usr: ntpdate + date - the Game Boy knows what day it isHEADmaster

An MBC5 cart has no RTC, so SNTP is the only route to a wall clock. ntpdate
does one 48-byte UDP exchange on port 123 and hands the stamp to the kernel;
SYS_SETTIME keeps it with the tick it arrived at, and SYS_GETTIME adds the
elapsed ticks back, so date(1) is just formatting.

    /# ntpdate
    ntpdate: clock set from pool.ntp.org
    /# date
    Sep 24 21:01:23 UTC 2026        (host said 21:01:26)

The clock does not survive a power cycle - which is exactly what /rc is for:
`ntpdate` there and the machine boots knowing the time.

All of it is bounded subtraction, no long division or shifts: SDCC pulls those
from sm83.lib, whose modules link into the $A000 RAM window. The month tables
are string literals for the same family of reason - an initialized array goes
to _INITIALIZED at $A000 and the image overflows its bank ("buffer too small"
out of makebin), which is how this first failed to link.

1 files changed, 81 insertions, 0 deletions

diff --git a/usr/ntpdate.c b/usr/ntpdate.c
new file mode 100644
index 0000000..bbeeb92
--- /dev/null
+++ b/usr/ntpdate.c
@@ -0,0 +1,81 @@
+#include "resolve.h"
+/* ntpdate [SERVER]: set the clock from SNTP (default pool.ntp.org).
+ *
+ * An MBC5 cart has no RTC, so this is the only way gbos can know the real
+ * time. One 48-byte UDP exchange with port 123: the reply's transmit
+ * timestamp is seconds since 1900, and unix time is that minus 2208988800.
+ * The kernel keeps the stamp plus the tick it arrived at (SYS_SETTIME), so
+ * date(1) can add the elapsed ticks back.
+ *
+ * The clock does not survive a power cycle - put ntpdate in /rc. */
+
+#define NTP_PORT      123
+#define UNIX_EPOCH_HI 0x8383 /* 2208988800 = 0x83AA7E80 */
+
+static unsigned char pkt[48];
+static unsigned char namebuf[64];
+static unsigned char srv[4];
+static unsigned char now[4];
+
+void main(void) {
+    char args[40];
+    unsigned char s, n, i;
+
+    {
+        char *a = getargs();
+        for (i = 0; a[i] && i < 39; i++) args[i] = a[i];
+        args[i] = 0;
+    }
+    if (!args[0]) {
+        const char *d = "pool.ntp.org";
+        for (i = 0; d[i]; i++) args[i] = d[i];
+        args[i] = 0;
+    }
+    for (i = 0; args[i] && i < 63; i++) namebuf[i] = (unsigned char)args[i];
+    namebuf[i] = 0;
+
+    if (!resolve((char *)namebuf, srv)) {
+        puts("ntpdate: cannot resolve ");
+        puts((char *)namebuf);
+        nl();
+        sexit(1);
+    }
+    for (i = 0; i < 48; i++) pkt[i] = 0;
+    pkt[0] = 0x1B; /* LI=0, VN=3, mode=3 (client) */
+
+    s = net_socket(SOCK_UDP);
+    if (s == 0xFF) {
+        puts("ntpdate: no socket");
+        nl();
+        sexit(1);
+    }
+    net_bind(s, 40123);
+    net_connect(s, srv, NTP_PORT);
+    net_send(s, pkt, 48);
+    n = net_recv(s, pkt, 48);
+    net_close(s);
+    if (n == 0xFF || n < 48) {
+        puts("ntpdate: no reply");
+        nl();
+        sexit(1);
+    }
+
+    /* transmit timestamp: seconds since 1900 at bytes 40..43, big-endian.
+       unix = ntp - 2208988800 ($83AA7E80), done bytewise - no long math. */
+    {
+        unsigned char b0 = pkt[43], b1 = pkt[42], b2 = pkt[41], b3 = pkt[40];
+        unsigned int t;
+        t = (unsigned int)b0 - 0x80;
+        now[0] = (unsigned char)t;
+        t = (unsigned int)b1 - 0x7E - ((t >> 8) & 1);
+        now[1] = (unsigned char)t;
+        t = (unsigned int)b2 - 0xAA - ((t >> 8) & 1);
+        now[2] = (unsigned char)t;
+        t = (unsigned int)b3 - 0x83 - ((t >> 8) & 1);
+        now[3] = (unsigned char)t;
+    }
+    settime(now);
+    puts("ntpdate: clock set from ");
+    puts((char *)namebuf);
+    nl();
+}