Add is_task_id_used

fox32os.def

@@ -12,6 +12,7 @@ yield_task:          jmp [0x00000A14]
 end_current_task:    jmp [0x00000A18]
 get_current_task_id: jmp [0x00000A1C]
 get_unused_task_id:  jmp [0x00000A20]
+is_task_id_used:     jmp [0x00000A24]
 
 ; memory jump table
 allocate_memory:     jmp [0x00000B10]

kernel/main.asm

@@ -28,6 +28,7 @@ jump_table:
     data.32 end_current_task
     data.32 get_current_task_id
     data.32 get_unused_task_id
+    data.32 is_task_id_used
 
     ; memory jump table
     org.pad 0x00000B10

kernel/task.asm

@@ -133,6 +133,15 @@ get_current_task_id:
     pop r2
     ret
 
+; check if a task ID is used
+; inputs:
+; r0: task ID
+; outputs:
+; Z flag: set if unused, reset if used
+is_task_id_used:
+    bts [task_id_bitmap], r0
+    ret
+
 task_load:
     mov r2, [r0] ; task ID
     add r0, 4