  R2,4(R3)
>         MOVW.P  R10,4(R3)
>         MOVW.P  R9,4(R3)
>         MOVW.P  R8,4(R3)
>         MOVW.P  R7,4(R3)
>         MOVW.P  R6,4(R3)
>         MOVW    R5,0(R3)
>         MOVW    R4,4(R3)
>         RET     ,
>         GLOBL   y$2<>+0(SB),R0,$256
>         END     ,
>
> The code for y() I can understand.
>
> but this:
>
>         MOVW    $16384,R3
>         MOVW    $1038,R2
>         MOVW.P  R2,4(R3)
>
> I don't get. How can R2 get stored to 16384, given the 4(R3)?
>
> Are we pretty much agreed that the MOVW.P is a post-increment mode?
>
> ron
>

Re: [9fans] ARM question

[Richard Miller]

> perhaps the argument to MOVW.P is the final
> (post-increment) value instead of the value used
> during the write.

Exactly.  MOVW.P x,d(Ry) means "store x at the address pointed to
by Ry, then increment Ry by d".  It's the exact equivalent of
"*Ry++ = x" in C, if d is sizeof(*Ry).

-- Richard

Re: [9fans] ARM question

[Bruce Ellis]

page 3-116 explains the flags. (jagger ref).

brucee

On Fri, 11 Mar 2005 08:59:52 +0000, Richard Miller <9fans@hamnavoe.com> wrote:
> > perhaps the argument to MOVW.P is the final
> > (post-increment) value instead of the value used
> > during the write.
>
> Exactly.  MOVW.P x,d(Ry) means "store x at the address pointed to
> by Ry, then increment Ry by d".  It's the exact equivalent of
> "*Ry++ = x" in C, if d is sizeof(*Ry).
>
> -- Richard
>
>

Re: [9fans] ARM question

[Charles Forsyth]

>>page 3-116 explains the flags. (jagger ref).

when reading it, note that 5c/5l uses .P to clear the P bit in ldr/str;
it's set by default.

Re: [9fans] ARM question

[boyd, rounin]

> "*Ry++ = x" in C, if d is sizeof(*Ry).

sounds good to me.
--
MGRS 31U DQ 52572 12604

Re: [9fans] ARM question

[Bruce Ellis]

And also note that 5i is very good.

brucee

On Fri, 11 Mar 2005 13:10:57 +0000, Charles Forsyth
<forsyth@terzarima.net> wrote:
> >>page 3-116 explains the flags. (jagger ref).
>
> when reading it, note that 5c/5l uses .P to clear the P bit in ldr/str;
> it's set by default.

Re: [9fans] ARM question

[Ronald G. Minnich]

On Fri, 11 Mar 2005, Richard Miller wrote:

> Exactly.  MOVW.P x,d(Ry) means "store x at the address pointed to
> by Ry, then increment Ry by d".  It's the exact equivalent of
> "*Ry++ = x" in C, if d is sizeof(*Ry).
>

gotcha. Thanks for the info. I knew the code was probably right, I just
couldn't make any sense of it.

ron

Re: [9fans] ARM question

[boyd, rounin]

> "*Ry++ = x" in C, if d is sizeof(*Ry).

sounds good to me.
--
MGRS 31U DQ 52572 12604