TMS320C6748: example4-4中，writer123为什么会按顺序执行？

example4-4 来自 bios user guide (SPRUEX3U) 4.1.1

代码：

/*.cfg*/
var Defaults = xdc.useModule('xdc.runtime.Defaults');
var Diags = xdc.useModule('xdc.runtime.Diags');
var Error = xdc.useModule('xdc.runtime.Error');
var Log = xdc.useModule('xdc.runtime.Log');
var LoggerBuf = xdc.useModule('xdc.runtime.LoggerBuf');
var Main = xdc.useModule('xdc.runtime.Main');
var Memory = xdc.useModule('xdc.runtime.Memory')
var SysMin = xdc.useModule('xdc.runtime.SysMin');
var System = xdc.useModule('xdc.runtime.System');
var Text = xdc.useModule('xdc.runtime.Text');
var BIOS = xdc.useModule('ti.sysbios.BIOS');
var Clock = xdc.useModule('ti.sysbios.knl.Clock');
var Task = xdc.useModule('ti.sysbios.knl.Task');
var Semaphore = xdc.useModule('ti.sysbios.knl.Semaphore');
var Hwi = xdc.useModule('ti.sysbios.hal.Hwi');
var HeapMem = xdc.useModule('ti.sysbios.heaps.HeapMem');
/* set heap and stack sizes */
BIOS.heapSize = 0x2000;
Program.stack = 0x1000;
SysMin.bufSize = 0x400;
/* set library type */
BIOS.libType = BIOS.LibType_Custom;
/* Set logger for the whole system */
var loggerBufParams = new LoggerBuf.Params();
loggerBufParams.numEntries = 32;
var logger0 = LoggerBuf.create(loggerBufParams);
Defaults.common$.logger = logger0;
Main.common$.diags_INFO = Diags.ALWAYS_ON;
/* Use Semaphore, and Task modules and set global properties */
var Semaphore = xdc.useModule('ti.sysbios.knl.Semaphore');
Program.global.sem = Semaphore.create(0);
var Task = xdc.useModule('ti.sysbios.knl.Task');
Task.idleTaskVitalTaskFlag = false;

/* Statically create reader and writer Tasks */
var reader = Task.create('&reader');
reader.priority = 5;
var writer0 = Task.create('&writer');
writer0.priority = 3;
writer0.arg0 = 0;
var writer1 = Task.create('&writer');
writer1.priority = 3;
writer1.arg0 = 1;
var writer2 = Task.create('&writer');
writer2.priority = 3;
writer2.arg0 = 2;
/* uses Queue module and create two instances statically */
var Queue = xdc.useModule('ti.sysbios.knl.Queue');
Program.global.msgQueue = Queue.create();
Program.global.freeQueue = Queue.create();




/* ======== semtest.c ======== */
#include <xdc/std.h>
#include <xdc/runtime/Memory.h>
#include <xdc/runtime/System.h>
#include <xdc/runtime/Error.h>
#include <ti/sysbios/BIOS.h>
#include <ti/sysbios/knl/Semaphore.h>
#include <ti/sysbios/knl/Task.h>
#include <ti/sysbios/knl/Queue.h>
#define NUMMSGS 3 /* number of messages */
#define NUMWRITERS 3 /* number of writer tasks created with */
/* Config Tool */
typedef struct MsgObj {
 Queue_Elem elem; /* first field for Queue */
 Int id; /* writer task id */
 Char val; /* message value */
} MsgObj, *Msg;
Void reader();
Void writer();
/* The following objects are created statically. */
extern Semaphore_Handle sem;
extern Queue_Handle msgQueue;
extern Queue_Handle freeQueue;
/* ======== main ======== */
Int main(Int argc, Char* argv[])
{
 Int i;
 MsgObj *msg;
 Error_Block eb;
 Error_init(&eb);
 
 msg = (MsgObj *) Memory_alloc(NULL, NUMMSGS * sizeof(MsgObj), 0, &eb);
 if (msg == NULL) {
 System_abort("Memory allocation failed");
 }
 
 /* Put all messages on freeQueue */
 for (i = 0; i < NUMMSGS; msg++, i++) {
 Queue_put(freeQueue, (Queue_Elem *) msg);
 }
 BIOS_start();
 return(0);
}
/* ======== reader ======== */
Void reader()
{
 Msg msg;
 Int i;
 for (i = 0; i < NUMMSGS * NUMWRITERS; i++) {
 /* Wait for semaphore to be posted by writer(). */
 Semaphore_pend(sem, BIOS_WAIT_FOREVER);
 /* get message */
 msg = Queue_get(msgQueue);
 /* print value */
 System_printf("read '%c' from (%d).\n", msg->val, msg->id);
 /* free msg */
 Queue_put(freeQueue, (Queue_Elem *) msg);
 }
 System_printf("reader done.\n");
}

/* ======== writer ======== */
Void writer(Int id)
{
 Msg msg;
 Int i;
 
 for (i = 0; i < NUMMSGS; i++) {
 /* Get msg from the free list. Since reader is higher
 * priority and only blocks on sem, list is never
 * empty. */
 msg = Queue_get(freeQueue);
 
 /* fill in value */
 msg->id = id;
 msg->val = (i & 0xf) + 'a';
 System_printf("(%d) writing '%c' ...\n", id, msg->val);
 /* put message */
 Queue_put(msgQueue, (Queue_Elem *) msg);
 /* post semaphore */
 Semaphore_post(sem);
 }
 
 System_printf("writer (%d) done.\n", id);
}




/*result*/
(0) writing 'a' ...
read 'a' from (0).
(0) writing 'b' ...
read 'b' from (0).
(0) writing 'c' ...
read 'c' from (0).
writer (0) done.
(1) writing 'a' ...
read 'a' from (1).
(1) writing 'b' ...
read 'b' from (1).
(1) writing 'c' ...
read 'c' from (1).
writer (1) done.
(2) writing 'a' ...
read 'a' from (2).
(2) writing 'b' ...
read 'b' from (2).
(2) writing 'c' ...
read 'c' from (2).
reader done.
writer (2) done.

#1  配置文件中，Program.global.sem = Semaphore.create(0); 说明信号量初始化为0？

#2 创建了三个writer，这三个task是按照顺序123执行的吗？还是说os没有“刻意”排序？

#3 信号量只是用来限制task的行为？

#4 如果用emaphore_pend(sem, BIOS_NO_WAIT);替换Semaphore_pend(sem, BIOS_WAIT_FOREVER);是不是就没有了限制效果？BIOS_NO_WAIT只是判断了一次信号量是否大于0？

#5 不是counting priority semaphore才会考虑task的优先级？这个信号量没有设置模式，默认的模式是simple counting。