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.
---
 src/blk.asm | 5 +++++
 1 file changed, 5 insertions(+)

(limited to 'src/blk.asm')

diff --git a/src/blk.asm b/src/blk.asm
index 2be961e..76561da 100644
--- a/src/blk.asm
+++ b/src/blk.asm
@@ -34,6 +34,8 @@ ENDM
 ; -----------------------------------------------------------------------------
 read_block::
     ld c, a
+    di                          ; the tick ISR pushes on SP, which points into
+                                ; the $A000 window we are about to remap
     srl a
     srl a
     srl a
@@ -54,6 +56,7 @@ read_block::
     dec b
     jr nz, .cp
     restore_bank                ; inline - no stack while the disk bank is mapped
+    ei
     ret
 
 ; -----------------------------------------------------------------------------
@@ -61,6 +64,7 @@ read_block::
 ; -----------------------------------------------------------------------------
 write_block::
     ld c, a
+    di                          ; see read_block
     srl a
     srl a
     srl a
@@ -81,6 +85,7 @@ write_block::
     dec b
     jr nz, .cp
     restore_bank
+    ei
     ret
 
 ; =============================================================================
-- 
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