net: SLIP framing over the link port + a host gateway (echo round-trip)

First step of link-port networking. The LCD terminal + OSK freed the serial
port from console duty, so it can be the network link.

Kernel (src/net.asm): raw link-port serial that bypasses the console/
terminal - sys_ssend (transmit, GB drives the clock) and sys_srecv (receive,
GB slave, blocks by yielding). Syscalls 29/30.

Userland: libc ssend()/srecv(); c/necho.c does SLIP (RFC 1055) framing over
them - send a packet, receive the reply, print it.

Host: tools/gateway.py wraps the emulator, owns its link serial, speaks SLIP,
and (for now) echoes every frame back - the "link cable adapter". Console
(ASCII) bytes on the same channel are printed for visibility.

Verified: `necho` sends a SLIP frame, the gateway decodes+echoes it, and gbos
prints the reply - a real framed round-trip over the Game Boy link port.
Next: swap the echo for actual network ops (DNS/HTTP or IRC/chat).

1 files changed, 44 insertions, 0 deletions

diff --git a/c/necho.c b/c/necho.c
new file mode 100644
index 0000000..b9dc9d5
--- /dev/null
+++ b/c/necho.c
@@ -0,0 +1,44 @@
+#include "gbos.h"
+/* necho: send a SLIP-framed packet over the link port and print the reply.
+   Talks to a host gateway (tools/gateway.py) that echoes frames back. */
+
+#define SLIP_END     0xC0
+#define SLIP_ESC     0xDB
+#define SLIP_ESC_END 0xDC
+#define SLIP_ESC_ESC 0xDD
+
+static void slip_send(const char *buf, unsigned char len) {
+    unsigned char i, c;
+    ssend(SLIP_END);
+    for (i = 0; i < len; i++) {
+        c = (unsigned char)buf[i];
+        if (c == SLIP_END)      { ssend(SLIP_ESC); ssend(SLIP_ESC_END); }
+        else if (c == SLIP_ESC) { ssend(SLIP_ESC); ssend(SLIP_ESC_ESC); }
+        else                      ssend(c);
+    }
+    ssend(SLIP_END);
+}
+
+static unsigned char slip_recv(char *buf, unsigned char max) {
+    unsigned char len = 0, c;
+    for (;;) {
+        c = srecv();
+        if (c == SLIP_END) { if (len) return len; continue; }
+        if (c == SLIP_ESC) {
+            c = srecv();
+            if (c == SLIP_ESC_END) c = SLIP_END;
+            else if (c == SLIP_ESC_ESC) c = SLIP_ESC;
+        }
+        if (len < max) buf[len++] = (char)c;
+    }
+}
+
+void main(void) {
+    char buf[64];
+    unsigned char len;
+    puts("net: send 'hello from gbos'"); nl();
+    slip_send("hello from gbos", 15);
+    len = slip_recv(buf, 63);
+    buf[len] = 0;
+    puts("net: recv '"); puts(buf); puts("'"); nl();
+}