; =============================================================================
; 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.
; -----------------------------------------------------------------------------
SchedYield::
    call FindNextReady          ; DE = &next PCB
    call hSwitchTo
    ret

; -----------------------------------------------------------------------------
; FindNextReady - round-robin from wSchedNext.  out: DE = &PCB (updates cursor)
; Scaffold: assumes at least one PS_READY task exists.
; -----------------------------------------------------------------------------
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
    ; nothing ready: fall back to slot 0 (idle behavior)
    xor a
    call PcbPtr
    ret
.found
    pop af                      ; A = chosen slot
    ld [wSchedNext], a
    call PcbPtr                 ; DE = &PCB
    ret

; -----------------------------------------------------------------------------
; TimerISR - preemption hook (TODO). A real preemptive switch must save the
; interrupted context and call hSwitchTo from here. For now the demo is
; cooperative (tasks call SYS_YIELD), so we just acknowledge and return.
; -----------------------------------------------------------------------------
TimerISR::
    reti