83C795

T he SMC 83C795 E thernet S ys tem Controller implements the IEEE 802.3 protocol for networks such as E thernet, Cheapernet, and 10BaseT. It is a highly integrated device that shrinks the es sence of a LAN adapter card onto a s ingle piece of s ilicon.

It includes the 802.3 Media Access Control (MAC) functions, the Phys ical Layer Interface (PLI) for 10BAS E -T media, and a hos t interface des igned for s imple connection to the Indus try S tandard Architecture (ISA) PC/AT bus. To create a LAN adapter only the 83C795, a single buffer RAM, an E E ROM chip, and an optional ROM for B IOS or IPL code s torage are required.

T ransformers and supporting analog components complete an adapter des ign. All necessary control logic is provided by the 83C795. T he resulting LAN adapter appears to the hos t as a block of I/O regis ters with a block of shared memory, unles s the I/O pipe is used.

T he base address for I/O regis ters is programmable as is the base addres s and s ize of the buffer memory. This device is similar to the SMC 83C790 LAN controller with three major differences: ` A small memory cache has been added for host accesses to shared memory. ` An I/O pipe mode has been added to access the buffer memory. ` Auto-configurability logic has been added in order to comply with the new ISA Plug and Play specification. As with the 83C790 chip, there are two basic modes of operation: normal and ALT EGO.

In the normal mode, the LAN controller operates much like the 83C690 LAN Controller with received frames being buffered in a ring of contiguous , fixed-s ize buffers .

When the ALT EGO feature is enabled, the device switches to a very different mode of operation. T he differences are summarized here and explained in detail throughout the specification:

1. Linked-list style of buffering instead of ring buffers.

2. Different register map for LAN controller, exposing new registers for the linked-list buffering.

Figure 1-1 depicts the 83C795's functionality.