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Unformatted text preview: the contents of two registers to the ALU, which adds the two words together and stores the result in a register, overwriting the previous contents of that register. I/O Read: Copy a byte or a word from an I/O device into a register. PC is also a commonly-used acronym for “Personal Computer”. However, the distinction between the two is always clear from the context. 8 CHAPTER 1. INTRODUCTION ¯ ¯ I/O Write: Copy a byte or a word from a register to an I/O device. Jump: Extract a word from the instruction itself and copy that word into the program counter (PC), overwriting the previous value of the PC. Chapter 4 has much more to say about how processors work. 1.4.2 Running the hello Program
Given this simple view of a system’s hardware organization and operation, we can begin to understand what happens when we run our example program. We must omit a lot of details here that will be ﬁlled in later, but for now we will be content with the big picture. Initially, the shell program is executing its instructions, waiting for us to type a command. As we type the char...
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