code:swtch - ikarishinjieva/unixV6-code-analyze-chs GitHub Wiki

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2170

2171

2172 /*

2173  * This routine is called to reschedule the CPU.

2174  * if the calling process is not in RUN state,

2175  * arrangements for it to restart must have

2176  * been made elsewhere, usually by calling via sleep.

2177  */

2178 swtch()

2179 {

2180     static struct proc *p;

2181     register i, n;

2182     register struct proc *rp;

2183

2184     if(p == NULL)

2185           p = &proc[0];

  • 若静态变量p为空,则使其指向0#进程
2186     /*

2187     * Remember stack of caller

2188     */

2189     savu(u.u_rsav);

  • 保存当前函数的r5,r6
2190     /*

2191     * Switch to scheduler's stack

2192     */

2193     retu(proc[0].p_addr);

  • 转入0#调度进程
2194

2195 loop:

2196     runrun = 0;

  • 清runrun标志
2197     rp = p;

2198     p = NULL;

2199     n = 128;

2200     /*

2201     * Search for highest-priority runnable process

2202     */

2203     i = NPROC;

2204     do {

2205           rp++;

2206           if(rp >= &proc[NPROC])

2207                     p = &proc[0];

2208           if(rp->p_stat==SRUN && (rp->p_flag&SLOAD)!=0) {

2209                    if(rp->p_pri < n) {

2210                          p = rp;

2211                          n = rp->p_pri;

2212                    }

2213           }

2214     } while(--i);

  • 遍历proc数组,寻找处于就绪状态,进程图像在内存中,且优先级最高的进程
2215     /*

2216     * If no process is runnable, idle.

2217     */

2218     if(p == NULL) {

2219           p = rp;

2220           idle();

2221           goto loop;

  • 若没能搜到,进入闲置状态,等待中断
  • 中断时若有进程醒来,则选中
2222     }

2223     rp = p;

2224     curpri = n;

2225     /* Switch to stack of the new process and set up

2226     * his segmentation registers.

2227     */

2228     retu(rp->p_addr);

  • 若搜到
    • 则将当前优先级置为选中进程的优先级
    • 将KISA6指向其ppda区首址
    • 恢复被选中进程的r5,r6
2229     sureg();
  • 为新选中进程设置用户态内存管理寄存器组
2230     /*

2231     * If the new process paused because it was

2232     * swapped out, set the stack level to the last call

2233     * to savu(u_ssav). This means that the return

2235     * actually returns from the last routine which did

2236     * the savu.

2237     *

2238     * You are not expected to understand this.

2239     */

2240     if(rp->p_flag&SSWAP) {

2241           p->p_flag =& ~SSWAP;

2242           aretu(u.u_ssav);

  • 若被选中进程带有SSWAP标志
    • 则清SSWAP标志
    • 调用aretu恢复r5,r6
2243     }

2244     /* The value returned here has many subtle implications.

2245     * See the newproc comments.

2246     */

2247     return(1);

2248 }

2249 /* -------------------------*/

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