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, 19 insertions, 0 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index bd58cc2..e3f0b1b 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -70,6 +70,7 @@ SyscallTable:
     dw sys_net                  ; 33 NET
     dw sys_sleep                ; 34 SLEEP
     dw sys_pollcon              ; 35 POLLCON
+    dw sys_uptime               ; 36 UPTIME
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -257,6 +258,24 @@ sys_yield:
     ret
 
 ; -----------------------------------------------------------------------------
+; sys_uptime(buf=DE) - copy the monotonic 64 Hz tick counter (4 bytes, LE)
+; into the caller's buffer. di/ei so the ISR can't split the 32-bit read.
+; -----------------------------------------------------------------------------
+sys_uptime:
+    di
+    ld hl, wTicks
+REPT 3
+    ld a, [hl+]
+    ld [de], a
+    inc de
+ENDR
+    ld a, [hl]
+    ld [de], a
+    ei
+    xor a
+    ret
+
+; -----------------------------------------------------------------------------
 ; sys_sleep(B = 1/64-second units) - cooperative delay driven by the DIV timer.
 ; DIV (FF04) free-runs at 16384 Hz, so 256 ticks = 1/64 s. We accumulate DIV
 ; deltas across SchedYields (other procs run) until enough units have elapsed.