add notes

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/1.md b/doc/1.md
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+++ b/doc/1.md
@@ -148,3 +148,10 @@ the users can interact with each program while it is running.
 A program laoded into memory and executing is called a process.
 When a process executes, it typically executes for only a short time
 before it either finishes or needs to perform I/O.
+
+Modern operating systems are interrupt driven.
+Events are almost always signaled by the occurrence of an interrupt or a trap.
+
+A trap, or an exception, is a software generated interrupt caused either by
+an error or by a specific request from a user program that an operating-system
+service be performed.