console/uart tweaks

kernel/console.c

@@ -39,7 +39,7 @@ consputc(int c)
   }
 
   if(c == BACKSPACE){
-    // if the user typed backspace, erase the character.
+    // if the user typed backspace, overwrite with a space.
     uartputc('\b'); uartputc(' '); uartputc('\b');
   } else {
     uartputc(c);
@@ -134,10 +134,10 @@ consoleread(int user_dst, uint64 dst, int n)
 }
 
 //
-// the uart interrupt handler, uartintr(), calls this
+// the console input interrupt handler.
-// for each input character. do erase/kill processing,
+// uartintr() calls this for input character.
-// append to cons.buf, wake up reader if a whole
+// do erase/kill processing, append to cons.buf,
-// line has arrived.
+// wake up consoleread() if a whole line has arrived.
 //
 void
 consoleintr(int c)
@@ -165,9 +165,16 @@ consoleintr(int c)
   default:
     if(c != 0 && cons.e-cons.r < INPUT_BUF){
       c = (c == '\r') ? '\n' : c;
-      cons.buf[cons.e++ % INPUT_BUF] = c;
+
+      // echo back to the user.
       consputc(c);
+
+      // store for consumption by consoleread().
+      cons.buf[cons.e++ % INPUT_BUF] = c;
+
       if(c == '\n' || c == C('D') || cons.e == cons.r+INPUT_BUF){
+        // wake up consoleread() if a whole line (or end-of-file)
+        // has arrived.
         cons.w = cons.e;
         wakeup(&cons.r);
       }

kernel/trap.c

@@ -180,7 +180,9 @@ devintr()
 
   if((scause & 0x8000000000000000L) &&
      (scause & 0xff) == 9){
-    // supervisor external interrupt, via PLIC.
+    // this is a supervisor external interrupt, via PLIC.
+
+    // irq indicates which device interrupted.
     int irq = plic_claim();
 
     if(irq == UART0_IRQ){

kernel/uart.c

@@ -43,6 +43,7 @@ uartinit(void)
   *R(1) = 0x01;
 }
 
+// write one output character to the UART.
 void
 uartputc(int c)
 {
@@ -52,6 +53,8 @@ uartputc(int c)
   *R(0) = c;
 }
 
+// read one input character from the UART.
+// return -1 if none is waiting.
 int
 uartgetc(void)
 {
@@ -63,6 +66,7 @@ uartgetc(void)
   }
 }
 
+// trap.c calls here when the uart interrupts.
 void
 uartintr(void)
 {