net: proper socket API in the kernel (SYS_NET) - no more SLIP in userland

The network stack moves into the kernel. src/socket.asm owns SLIP framing,
IPv4, RFC1071 checksums, and ICMP; programs now speak a socket API through one
syscall (SYS_NET, DE=&netreq dispatched by op): net_socket/connect/send/recv/
close/poll (c/sock.h). No program touches SLIP, IP headers, or checksums.

- Socket table (4 sockets) + tx/rx buffers in WRAM0; our IP = 10.0.0.2.
- net_pump: drains the link, reassembles SLIP frames, demuxes IPv4. Inbound
  ICMP echo requests are auto-answered in-kernel, so the GB replies to pings
  whenever any process pumps RX.
- ICMP sockets: send() emits an echo request to the connected peer; recv()
  returns the matching reply (with a spin/yield timeout).

ping.c is now a ~15-line socket client; netd.c is just `for(;;){net_poll();
yield();}`. Verified over tunbridge:
    /# ping 1.1.1.1     -> replies from the real internet (kernel builds it all)
    host# ping 10.0.0.2 -> 4/4, 0% loss (kernel auto-answers)

Gotchas recorded: gbos.inc isn't a make dep (touch asm after editing); this
crt0 doesn't copy initializers (fill arrays at runtime); and the arg string at
0xA000 overlaps _DATA, so parse targets must sit past it (big buffer first).
UDP and TCP sockets build on this same core next.

1 files changed, 52 insertions, 0 deletions

diff --git a/c/sock.h b/c/sock.h
new file mode 100644
index 0000000..9af2351
--- /dev/null
+++ b/c/sock.h
@@ -0,0 +1,52 @@
+#ifndef SOCK_H
+#define SOCK_H
+/* gbos socket API - the kernel owns SLIP/IP/ICMP/UDP/TCP; programs just do
+   socket()/connect()/send()/recv()/close(). Header-only: one copy per program.
+   The netreq layout must match include/gbos.inc (NR_* / NET_* / SOCK_*). */
+#include "gbos.h"
+
+#define SOCK_ICMP 1
+#define SOCK_UDP  2
+#define SOCK_TCP  3
+
+struct netreq {
+    unsigned char op;
+    unsigned char sock;
+    unsigned char *buf;
+    unsigned char len;
+    unsigned char ip[4];
+    unsigned char port_hi, port_lo;
+};
+static struct netreq _nr;
+
+/* -> socket id, or 0xFF if the table is full */
+static unsigned char net_socket(unsigned char type) {
+    _nr.op = 0; _nr.sock = type;
+    return sys_net(&_nr);
+}
+/* set the remote address/port for send()/recv() */
+static unsigned char net_connect(unsigned char s, const unsigned char *ip, unsigned int port) {
+    unsigned char i;
+    _nr.op = 1; _nr.sock = s;
+    for (i = 0; i < 4; i++) _nr.ip[i] = ip[i];
+    _nr.port_hi = (unsigned char)(port >> 8); _nr.port_lo = (unsigned char)port;
+    return sys_net(&_nr);
+}
+/* send 'len' payload bytes to the connected peer -> 0 ok / 0xFF error */
+static unsigned char net_send(unsigned char s, void *buf, unsigned char len) {
+    _nr.op = 2; _nr.sock = s; _nr.buf = (unsigned char *)buf; _nr.len = len;
+    return sys_net(&_nr);
+}
+/* receive up to 'max' payload bytes -> length, or 0xFF on timeout */
+static unsigned char net_recv(unsigned char s, void *buf, unsigned char max) {
+    _nr.op = 3; _nr.sock = s; _nr.buf = (unsigned char *)buf; _nr.len = max;
+    return sys_net(&_nr);
+}
+static void net_close(unsigned char s) {
+    _nr.op = 4; _nr.sock = s; sys_net(&_nr);
+}
+/* pump the RX path once (answers inbound pings) without blocking */
+static void net_poll(void) {
+    _nr.op = 6; sys_net(&_nr);
+}
+#endif