kernel: a real timer source (64 Hz tick IRQ) + uptime(1)

The kernel had no clock: TimerISR was a reti stub, IEF_TIMER masked, and
IME was never enabled - the vectors were decorative. sys_sleep just polls
DIV deltas per-process; nothing counted globally.

Now: TAC runs the hardware timer at 16384 Hz with TMA=0, so TIMA overflows
at exactly 64 Hz; TimerISR increments a monotonic 32-bit wTicks (wraps
after ~2.1 years). Scheduling stays cooperative - the ISR is transparent.

Enabling IME in a kernel written for zero interrupts needs care wherever
SP points into memory whose bank is being switched (an IRQ pushes onto SP):
  - read_block/write_block map the disk bank over the $A000 window that
    holds the caller's stack -> di/ei around the transfer (~2ms, well under
    the 15.6ms tick period, so no tick is ever lost)
  - hSwitchTo switches SVBK + cart-RAM banks under the outgoing stack ->
    di on entry, ei once the incoming stack is mapped
  - fork already runs on KSTACK_TOP2 (fixed WRAM) - safe as-is
  - term_putc's SVBK switch only remaps $Dxxx, stacks live in $Axxx/$Cxxx

SYS_UPTIME (36) copies the counter (4B LE, di/ei so the read can't tear)
to a user buffer; libc gticks(); usr/uptime.c formats 'up [Nd] H:MM:SS'.
uptime avoids SDCC long div/shift entirely: sm83.lib modules link into
their own areas that land in the $A000 RAM window (latent build.sh trap,
documented there) - bytewise >>6 plus bounded subtraction loops instead.

4 files changed, 54 insertions, 2 deletions

diff --git a/usr/build.sh b/usr/build.sh
index 93815d2..5d494cf 100755
--- a/usr/build.sh
+++ b/usr/build.sh
@@ -10,7 +10,12 @@ sdasgb -o "$B/crt0.rel" usr/crt0.s
 sdasgb -o "$B/libc.rel" usr/libc.s
 sdcc -msm83 -c -Iusr usr/libc.c -o "$B/libc_c.rel"
 sdcc -msm83 -c -Iusr "$SRC" -o "$B/prog.rel"
-# link: crt0 first (=> _start at 0x4000), then libc, program, and sm83 lib
+# link: crt0 first (=> _start at 0x4000), then libc, program, and sm83 lib.
+# TRAP: only _CODE/_DATA have pinned bases. sm83.lib modules (long div/mod/
+# shift helpers) link into their own areas, which sdldgb places after _DATA -
+# i.e. code in the $A000 RAM window ("buffer too small" from makebin, or
+# garbage at runtime). Don't use long division/shifts in programs; see
+# usr/uptime.c for the inlined-arithmetic workaround.
 sdldgb -n -m -w -j -i "$B/prog.ihx" \
        -b _CODE=0x4000 -b _DATA=0xa000 \
        -k "$LIBDIR" -l sm83 \
diff --git a/usr/gbos.h b/usr/gbos.h
index 2350665..5fd897a 100644
--- a/usr/gbos.h
+++ b/usr/gbos.h
@@ -21,6 +21,7 @@ unsigned char pollcon(void);            /* poll console ring (link-injected
                                            input, filled by net pumps), or 0 */
 unsigned char sys_net(void *req);       /* socket layer trap (see sock.h)  */
 void          gsleep(unsigned char units); /* delay units*(1/64)s (raw trap)  */
+void          gticks(unsigned char buf[4]); /* 64 Hz uptime ticks, 32-bit LE  */
 void          msleep(unsigned int ms);     /* delay ~ms milliseconds          */
 
 /* helpers (c/libc.c) */
diff --git a/usr/libc.s b/usr/libc.s
index 5b176d9..2366cd8 100644
--- a/usr/libc.s
+++ b/usr/libc.s
@@ -1,7 +1,7 @@
 	.module libc
 	.globl _writes, _putc, _puts, _nl, _strlen, _readc, _sexit, _getpid, _getargs
 	.globl _open, _close, _fgetc, _fputc, _flist, _fremove, _pipe, _ssend, _srecv, _srecv_nb, _pollin, _pollcon
-	.globl _sys_net, _gsleep
+	.globl _sys_net, _gsleep, _gticks
 	;; SDCC sm83 sdcccall(1): 1st arg -> A (byte) or DE (pointer); ret A / BC.
 	;; rst $30 (the trap) clobbers BC/DE/HL; SDCC treats them caller-saved, so
 	;; wrappers need not preserve them - but we compute cleanly regardless.
@@ -130,6 +130,12 @@ _gsleep:
 	rst	#0x30
 	ret
 
+	;; void gticks(unsigned char buf[4] /*DE*/) - 64 Hz uptime ticks, LE
+_gticks:
+	ld	c, #36			; SYS_UPTIME
+	rst	#0x30
+	ret
+
 	;; unsigned char pollin(void) -> A  (typed OSK char, or 0)
 _pollin:
 	ld	c, #32			; SYS_POLLIN
diff --git a/usr/uptime.c b/usr/uptime.c
new file mode 100644
index 0000000..1062217
--- /dev/null
+++ b/usr/uptime.c
@@ -0,0 +1,40 @@
+#include "gbos.h"
+/* uptime: time since boot, from the kernel's 64 Hz tick (SYS_UPTIME).
+
+   No long division/shifts: SDCC pulls those from sm83.lib, whose modules
+   link into their own areas that land in the $A000 RAM window (see
+   usr/build.sh). Everything here is inlined 8/32-bit add/sub/compare:
+   ticks>>6 done bytewise, then day/hour/minute by bounded subtraction. */
+
+static void two(unsigned int v) {       /* zero-padded 2-digit field */
+    if (v < 10) putc('0');
+    putu(v);
+}
+
+void main(void) {
+    unsigned char t[4];
+    union { unsigned long l; unsigned char b[4]; } u;  /* sm83 is LE */
+    unsigned long sec;
+    unsigned int d, h, m;
+
+    gticks(t);                          /* 32-bit LE tick count, 64 Hz */
+
+    /* seconds = ticks >> 6, composed a byte at a time (24 bits is 194 days) */
+    u.b[0] = (unsigned char)((t[0] >> 6) | (t[1] << 2));
+    u.b[1] = (unsigned char)((t[1] >> 6) | (t[2] << 2));
+    u.b[2] = (unsigned char)((t[2] >> 6) | (t[3] << 2));
+    u.b[3] = 0;
+    sec = u.l;
+
+    d = 0;
+    while (sec >= 86400UL) { sec -= 86400UL; d++; }   /* <= 194 rounds */
+    h = 0;
+    while (sec >= 3600UL)  { sec -= 3600UL;  h++; }   /* <= 23 rounds  */
+    m = 0;
+    while (sec >= 60UL)    { sec -= 60UL;    m++; }   /* <= 59 rounds  */
+
+    puts("up ");
+    if (d) { putu(d); puts("d "); }
+    putu(h); putc(':'); two(m); putc(':'); two((unsigned int)sec);
+    nl();
+}