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, 22 insertions, 3 deletions

diff --git a/src/sched.asm b/src/sched.asm
index a540c49..0369ff4 100644
--- a/src/sched.asm
+++ b/src/sched.asm
@@ -67,9 +67,28 @@ FindNextReady::
     ret
 
 ; -----------------------------------------------------------------------------
-; TimerISR - preemption hook (TODO). A real preemptive switch must save the
-; interrupted context and call hSwitchTo from here. For now the demo is
-; cooperative (tasks call SYS_YIELD), so we just acknowledge and return.
+; TimerISR - the system tick. TAC runs the timer at 16384 Hz with TMA=0, so
+; TIMA overflows (and this fires) at 64 Hz: wTicks is a monotonic 32-bit
+; 1/64-second counter (SYS_UPTIME reads it; wraps after ~2.1 years).
+; Scheduling stays cooperative - this ISR is deliberately transparent (all
+; registers preserved, fixed-WRAM only), so it can interrupt anything,
+; including kernel code mid-syscall. Preemption would hook in here later.
 ; -----------------------------------------------------------------------------
 TimerISR::
+    push af
+    push hl
+    ld hl, wTicks
+    inc [hl]
+    jr nz, .done                ; no carry out of byte 0
+    inc hl
+    inc [hl]
+    jr nz, .done
+    inc hl
+    inc [hl]
+    jr nz, .done
+    inc hl
+    inc [hl]
+.done
+    pop hl
+    pop af
     reti