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, 1 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 9a7875b..fbdf10a 100644
--- a/Makefile
+++ b/Makefile
@@ -21,7 +21,7 @@ $(BUILD)/%.o: src/%.asm | $(BUILD)
 # $(BUILD)/usr and INCBIN'd by programs.asm
 PROGS  := chello echo true false uname pid cat wc head args \
           ls save rm sh ps kill spin blk mkdir count ptest necho \
-          wget chat netd ping nslookup dhcp irc
+          wget chat netd ping nslookup dhcp irc uptime
 CBLOBS := $(patsubst %,$(BUILD)/usr/%.bin,$(PROGS))
 $(BUILD)/programs.o: $(CBLOBS)