Suggest a free RTOS for 8051 based application

; EXEC is the kernel entry for tasks.  The kernel assumes exclusive use
; of registers R6 and R7 in each task register bank.  Up to 4 tasks are
; supported by the kernel, as the 8051 has 4 register banks.
;       Entry:	R6 contains task blocking-time request.
exec:
        mov	r7,sp                   ;save task stack state
        mov	a,psw                   ;fetch current tsk reg bank ptr
bump_bank:
        add	a,#8                    ;  & bump it to next task's bank
        anl	a,#18h
        mov	psw,a
        jnz	bump_task_timer         ;branch fwd if not last tsk in list
        jnb	new_tick,$              ;wait for next realtime tick
        clr	new_tick                ;  then reset realtime tick flag
bump_task_timer:
        djnz	r6,bump_bank
        mov	sp,r7                   ;restore task stack state
        ret                             ;  & exit to task