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.

1 files changed, 7 insertions, 1 deletions

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