Added first draft of a part15-tcpip write-up

part15-tcpip/README.md

@@ -0,0 +1,63 @@
+Writing a "bare metal" operating system for Raspberry Pi 4 (Part 15)
+====================================================================
+
+Adding a TCP/IP stack
+---------------------
+Having achieved "proof of life" from our Ethernet module in _part14-spi-ethernet_, you're doubtless wondering how to go from there to serving web pages, posting tweets on Twitter or perhaps even just simply responding to a ping!
+
+This is where you'll need a fully-fledged TCP/IP stack that goes way beyond handcrafted ARPs, implementing many more protocols to achieve efficient bi-directional communication.
+
+In this part we make use of some code from Guido Socher of [tuxgraphics.org](http://tuxgraphics.org/), designed to be a lightweight TCP/IP stack for embedded devices. I chose this because it was super simple to get working (or "port"), but you might want to look at [LwIP](https://en.wikipedia.org/wiki/LwIP) if you need something more advanced.
+
+The code
+--------
+Most of the new code is in the _tcpip/_ subdirectory. I actually came across it in [this Github repository](https://github.com/ussserrr/maglev-ti-rtos) and, again, made only a very few cosmetic changes (`diff` is your friend!).
+
+It did require me to expose the `strlen()` function we implemented in _lib/fb.c_, so that's added to _include/fb.h_. Similarly, we expose the `memcpy()` function we implemented in _kernel/kernel.c_, so that's added to _kernel/kernel.h_.
+
+I also needed a single function that tells the ENC to send a packet. Nothing new here, just different packaging:
+
+```c
+void enc28j60PacketSend(unsigned short buflen, void *buffer) {
+   if (ENC_RestoreTXBuffer(&handle, buflen) == 0) {
+      ENC_WriteBuffer((unsigned char *) buffer, buflen);
+      handle.transmitLength = buflen;
+      ENC_Transmit(&handle);
+   }
+}
+```
+
+This was also added to _kernel/kernel.h_. Finally, _kernel/kernel.c_ now calls a function called `net_test()` instead of our original `arp_test()`.
+
+The changes to _arp.c_
+----------------------
+We initialise the network card in exactly the same way but, when we're done, we call this function in Guido's code:
+
+```c
+init_udp_or_www_server(myMAC, deviceIP);
+```
+
+This tells the TCP/IP library who we are, so we're all on the same page!
+
+Finally, and aside from a little cleanup, the major change is the new `net_test()` function:
+
+```c
+void net_test(void)
+{
+   while (1) {
+      while (!ENC_GetReceivedFrame(&handle));
+
+      uint16_t len = handle.RxFrameInfos.length;
+      uint8_t *buffer = (uint8_t *)handle.RxFrameInfos.buffer;
+      packetloop_arp_icmp_tcp(buffer, len);
+   }
+}
+```
+
+This is an infinite loop which waits for an incoming packet and then simply passes it to Guido's `packetloop_arp_icmp_tcp()` function. This function implements some useful things, like responding to pings. I modified the routine to print a message to the screen when it sends a "pong" (look from line 1381), so we can see when it's in action!
+
+_Imagine my excitement when I built, ran and could ping my RPi4 at 192.168.0.66 and get a response to both my laptop and my iPhone!_
+
+I recommend reading [this page](http://tuxgraphics.org/electronics/200905/embedded-tcp-ip-stack.shtml) to give you some ideas about what else you might achieve with Guido's library...
+
+![Pinging from my iPhone](images/15-tcpip-pinging.jpg)

part15-tcpip/tcpip/ip_arp_udp_tcp.c

@@ -15,7 +15,6 @@
  * packet. The client "web browser" as implemented here can also receive
  * large pages.
  *
- * Chip type           : ATMEGA88/168/328/644 with ENC28J60
  *********************************************/
 
 #include "ip_config.h"
@@ -27,7 +26,7 @@
 //
 static uint8_t macaddr[6];
 static uint8_t ipaddr[4]={0,0,0,0};
-static uint8_t seqnum=0xa; // my initial tcp sequence number
+//static uint8_t seqnum=0xa; // my initial tcp sequence number
 static void (*icmp_callback)(uint8_t *ip);
 
 //
@@ -1319,7 +1318,7 @@ uint8_t packetloop_icmp_checkreply(uint8_t *buf,uint8_t *ip_monitoredhost)
 // of the tcp data if there is tcp data part
 uint16_t packetloop_arp_icmp_tcp(uint8_t *buf,uint16_t plen)
 {
-        uint16_t len;
+//        uint16_t len;
 #if defined (TCP_client)
         uint8_t send_fin=0;
         uint16_t tcpstart;

part15-tcpip/tcpip/ip_arp_udp_tcp.h

@@ -7,7 +7,6 @@
  *
  * IP/ARP/UDP/TCP functions
  *
- * Chip type           : ATMEGA88/168/328/644 with ENC28J60
  *********************************************/
 //@{
 #ifndef IP_ARP_UDP_TCP_H

part15-tcpip/tcpip/net.h

@@ -130,3 +130,4 @@
 //
 #endif
 //@}
+