void pcnet_ioport_writel(void *opaque, uint32_t addr, uint32_t val)
{
    PCNetState *s = opaque;
    pcnet_poll_timer(s);
#ifdef PCNET_DEBUG_IO
    printf("pcnet_ioport_writel addr=0x%08x val=0x%08x\n", addr, val);
#endif
    if (BCR_DWIO(s)) {
        switch (addr & 0x0f) {
        case 0x00: /* RDP */
            pcnet_csr_writew(s, s->rap, val & 0xffff);
            break;
        case 0x04:
            s->rap = val & 0x7f;
            break;
        case 0x0c:
            pcnet_bcr_writew(s, s->rap, val & 0xffff);
            break;
        }
    } else
    if ((addr & 0x0f) == 0) {
        /* switch device to dword i/o mode */
        pcnet_bcr_writew(s, BCR_BSBC, pcnet_bcr_readw(s, BCR_BSBC) | 0x0080);
#ifdef PCNET_DEBUG_IO
        printf("device switched into dword i/o mode\n");
#endif
    }
    pcnet_update_irq(s);
}

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.