jobs: background execution (&), non-blocking reap, spin demo

- sys_reap: nohang reap of a finished zombie child (-> pid or 0).
- shell: 'cmd &' launches in the background (prints [pid], no wait); reaps
  finished jobs at the prompt ([pid done]). Foreground now waits for its own
  child specifically (loops wait(), reporting bg completions meanwhile) so kill
  reports the right pid.
- spin: a process that yields forever - a target for ps/kill.
- verified: spin &; ps shows it; kill <pid> removes just that one; immortal
  init; self-finishing bg worker reaped with [pid done].

1 files changed, 58 insertions, 0 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index 0b47c05..2fc66b6 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -56,6 +56,7 @@ SyscallTable:
     dw sys_lookup               ; 19 LOOKUP
     dw sys_ps                   ; 20 PS
     dw sys_progname             ; 21 PROGNAME
+    dw sys_reap                 ; 22 REAP
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -554,3 +555,60 @@ sys_progname::
     xor a
     ld [de], a
     ret
+
+; =============================================================================
+; sys_reap() -> A = pid of a reaped zombie child, or 0 (never blocks).
+; Used by the shell to clean up finished background jobs.
+; =============================================================================
+sys_reap::
+    ld a, [wCurProc]
+    add PROC_PID
+    ld l, a
+    ld a, [wCurProc+1]
+    adc 0
+    ld h, a
+    ld a, [hl]
+    ld [wWaitMyPid], a
+    ld c, 0
+.scan
+    ld a, c
+    call PcbPtr                 ; DE = &PCB[c]
+    ld a, [de]
+    cp PS_ZOMBIE
+    jr nz, .next
+    ld h, d
+    ld l, e
+    ld a, l
+    add PROC_PARENT
+    ld l, a
+    ld a, h
+    adc 0
+    ld h, a
+    ld a, [wWaitMyPid]
+    cp [hl]
+    jr nz, .next
+    ; reap: pid, free bank, free slot
+    ld h, d
+    ld l, e
+    inc hl                      ; PROC_PID
+    ld a, [hl]
+    ld [wWaitRetPid], a
+    ld a, e
+    add PROC_RAMB
+    ld l, a
+    ld a, d
+    adc 0
+    ld h, a
+    ld a, [hl]
+    call FreeRamBank
+    ld a, PS_FREE
+    ld [de], a
+    ld a, [wWaitRetPid]
+    ret
+.next
+    inc c
+    ld a, c
+    cp MAX_PROCS
+    jr c, .scan
+    xor a
+    ret