1.start item
1.start_item();sequencer.wait_for_grant(prior);this.pre_do(1);
需要指出，这里明确说明了wait_for_grant和send_request之间不能有任何延迟，所以在mid_do这个任务里千万不能有任何延迟。
task uvm_sequencer_base::wait_for_grant(uvm_sequence_base sequence_ptr,int item_priority = -1,bit lock_request = 0);uvm_sequence_request req_s;int my_seq_id;if (sequence_ptr == null)uvm_report_fatal("uvm_sequencer","wait_for_grant passed null sequence_ptr", UVM_NONE);my_seq_id = m_register_sequence(sequence_ptr);// If lock_request is asserted, then issue a lock. Don't wait for the response, since// there is a request immediately following the lock requestif (lock_request == 1) beginreq_s = new();req_s.grant = 0;req_s.sequence_id = my_seq_id;req_s.request = SEQ_TYPE_LOCK;req_s.sequence_ptr = sequence_ptr;req_s.request_id = g_request_id++;req_s.process_id = process::self();arb_sequence_q.push_back(req_s);end// Push the request onto the queuereq_s = new();req_s.grant = 0;req_s.request = SEQ_TYPE_REQ;req_s.sequence_id = my_seq_id;req_s.item_priority = item_priority;req_s.sequence_ptr = sequence_ptr;req_s.request_id = g_request_id++;req_s.process_id = process::self();arb_sequence_q.push_back(req_s);m_update_lists();// Wait until this entry is granted// Continue to point to the element, since location in queue will changem_wait_for_arbitration_completed(req_s.request_id);// The wait_for_grant_semaphore is used only to check that send_request// is only called after wait_for_grant. This is not a complete check, since// requests might be done in parallel, but it will catch basic errorsreq_s.sequence_ptr.m_wait_for_grant_semaphore++;//这个变量会用在send_request那里，发了一个减少一个，这里得到一个grant加一个endtask
需要注意几点：
（1）uvm_sequence_base,相当于一个seq或者item的合集
（2） uvm_sequence_request
class uvm_sequence_request;bit grant;int sequence_id;int request_id;int item_priority;process process_id;uvm_sequencer_base::seq_req_t request;uvm_sequence_base sequence_ptr;
endclass
(3)m_register_sequence,这个任务核心就是输出sequence_ptr的id，同时将sequence_ptr记录到自己定义的动态数组reg_sequence中
function int uvm_sequencer_base::m_register_sequence(uvm_sequence_base sequence_ptr);if (sequence_ptr.m_get_sqr_sequence_id(m_sequencer_id, 1) > 0)return sequence_ptr.get_sequence_id();sequence_ptr.m_set_sqr_sequence_id(m_sequencer_id, g_sequence_id++);reg_sequences[sequence_ptr.get_sequence_id()] = sequence_ptr;return sequence_ptr.get_sequence_id();
endfunction-------------------------------------------------------------------------------
m_get_sqr_sequence_id主要做的内容就是：定义了一个存放id的动态数组m_sqr_seq_ids，如果这个动态数组中存在输入的id，同时需要更新这个id，那就将这个id更新到对应item中function int m_get_sqr_sequence_id(int sequencer_id, bit update_sequence_id);if (m_sqr_seq_ids.exists(sequencer_id)) beginif (update_sequence_id == 1) beginset_sequence_id(m_sqr_seq_ids[sequencer_id]);endreturn m_sqr_seq_ids[sequencer_id];endif (update_sequence_id == 1)set_sequence_id(-1);return -1;endfunction
------------------------------------------------------------------------------------
function void set_sequence_id(int id);//uvm_sequence_item中的任务m_sequence_id = id;//这个m_sequence_id是item的idendfunction
------------------------------------------------------------------------------------function int get_sequence_id();//get_sequence_id是uvm_sequence_item中的任务return (m_sequence_id);//就是返回当前item的idendfunction
------------------------------------------------------------------------------------reg_sequences是定义的uvm_sequence_base的动态数组-> protected uvm_sequence_base reg_sequences[int];
------------------------------------------------------------------------------------（4）
protected uvm_sequence_request arb_sequence_q[$];（5）
function void uvm_sequencer_base::m_update_lists();m_lock_arb_size++;
endfunction（6）注意这里有几个变量，第一个变量是lock_arb_size，第二个变量是m_lock_arb_size变量，m_lock_arb_size这个变量在每次进行m_update_lists()任务的时候会加一次，在一开始的时候会将这两个值相当，随后去等执行m_update_lists()任务。注意这里的arb_completed,原型是 protected bit arb_completed[int]; 删除这个index相当于从仲裁的q（arb_completed）将仲裁完毕的id删除。
task uvm_sequencer_base::m_wait_for_arbitration_completed(int request_id);int lock_arb_size;// Search the list of arb_wait_q, see if this item is doneforever beginlock_arb_size = m_lock_arb_size;if (arb_completed.exists(request_id)) beginarb_completed.delete(request_id);return;endwait (lock_arb_size != m_lock_arb_size);end
endtask2.finish_item
finish_item();
(1)this.mid_do(item);
(2)sequencer.send_request(item);
(3)sequencer.wait_for_item_done();
(4)this.post_do(item);
注意，实际上会存在三个fifo，一个fifo是 uvm_tlm_fifo #(REQ) m_req_fifo;另两个是
REQ m_last_req_buffer[$];
RSP m_last_rsp_buffer[$];
uvm_send实际上是不断的往m_req_fifo去放入内容。
function void uvm_sequencer_param_base::send_request(uvm_sequence_base sequence_ptr,uvm_sequence_item t,bit rerandomize = 0);REQ param_t;if (sequence_ptr == null) beginuvm_report_fatal("SNDREQ", "Send request sequence_ptr is null", UVM_NONE);end//没有的化报fatalif (sequence_ptr.m_wait_for_grant_semaphore < 1) beginuvm_report_fatal("SNDREQ", "Send request called without wait_for_grant", UVM_NONE);end//注意，在上面wait_for_grant任务中得到一个grant就会加一个，这里减少sequence_ptr.m_wait_for_grant_semaphore--;if ($cast(param_t, t)) beginif (rerandomize == 1) begin//如果给了随机的选项，需要进行随机if (!param_t.randomize()) beginuvm_report_warning("SQRSNDREQ", "Failed to rerandomize sequence item in send_request");endendif (param_t.get_transaction_id() == -1) beginparam_t.set_transaction_id(sequence_ptr.m_next_transaction_id++);endm_last_req_push_front(param_t);//往别的q里去推end else beginuvm_report_fatal(get_name(),$sformatf("send_request failed to cast sequence item"), UVM_NONE);endparam_t.set_sequence_id(sequence_ptr.m_get_sqr_sequence_id(m_sequencer_id, 1));t.set_sequencer(this);if (m_req_fifo.try_put(param_t) != 1) beginuvm_report_fatal(get_full_name(), "Concurrent calls to get_next_item() not supported. Consider using a semaphore to ensure that concurrent processes take turns in the driver", UVM_NONE);endm_num_reqs_sent++;//记录send了多少内容// Grant any locks as soon as possiblegrant_queued_locks();
endfunctiongrant_queued_locks（）任务有点复杂，没看完，留个坑
注意这个任务 ，前期会一直往m_last_req_buffer里push，注意这里是从前推入。这个q是uvm_sequencer_param_base类中定义的q，他的类型采用了参数化的类REQ，定义方式如下：
REQ m_last_req_buffer[$];
function void uvm_sequencer_param_base::m_last_req_push_front(REQ item);if(!m_num_last_reqs)return;if(m_last_req_buffer.size() == m_num_last_reqs)void'(m_last_req_buffer.pop_back());this.m_last_req_buffer.push_front(item);
endfunction其中m_num_last_reqs这个变量是通过set_num_last_reqs任务实现的，也就是说m_last_req_buffer的buf含量最大是1024个。
function void uvm_sequencer_param_base::set_num_last_reqs(int unsigned max);if(max > 1024) beginuvm_report_warning("HSTOB", $sformatf("Invalid last size; 1024 is the maximum and will be used"));max = 1024;end//shrink the buffer if necessarywhile((m_last_req_buffer.size() != 0) && (m_last_req_buffer.size() > max))void'(m_last_req_buffer.pop_back());m_num_last_reqs = max;num_last_items = max;endfunction这里就有个问题了，可以看到，实际上源码 并没有用m_last_req_buffer，而是用的是m_req_fifo，那这个buffer到底是做什么用的呢？其实查阅uvm手册可以发现，针对m_last_req_buffer的相关任务大多是以API的形式存在，所以个人理解还是封装了一下以便上层可以根据API实现更为负载的sequencer。
下面介绍结构和m_last_req_buffer/m_last_rsp_buffer相关的方法，可以在sequencer中实现。
（1）last_req，返回最后一个req，虽然叫last_req，但是实际可以返回任何顺序的。
function REQ last_req(int unsigned n = 0);if(n > m_num_last_reqs) beginuvm_report_warning("HSTOB",$sformatf("Invalid last access (%0d), the max history is %0d", n,m_num_last_reqs));return null;endif(n == m_last_req_buffer.size())return null;return m_last_req_buffer[n];
endfunctionget_num_last_reqs，相当于返回m_last_req_buffer的size
function int unsigned uvm_sequencer_param_base::get_num_last_reqs();return m_num_last_reqs;
endfunctionset_num_last_reqs,约束m_last_req_buffer的size，多的部分会被pop出去。
function void uvm_sequencer_param_base::set_num_last_reqs(int unsigned max);if(max > 1024) beginuvm_report_warning("HSTOB", $sformatf("Invalid last size; 1024 is the maximum and will be used"));max = 1024;end//shrink the buffer if necessarywhile((m_last_req_buffer.size() != 0) && (m_last_req_buffer.size() > max))void'(m_last_req_buffer.pop_back());m_num_last_reqs = max;num_last_items = max;endfunctionm_last_rsp_buffer的方法和上面类似，但是方法名称有所不同，分别是last_rsp（）；set_num_last_rsps（）；get_num_last_rsps（）；
function int uvm_sequencer_param_base::get_num_rsps_received();return m_num_rsps_received;//相当于m_last_rsp_buffer的size
endfunctionwait_for_item_done针对这个sequence_ptr，如果没有定义了transaction_id，那么仅仅需要等sequence_id，如果定义了的化，还需要多等一个 transaction_id。
task uvm_sequencer_base::wait_for_item_done(uvm_sequence_base sequence_ptr,int transaction_id);int sequence_id;sequence_id = sequence_ptr.m_get_sqr_sequence_id(m_sequencer_id, 1);m_wait_for_item_sequence_id = -1;m_wait_for_item_transaction_id = -1;if (transaction_id == -1)wait (m_wait_for_item_sequence_id == sequence_id);elsewait ((m_wait_for_item_sequence_id == sequence_id &&m_wait_for_item_transaction_id == transaction_id));
endtask3.uvm_do做了上面的两部分：
4.driver，这里介绍和driver相关的两个方法：（1）get_next_item（2）try_next_item
需要注意的是，这两个方法都是是从前面说的m_req_fifo里去拿。
注意这里get_next_item任务，在执行这个任务的时候，get_next_item_called变量置为1，这个变量会在item_done的任务里再置为0，这个变量是为了避免多次get item，但是不执行item_done的情况。
task uvm_sequencer::get_next_item(output REQ t);REQ req_item;// If a sequence_item has already been requested, then get_next_item()// should not be called again until item_done() has been called.if (get_next_item_called == 1)uvm_report_error(get_full_name(),"Get_next_item called twice without item_done or get in between", UVM_NONE);if (!sequence_item_requested)m_select_sequence();// Set flag indicating that the item has been requested to ensure that item_done or get// is called between requestssequence_item_requested = 1;get_next_item_called = 1;m_req_fifo.peek(t);
endtasktry_next_item
task uvm_sequencer::try_next_item(output REQ t);int selected_sequence;time arb_time;uvm_sequence_base seq;if (get_next_item_called == 1) beginuvm_report_error(get_full_name(), "get_next_item/try_next_item called twice without item_done or get in between", UVM_NONE);return;end//前面就是做一些列的判断，防止出现多次get item的情况// allow state from last transaction to settle such that sequences'// relevancy can be determined with up-to-date informationwait_for_sequences();//这个任务实际上就是等一个时间片// choose the sequence based on relevancyselected_sequence = m_choose_next_request();//仲裁出到底使用哪个sequence// return if none availableif (selected_sequence == -1) begint = null;return;end// now, allow chosen sequence to resumem_set_arbitration_completed(arb_sequence_q[selected_sequence].request_id);seq = arb_sequence_q[selected_sequence].sequence_ptr;arb_sequence_q.delete(selected_sequence);m_update_lists();sequence_item_requested = 1;get_next_item_called = 1;// give it one NBA to put a new item in the fifowait_for_sequences();// attempt to get the item; if it fails, produce an error and returnif (!m_req_fifo.try_peek(t))uvm_report_error("TRY_NEXT_BLOCKED", {"try_next_item: the selected sequence '",seq.get_full_name(), "' did not produce an item within an NBA delay. ","Sequences should not consume time between calls to start_item and finish_item. ","Returning null item."}, UVM_NONE);endtask具体看m_choose_next_request()任务
function int uvm_sequencer_base::m_choose_next_request();int i, temp;int avail_sequence_count;int sum_priority_val;integer avail_sequences[$];integer highest_sequences[$];int highest_pri;string s;avail_sequence_count = 0;grant_queued_locks();i = 0;while (i < arb_sequence_q.size()) beginif ((arb_sequence_q[i].process_id.status == process::KILLED) ||(arb_sequence_q[i].process_id.status == process::FINISHED)) begin`uvm_error("SEQREQZMB", $sformatf("The task responsible for requesting a wait_for_grant on sequencer '%s' for sequence '%s' has been killed, to avoid a deadlock the sequence will be removed from the arbitration queues", this.get_full_name(), arb_sequence_q[i].sequence_ptr.get_full_name()))remove_sequence_from_queues(arb_sequence_q[i].sequence_ptr);continue;endif (i < arb_sequence_q.size())if (arb_sequence_q[i].request == SEQ_TYPE_REQ)if (is_blocked(arb_sequence_q[i].sequence_ptr) == 0)if (arb_sequence_q[i].sequence_ptr.is_relevant() == 1) beginif (m_arbitration == UVM_SEQ_ARB_FIFO) beginreturn i;//如果是默认的仲裁方式，实际上在这里就会返回ID号endelse avail_sequences.push_back(i);endi++;end//这个任务实际就是在seq不是lock的，同时这个seq是有效的（is_relevant=1）的情况下，如果不是fifo的仲裁方式的化，将arb_sequence_q中的索引值推入到avail_sequences中去。// Return immediately if there are 0 or 1 available sequencesif (m_arbitration == UVM_SEQ_ARB_FIFO) beginreturn -1;endif (avail_sequences.size() < 1) begin//这两个函数我理解只是做保护的return -1;endif (avail_sequences.size() == 1) begin//size为1的时候立刻返回return avail_sequences[0];end// If any locks are in place, then the available queue must// be checked to see if a lock prevents any sequence from proceedingif (lock_list.size() > 0) begin//这个函数没懂for (i = 0; i < avail_sequences.size(); i++) beginif (is_blocked(arb_sequence_q[avail_sequences[i]].sequence_ptr) != 0) beginavail_sequences.delete(i);i--;endendif (avail_sequences.size() < 1)return -1;if (avail_sequences.size() == 1)return avail_sequences[0];end// Weighted Priority Distribution// Pick an available sequence based on weighted priorities of available sequences
//权重约束，这里我理解是把所有seq的权重加在一起得到一个总的门槛值，然后随机一个在门槛值和0之间的值，如果当前seq的权重大于这个随机值，就认为仲裁成功，在此后的循环过程中，新的seq的权重总是会加上之前seq的权重，把这个权重的和与门槛值进行比较。if (m_arbitration == UVM_SEQ_ARB_WEIGHTED) beginsum_priority_val = 0;for (i = 0; i < avail_sequences.size(); i++) beginsum_priority_val += m_get_seq_item_priority(arb_sequence_q[avail_sequences[i]]);endtemp = $urandom_range(sum_priority_val-1, 0);sum_priority_val = 0;for (i = 0; i < avail_sequences.size(); i++) beginif ((m_get_seq_item_priority(arb_sequence_q[avail_sequences[i]]) + sum_priority_val) > temp) beginreturn avail_sequences[i];endsum_priority_val += m_get_seq_item_priority(arb_sequence_q[avail_sequences[i]]);enduvm_report_fatal("Sequencer", "UVM Internal error in weighted arbitration code", UVM_NONE);end//这里相当于随机给一个// Random Distributionif (m_arbitration == UVM_SEQ_ARB_RANDOM) begini = $urandom_range(avail_sequences.size()-1, 0);return avail_sequences[i];end// Strict Fifoif ((m_arbitration == UVM_SEQ_ARB_STRICT_FIFO) || m_arbitration == UVM_SEQ_ARB_STRICT_RANDOM) beginhighest_pri = 0;// Build a list of sequences at the highest priority
//这个for循环相当于找最大优先级的seq，把最大优先级的seq都放到highest_sequences，注意这里的delete动作，保证了只会存在最大优先级的seqfor (i = 0; i < avail_sequences.size(); i++) beginif (m_get_seq_item_priority(arb_sequence_q[avail_sequences[i]]) > highest_pri) begin// New highest priority, so start new listhighest_sequences.delete();highest_sequences.push_back(avail_sequences[i]);highest_pri = m_get_seq_item_priority(arb_sequence_q[avail_sequences[i]]);endelse if (m_get_seq_item_priority(arb_sequence_q[avail_sequences[i]]) == highest_pri) beginhighest_sequences.push_back(avail_sequences[i]);endend
//如果是严格fifo，会把最大优先级的，同时最先进入fifo的，先给出出去// Now choose one based on arbitration typeif (m_arbitration == UVM_SEQ_ARB_STRICT_FIFO) beginreturn(highest_sequences[0]);end
//如果不是严格fifo的化，就是在最大优先级的各个seq里去随机 i = $urandom_range(highest_sequences.size()-1, 0);return highest_sequences[i];end
//这个仲裁没看if (m_arbitration == UVM_SEQ_ARB_USER) begini = user_priority_arbitration( avail_sequences);// Check that the returned sequence is in the list of available sequences. Failure to// use an available sequence will cause highly unpredictable results.highest_sequences = avail_sequences.find with (item == i);if (highest_sequences.size() == 0) beginuvm_report_fatal("Sequencer",$sformatf("Error in User arbitration, sequence %0d not available\n%s",i, convert2string()), UVM_NONE);endreturn(i);enduvm_report_fatal("Sequencer", "Internal error: Failed to choose sequence", UVM_NONE);endfunction
未完。。。。
