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

8495 /* Start output on the typewriter. It is used from the top

8496  * half after some characters have been put on the output

8497  * queue, from the interrupt routine to transmit the next

8498  * character, and after a timeout has finished.

8499  * If the SSTART bit is off for the tty the work is done

8500  * here, using the protocol of the single-line interfaces

8501  * (kl, dl, dc); otherwise the address word of the tty

8502  * structure is taken to be the name of the device-dependent

8503  * start-up routine.

8504  */

8505 ttstart(atp)

8506 struct tty *atp;

8507 {

8508     register int *addr, c;

8509     register struct tty *tp;

8510     struct { int (*func)(); };

8511

8512     tp = atp;

8513     addr = tp->t_addr;

8514     if (tp->t_state&SSTART) {

8515          (*addr.func)(tp);

8516          return;

8517     }

• 若 tty 有特殊启动函数，调用函数，并返回

8518     if ((addr->tttcsr&DONE)==0 || tp->t_state&TIMEOUT)

8519          return;

• 接收器 作业未完成 或 TIMEOUT标志被置(参看后面的说明)，函数返回

8520     if ((c=getc(&tp->t_outq)) >= 0) {

8521          if (c<=0177)

8522           addr->tttbuf = c | (partab[c]&0200);

8523          else {

8524           timeout(ttrstrt, tp, c&0177);

8525           tp->t_state =| TIMEOUT;

8526          }

8527     }

• tty 输出队列不为空，则取出头元素c
  • 若 c 为普通字符，则
    • c 送入 发送器数据缓存寄存器，用于向tty输出
    • partab[c] & 200 用于置奇偶位 (引自LIONS代码)
  • 若 c 为延迟字符(ttyoutput置)，则插入延迟
    • TIMEOUT标志 在 ttrstrt 中清位

该标志用于保证ttstart两次连续的调用，第二次再不调用ttrstrt，避免嵌套调用

8528 }

8529 /* ------------------------- */

• code:klxint
• code:ttyinput
• code:ttrstrt
• code:ttwrite