HelenOS sources

TX_BUFF_SIZE      888 uspace/drv/nic/rtl8139/driver.c 		uint32_t addr = PTR2U32(rtl8139->tx_buff_phys + i * TX_BUFF_SIZE);
TX_BUFF_SIZE     1115 uspace/drv/nic/rtl8139/driver.c 		rtl8139->tx_buff[i] = rtl8139->tx_buff_virt + i * TX_BUFF_SIZE;
HelenOS homepage, sources at GitHub