Abstract: A network interface card (NIC) is provided with a transmit unload block for asynchronously segmenting packet data into ATM cells for packets of multiple channels. The transmit unload block comprises a cellification block and a cellification and DMA scheduler. The cellification block is used to perform the actual cellification of the packet data into ATM cells, one ATM cell at a time, and management of the packet control data associated with the ATM cell's packet as well as management of the buffer control data associated with the ATM cell's channel. The construction of the current ATM cell is overlapped with the management of the packet and buffer control data associated with the immediately preceding ATM cell. The cellification and DMA scheduler is used to control the operation of the cellification block. The cellification and DMA scheduler is also used to schedule DMAs to obtain additional packet data for the channels.