From 0cc9b0a8bdf774ee376937d5a6767ccdced51245 Mon Sep 17 00:00:00 2001
From: user <user@clank>
Date: Sat, 18 Jul 2026 10:10:04 +0200
Subject: 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.
---
 usr/build.sh | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

(limited to 'usr/build.sh')

diff --git a/usr/build.sh b/usr/build.sh
index 93815d2..5d494cf 100755
--- a/usr/build.sh
+++ b/usr/build.sh
@@ -10,7 +10,12 @@ sdasgb -o "$B/crt0.rel" usr/crt0.s
 sdasgb -o "$B/libc.rel" usr/libc.s
 sdcc -msm83 -c -Iusr usr/libc.c -o "$B/libc_c.rel"
 sdcc -msm83 -c -Iusr "$SRC" -o "$B/prog.rel"
-# link: crt0 first (=> _start at 0x4000), then libc, program, and sm83 lib
+# link: crt0 first (=> _start at 0x4000), then libc, program, and sm83 lib.
+# TRAP: only _CODE/_DATA have pinned bases. sm83.lib modules (long div/mod/
+# shift helpers) link into their own areas, which sdldgb places after _DATA -
+# i.e. code in the $A000 RAM window ("buffer too small" from makebin, or
+# garbage at runtime). Don't use long division/shifts in programs; see
+# usr/uptime.c for the inlined-arithmetic workaround.
 sdldgb -n -m -w -j -i "$B/prog.ihx" \
        -b _CODE=0x4000 -b _DATA=0xa000 \
        -k "$LIBDIR" -l sm83 \
-- 
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