uvm_do中不同的第一个参数时的用法

当uvm_do的第一个参数是某个transaction的指针时，uvm_do主要包括实例化+随机化和约束+发送；
当uvm_do的第一个参数是某个sequence的指针时，uvm_do主要包括实例化+随机化和约束+启动；

// for a sequence item, the following are called, in order
//| `uvm_create(item)//
//| sequence.wait_for_grant(prior) (task)//
//| this.pre_do(1)                 (task)
//| item.randomize()//
//| this.mid_do(item)              (func) 
//| sequencer.send_request(item)   (func)//
//| sequencer.wait_for_item_done   (taks)//
//| this.post_do(item)

// for a sequence, the following are called in order
//| `uvm_create(sub_seq)//
//| sub_seq.randomize()//
//| sub_seq.pre_start()            (task)//
//| this.pre_do(0)                 (task)
//| this.mid_do(sub_seq)           (func)
//| sub_seq.body()                 (task)
//| this.post_do(sub_seq)          (func)
//| sub_seq.post_start()           (task)//