diff options
Diffstat (limited to '')
| -rw-r--r-- | src/sched.asm | 12 |
1 files changed, 9 insertions, 3 deletions
diff --git a/src/sched.asm b/src/sched.asm index 0369ff4..b4b5a30 100644 --- a/src/sched.asm +++ b/src/sched.asm @@ -67,9 +67,10 @@ FindNextReady:: ret ; ----------------------------------------------------------------------------- -; 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). +; TimerISR - the system tick. TAC runs the timer at 16384 Hz with TMA=0 - +; but the CPU is in DOUBLE-SPEED mode, so the input clock is really 32768 Hz +; and this fires at 128 Hz. A toggle halves that back down: wTicks stays a +; monotonic 32-bit 1/64-second counter (SYS_UPTIME reads it; wraps ~2.1 yr). ; 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. @@ -77,6 +78,11 @@ FindNextReady:: TimerISR:: push af push hl + ld hl, wTickTog + ld a, [hl] + xor 1 + ld [hl], a + jr nz, .done ; first IRQ of the 128 Hz pair: count nothing ld hl, wTicks inc [hl] jr nz, .done ; no carry out of byte 0 |