; =============================================================================
; sched.asm - round-robin scheduler + yield + timer stub
; =============================================================================
INCLUDE "include/gbos.inc"

SECTION "sched", ROM0

; -----------------------------------------------------------------------------
; SchedInit - prime the round-robin cursor so the first pick lands on slot 0.
; -----------------------------------------------------------------------------
SchedInit::
    ld a, MAX_PROCS - 1
    ld [wSchedNext], a
    ret

; -----------------------------------------------------------------------------
; SchedRunFirst - hand control to the first runnable task. Never returns; the
; kernel-init context is saved into wKernelPCB and simply abandoned.
; -----------------------------------------------------------------------------
SchedRunFirst::
    ld hl, wKernelPCB
    ld a, l
    ld [wCurProc], a
    ld a, h
    ld [wCurProc+1], a
    call FindNextReady          ; DE = &first ready PCB
    jp hSwitchTo

; -----------------------------------------------------------------------------
; SchedYield - cooperative switch to the next runnable task.  Returns to the
; caller when this task is eventually rescheduled.
;
; PRESERVES ALL REGISTERS, on both paths. The switch path always did, because
; hSwitchTo saves and restores the full context - but the "nobody else is
; runnable" path used to `ret c` straight out of FindNextReady, which leaves
; HL pointing into wProcTable (via PcbPtr). A caller holding a pointer in HL
; across the call then walked the process table instead of its own variable.
;
; That is what warm-rebooted a link-less gbos every ~11s: with no DHCP server,
; the boot-time `dhcp` spun in net_op_recv's timeout loop and, with the shell
; blocked on tty input, every yield took the no-switch path - so `inc hl /
; inc [hl]` incremented PCB bytes instead of wNetTO until a saved return
; address rolled to $0100 and the cart restarted. It only ever bit with the
; network absent, which is why it hid for so long.
; -----------------------------------------------------------------------------
SchedYield::
    push af
    push bc
    push de
    push hl
    call FindNextReady          ; DE = &next PCB, CF if none runnable
    jr c, .none                 ; nobody else ready: keep running current
    call hSwitchTo              ; (returns here when we are rescheduled; the
                                ;  pushes above sit on our own stack)
.none
    pop hl
    pop de
    pop bc
    pop af
    ret

; -----------------------------------------------------------------------------
; FindNextReady - round-robin from wSchedNext.
;   out: DE = &PCB and CF clear on success; CF set if nothing is PS_READY.
; -----------------------------------------------------------------------------
FindNextReady::
    ld a, [wSchedNext]
    ld b, MAX_PROCS
.scan
    inc a
    cp MAX_PROCS
    jr c, .nowrap
    xor a
.nowrap
    push af                     ; save candidate slot
    call PcbPtr                 ; DE = &PCB[slot]  (preserves BC)
    ld a, [de]                  ; state
    cp PS_READY
    jr z, .found
    pop af                      ; restore candidate slot
    dec b
    jr nz, .scan
    scf                         ; nothing runnable
    ret
.found
    pop af                      ; A = chosen slot
    ld [wSchedNext], a
    call PcbPtr                 ; DE = &PCB
    and a                       ; clear carry
    ret

; -----------------------------------------------------------------------------
; 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.
; -----------------------------------------------------------------------------
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
    inc hl
    inc [hl]
    jr nz, .done
    inc hl
    inc [hl]
    jr nz, .done
    inc hl
    inc [hl]
.done
    pop hl
    pop af
    reti