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 request
if (lock_request == 1) begin
req_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 queue
req_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 change
m_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 errors
req_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)) begin
if (update_sequence_id == 1) begin
set_sequence_id(m_sqr_seq_ids[sequencer_id]);
end
return m_sqr_seq_ids[sequencer_id];
end
if (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id
endfunction
------------------------------------------------------------------------------------
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id
endfunction
------------------------------------------------------------------------------------
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[$];（5）
function void uvm_sequencer_base::m_update_lists();
m_lock_arb_size++;
endfunction（6）注意这里有几个变量，第一个变量是lock_arb_size，第二个变量是m_lock_arb_size变量，m_lock_arb_size这个变量在每次进行m_update_lists()任务的时候会加一次，在一开始的时候会将这两个值相当，随后去等执行m_update_lists()任务。注意这里的arb_completed,原型是 protected bit arb_completed[int]; 删除这个index相当于从仲裁的q（arb_completed）将仲裁完毕的id删除。
task uvm_sequencer_base::m_wait_for_arbitration_completed(int request_id);
int lock_arb_size;
// Search the list of arb_wait_q, see if this item is done
forever
begin
lock_arb_size = m_lock_arb_size;
if (arb_completed.exists(request_id)) begin
arb_completed.delete(request_id);
return;
end
wait (lock_arb_size != m_lock_arb_size);
end
endtask2.finish_item
finish_item();
(1)this.mid_do(item);
(2)sequencer.send_request(item);
(3)sequencer.wait_for_item_done();
(4)this.post_do(item);注意，实际上会存在三个fifo，一个fifo是 uvm_tlm_fifo #(REQ) m_req_fifo;另两个是
REQ m_last_req_buffer[$];
RSP m_last_rsp_buffer[$];
uvm_send实际上是不断的往m_req_fifo去放入内容。
function void uvm_sequencer_param_base::send_request(uvm_sequence_base sequence_ptr,
uvm_sequence_item t,
bit rerandomize = 0);
REQ param_t;
if (sequence_ptr == null) begin
uvm_report_fatal("SNDREQ", "Send request sequence_ptr is null", UVM_NONE);
end//没有的化报fatal
if (sequence_ptr.m_wait_for_grant_semaphore < 1) begin
uvm_report_fatal("SNDREQ", "Send request called without wait_for_grant", UVM_NONE);
end//注意，在上面wait_for_grant任务中得到一个grant就会加一个，这里减少
sequence_ptr.m_wait_for_grant_semaphore--;
if ($cast(param_t, t)) begin
if (rerandomize == 1) begin//如果给了随机的选项，需要进行随机
if (!param_t.randomize()) begin
uvm_report_warning("SQRSNDREQ", "Failed to rerandomize sequence item in send_request");
end
end
if (param_t.get_transaction_id() == -1) begin
param_t.set_transaction_id(sequence_ptr.m_next_transaction_id++);
end
m_last_req_push_front(param_t);//往别的q里去推
end else begin
uvm_report_fatal(get_name(),$sformatf("send_request failed to cast sequence item"), UVM_NONE);
end
param_t.set_sequence_id(sequence_ptr.m_get_sqr_sequence_id(m_sequencer_id, 1));
t.set_sequencer(this);
if (m_req_fifo.try_put(param_t) != 1) begin
uvm_report_fatal(get_full_name(), "Concurrent calls to get_next_item() not supported. Consider using a semaphore to ensure that concurrent processes take turns in the driver", UVM_NONE);
end
m_num_reqs_sent++;//记录send了多少内容
// Grant any locks as soon as possible
grant_queued_locks();
endfunctiongrant_queued_locks（）任务有点复杂，没看完，留个坑
注意这个任务 ，前期会一直往m_last_req_buffer里push，注意这里是从前推入。这个q是uvm_sequencer_param_base类中定义的q，他的类型采用了参数化的类REQ，定义方式如下：
REQ m_last_req_buffer[$];
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) begin
uvm_report_warning("HSTOB",
$sformatf("Invalid last size; 1024 is the maximum and will be used"));
max = 1024;
end
//shrink the buffer if necessary
while((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) begin
uvm_report_warning("HSTOB",
$sformatf("Invalid last access (%0d), the max history is %0d", n,
m_num_last_reqs));
return null;
end
if(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) begin
uvm_report_warning("HSTOB",
$sformatf("Invalid last size; 1024 is the maximum and will be used"));
max = 1024;
end
//shrink the buffer if necessary
while((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);
else
wait ((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
需要注意的是，get_next_item是从前面说的m_req_fifo里去拿
未完。。。。