Alignment puzzle

[JFG]

Hi -

I have this program to maintain and I'm puzzling over the following
function -

void
lance_copytobuf_gap2(sc, fromv, boff, len)
struct lance_softc *sc;
void *fromv;
int boff;
register int len;
{
volatile caddr_t buf = sc->sc_mem;
register caddr_t from = fromv;
register volatile u_int16_t *bptr;
if (boff & 0x1) {
/* handle unaligned first byte */
bptr = ((volatile u_int16_t *)buf) + (boff - 1);
*bptr = (*from++ << 8) | (*bptr & 0xff);
bptr += 2;
len--;
} else
bptr = ((volatile u_int16_t *)buf) + boff;
while (len 1) {
*bptr = (from[1] << 8) | (from[0] & 0xff);
bptr += 2;
from += 2;
len -= 2;
}
if (len == 1)
*bptr = (u_int16_t)*from;
}

What exactly is happening in the "handle unaligned first byte" part? It
seems quite opaque to me.

--
We all live in a yellow submarine.

[JFG]

Can no one help with this? I'd have thought it's bread and butter for
the experts!

On 20 Sep 2007 at 0:16, JFG wrote:

Hi -

I have this program to maintain and I'm puzzling over the following
function -

void
lance_copytobuf_gap2(sc, fromv, boff, len)
struct lance_softc *sc;
void *fromv;
int boff;
register int len;
{
volatile caddr_t buf = sc->sc_mem;
register caddr_t from = fromv;
register volatile u_int16_t *bptr;
if (boff & 0x1) {
/* handle unaligned first byte */
bptr = ((volatile u_int16_t *)buf) + (boff - 1);
*bptr = (*from++ << 8) | (*bptr & 0xff);
bptr += 2;
len--;
} else
bptr = ((volatile u_int16_t *)buf) + boff;
while (len 1) {
*bptr = (from[1] << 8) | (from[0] & 0xff);
bptr += 2;
from += 2;
len -= 2;
}
if (len == 1)
*bptr = (u_int16_t)*from;
}

What exactly is happening in the "handle unaligned first byte" part? It
seems quite opaque to me.

--
We all live in a yellow submarine.

--
We all live in a yellow submarine.