kernel: complete the process lifecycle (exit/wait/reap + bank free list)

- exit(): zombie + status, reparent orphans to init, wake blocked parent,
  schedule away forever (resources reclaimed by the reaper)
- wait(): scan for a zombie child; reap (free cart-RAM bank + PCB slot) and
  return pid/code; block PS_BLOCKED + yield if children still live; $FF if none
- cart-RAM banks are now a free list (wBankUsed bitmap): AllocRamBank/FreeRamBank
- scheduler: FindNextReady returns carry when nothing runnable; yield idles
  instead of blindly picking slot 0
- helpers: FindPcbByPid, ReparentToInit
- demo: init forks 3 workers -> exec -> exit(pid) -> wait/reap => wwwR2R3R4!
  verified bank recycling with 6 workers over 3 banks (w2w3w4w5w6w7)
- README: document the lifecycle + syscall status

1 files changed, 100 insertions, 10 deletions

diff --git a/src/proc.asm b/src/proc.asm
index 114cc9c..a2b1368 100644
--- a/src/proc.asm
+++ b/src/proc.asm
@@ -11,26 +11,116 @@ SECTION "proc", ROM0
 ProcInit::
     ld a, 1
     ld [wNextPid], a
-    ld a, 1                 ; bank 0 is the init task's; allocate from 1 up
-    ld [wNextRamBank], a
+    ; bank 0 belongs to the init task; the rest start free
+    ld a, 1
+    ld [wBankUsed + 0], a
+    ld hl, wBankUsed + 1
+    ld b, NUM_RAM_BANKS - 1
+    xor a
+.bclr
+    ld [hl+], a
+    dec b
+    jr nz, .bclr
     ret
 
 ; -----------------------------------------------------------------------------
-; AllocRamBank - bump-allocate a cart-RAM bank.  out: A=bank, CF set if none.
+; AllocRamBank - claim a free cart-RAM bank.  out: A=bank, CF set if none.
 ; -----------------------------------------------------------------------------
 AllocRamBank::
-    ld a, [wNextRamBank]
+    ld hl, wBankUsed
+    ld c, 0
+.scan
+    ld a, [hl]
+    or a
+    jr z, .free
+    inc hl
+    inc c
+    ld a, c
     cp NUM_RAM_BANKS
-    jr nc, .none
-    ld b, a
-    inc a
-    ld [wNextRamBank], a
-    ld a, b
+    jr c, .scan
+    scf
+    ret
+.free
+    ld a, 1
+    ld [hl], a              ; mark used
+    ld a, c
     and a                   ; clear carry
     ret
-.none
+
+; -----------------------------------------------------------------------------
+; FreeRamBank - release a cart-RAM bank.  in: A=bank.
+; -----------------------------------------------------------------------------
+FreeRamBank::
+    ld hl, wBankUsed
+    add l
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    xor a
+    ld [hl], a
+    ret
+
+; -----------------------------------------------------------------------------
+; FindPcbByPid - locate a live PCB by pid.
+;   in: A=pid  out: DE=&PCB, CF set if not found.
+; -----------------------------------------------------------------------------
+FindPcbByPid::
+    ld b, a                 ; target pid
+    ld c, 0
+.l
+    ld a, c
+    call PcbPtr             ; DE=&PCB[c] (preserves BC)
+    ld a, [de]
+    cp PS_FREE
+    jr z, .n
+    ld h, d
+    ld l, e
+    inc hl                  ; ->PROC_PID
+    ld a, [hl]
+    cp b
+    jr z, .found
+.n
+    inc c
+    ld a, c
+    cp MAX_PROCS
+    jr c, .l
     scf
     ret
+.found
+    and a                   ; clear carry (pids are >= 1)
+    ret
+
+; -----------------------------------------------------------------------------
+; ReparentToInit - give every live child of wExitMyPid to pid INIT_PID.
+; -----------------------------------------------------------------------------
+ReparentToInit::
+    ld c, 0
+.l
+    ld a, c
+    call PcbPtr             ; DE=&PCB (preserves BC)
+    ld a, [de]
+    cp PS_FREE
+    jr z, .n
+    ld h, d
+    ld l, e
+    ld a, l
+    add PROC_PARENT
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a                 ; HL=&PROC_PARENT
+    ld a, [wExitMyPid]
+    cp [hl]
+    jr nz, .n
+    ld a, INIT_PID
+    ld [hl], a
+.n
+    inc c
+    ld a, c
+    cp MAX_PROCS
+    jr c, .l
+    ret
 
 ; -----------------------------------------------------------------------------
 ; FindFreeSlot - scan the proc table for a PS_FREE slot.