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, 11 insertions, 3 deletions

diff --git a/src/boot.asm b/src/boot.asm
index c2d461a..4049f9d 100644
--- a/src/boot.asm
+++ b/src/boot.asm
@@ -81,11 +81,19 @@ KernelInit:
 
     call SchedInit              ; pick first task, set wCurProc
 
-    ; interrupts: enable timer for preemption
+    ; system tick: hardware timer at 16384 Hz input, TMA=0 -> TIMA overflow
+    ; IRQ at 64 Hz. TimerISR just counts (wTicks); scheduling stays
+    ; cooperative. Only the timer bit is enabled in IE.
+    xor a
+    ld [rTIMA], a
+    ld [rTMA], a
+    ld a, %00000111             ; enable | 16384 Hz
+    ld [rTAC], a
     xor a
     ld [rIF], a
-    ; ld a, IEF_TIMER  ; (timer bit) - leave masked for cooperative demo
-    ; ld [rIE], a
+    ld a, IEF_TIMER
+    ld [rIE], a
+    ei
 
     ; hand control to the first task via a context switch-in
     jp SchedRunFirst