Ne2000

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The Ne2000 network card chipset was a reference design that was never meant to go into mainstream production, but after Novell used it as cheap hardware to go with their software it became popular. It is a good first network card to program because it follows a simple design (making it helpful for learning), they're probably dirt-cheap, and it is supported by most PC emulators. Both Bochs and QEMU provide ISA and PCI implementations.

Ne2000 is not technically a card, it is a standard that several implementors follow. The best available description of the initial standard was located DP8390D/NS32490D NIC Network Interface Controller (archive) and was published by National Semiconductor.

Quick Overview of the NIC design

The Ne2000 network card uses two ring buffers for packet handling. These are circular buffers made of 256-byte pages that the chip's DMA logic will use to store received packets or to get received packets.

Note that a packet will always start on a page boundary, thus there may be unused bytes at the end of a page.

Ring Buffer

Two registers PSTART and PSTOP define a set of 256-byte pages in the buffer memory that will be used for the ring buffer. As soon as the DMA attempts to read/write to PSTOP, it will be sent back to PSTART 

PSTART                                                                       PSTOP
####+-8------+-9------+-a------+-b------+-c------+-d------+-e------+-f------+####
####| Packet 3 (cont) |########|########|Packet1#|   Packet  2#####|Packet 3|####
####+--------+--------+--------+--------+--------+--------+--------+--------+####

An 8-page ring buffer with 3 packets and 2 free slots.

While receiving, the NIC has 2 additional registers that point to the first packet that's still to be read and to the start of the currently written packet (named boundary pointer and current page respectively).

Register Pages

Programming registers of the NE2000 are collected in pages. Page 0 contains most of the control and status registers while page 1 contains physical (PAR0..PAR5) and multicast addresses (MAR0..MAR7) to be checked by the card.

Note that the same register number could have a different meaning depending whether you read or write to it. For instance, register 0x0C on page 0 is the receive configuration register in write mode and "receive status register" in read mode. Most of the configuration registers can still be read on page 2 though. Each register is a single byte and the page is selected by highest 2 bits of the COMMAND register (which is available in all pages) 

Ne2K_registers (page=0, read) {
   COMMAND=0,          //!< the master command register
   CLDA0,              //!< Current Local DMA Address 0
   CLDA1,              //!< Current Local DMA Address 1
   BNRY,               //!< Boundary Pointer (for ringbuffer)
   TSR,                //!< Transmit Status Register
   NCR,                //!< collisions counter
   FIFO,               //!< (for what purpose ??)
   ISR,                //!< Interrupt Status Register
   CRDA0,              //!< Current Remote DMA Address 0
   CRDA1,              //!< Current Remote DMA Address 1
   RSR=0x0c,           //!< Receive Status Register
};

/*Registers that are the same in read & write are omitted.*/
Ne2K_registers (page=0, write) {
   PTART=1,            //!< page start (init only)
   PSTOP,              //!< page stop  (init only)
   TPSR=4,             //!< transmit page start address
   TBCR0,              //!< transmit byte count (low)
   TBCR1,              //!< transmit byte count (high)
   RSAR0=8,            //!< remote start address (lo)
   RSAR1,              //!< remote start address (hi)
   RBCR0,              //!< remote byte count (lo)
   RBCR1,              //!< remote byte count (hi)
   RCR,                //!< receive config register
   TCR,                //!< transmit config register
   DCR,                //!< data config register    (init)
   IMR,                //!< interrupt mask register (init)
};

Initialization and MAC Address

This wasn't exactly obvious, but by looking at the ne2k-pci module from Linux I managed to figure out how to initilize the card and read its MAC address: 

nif->iobase = nif->pcidev->bar[0] & ~0x3;

outb(nif->iobase + 0x1F, inb(nif->iobase + 0x1F));  // write the value of RESET into the RESET register
while ((inb(nif->iobase + 0x07) & 0x80) == 0);      // wait for the RESET to complete
outb(nif->iobase + 0x07, 0xFF);                     // mask interrupts

uint8_t prom[32];
outb(nif->iobase, (1 << 5) | 1);	// page 0, no DMA, stop
outb(nif->iobase + 0x0E, 0x49);		// set word-wide access
outb(nif->iobase + 0x0A, 0);		// clear the count regs
outb(nif->iobase + 0x0B, 0);
outb(nif->iobase + 0x0F, 0);		// mask completion IRQ
outb(nif->iobase + 0x07, 0xFF);
outb(nif->iobase + 0x0C, 0x20);		// set to monitor
outb(nif->iobase + 0x0D, 0x02);		// and loopback mode.
outb(nif->iobase + 0x0A, 32);		// reading 32 bytes
outb(nif->iobase + 0x0B, 0);		// count high
outb(nif->iobase + 0x08, 0);		// start DMA at 0
outb(nif->iobase + 0x09, 0);		// start DMA high
outb(nif->iobase, 0x0A);		// start the read

int i;
for (i=0; i<32; i++)
{
  prom[i] = inb(nif->iobase + 0x10);
};

// program the PAR0..PAR5 registers to listen for packets to our MAC address!		
for (i=0; i<6; i++)
{
  writeRegister(nif, 1, 0x01+i, prom[i]);
};

The first 6 bytes of "prom" extracted here are the MAC address.

Sending a Packet

The following sequence is the one observed by the ne2k-pci module in linux. Note that some odd cards needs a patch (read-before-write) that isn't covered here. The data configuration is initialized at 0x49 (word transfer, 8086 byte order, dual 16bit DMA, loopback disabled). Note that the weird driver doesn't seem to use interrupts for completion notification.

  1. COMMAND register set to "start" and "nodma" (0x22)
  2. RBCRx are loaded with the packet size
  3. "Remote DMA complete?" bit is cleared by writing a 1 in bit 6 of ISR (that's odd, but that's the way it works)
  4. RSARx are loaded with 0x00 (low) and target page number (high) respectively. At this stage, the chip is ready receiving packet data and storing it in the ring buffer for emission.
  5. COMMAND register set to "start" and "remote write DMA" (0x12)
  6. Packets data is now written to the "data port" (that is register 0x10) of the NIC in a loop (or using an "outsx" if available). The NIC will then update its remote DMA logic after each written 16-bit value/32-bit value and places bytes in the transmit ring buffer.
  7. Poll ISR register until bit 6 (Remote DMA completed) is set.

ISA configuration information

Ne2000 Registers

The base register number can be anywhere from 0x280 to 0x380, as I've found, but I usually configure bochs to operate with port 0x300 as a base.

Ne2000 Interrupts

I have configured my ne2000 card in bochs to signal interrupts on IRQ 3.

Ne2000 Reset

Before transmitting data with the ne2000, it must be reset and data in the ring buffer cleared. This can be done by writing out the contents of the reset register to the reset register.

See Also

External Links

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