Fall 1998 Computers and Society
Tuesdays 6 - 9:40 PM in 113 Rackham: GST 2710, Section 990 and AGS 3360, Section 983
Mondays 5:30 - 9:10 PM at NWAC: GST 2710, Section 984
Course web site: http://www.cll.wayne.edu/isp/drbowen/casf98/
Last updated: 6/13/01
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Paper and Pencil Computer: a Detailed Example

Background:
During the semester, we have been going step by step into how computers work. The path so far has been:

1. Structure of a computer - the major parts
2. Binary numbers - how computers do arithmetic
3. Doing arithmetic with switches - how electrical circuits can do arithmetic
4. Logic gates - arithmetic circuits, and steering numbers to different circuits

Now, for the fifth and last step along this particular path; putting it all into action. We will use a common example called The Paper and Pencil Computer. This example is as simple a computer as can be, and still work like a computer. While it is drastically simplified, it still illustrates the basic way that all computers work. First, the architecture, or what the parts are and how they are connected. In our diagram of a computer, this one has only four components:

1. Inputs - only a keyboard
2. CPU - a really simple one, but we will look at it in detail
3. Outputs - only a monitor
4. Primary storage - 32 bytes of RAM (not 32MB!)
5. Secondary storage - well, something had to go. No secondary storage.

So, here it is:

The 32 bytes of memory are shown as eight rows of 4. They are numbered 0 through 31, starting at the upper left and going row by row. That number is the address of the memory location. Each byte also has its contents, which is the one byte or 8-bit number stored there.

• Lowest value in 8 bits?
• Highest value in 8 bits?
• If the addresses go from 0 to 31 as stated, how many bits are required to hold the address?

Remember, both the instructions (program) and the data are stored in RAM while the program is being executed. without a disk drive, there is a problem in getting the program and data into the memory locations, but we will ignore that. In the examples and problems here, the contents of the memory location will be given as part of the example / problem statement. How do they get there? Somebody else's problem.

This document describes the parts in more detail and how they work together. Other documents will give examples.

The CPU:
The Central Processing Unit has two parts; the Control Unit and the the Arithmetic and Logic Unit

• Control Unit. This sets basic pattern of how the computer gets instructions from the memory, decodes them, executes them and then gets the next instruction. The logic gates that steer bits to different parts of the computer are in here.
• Arithmetic and Logic Unit. This is a scratchpad where the computer does arithmetic (addition and subtraction only for this one; no multiplication) and makes logical choices (if A is true then do B). The Logic gates that do addition and carry are in here.

First, the Control unit. It follows the same cycle, time after time, but very quickly. Here is the basic cycle:

This is the master cycle that the computer follows. The desired location that the program starts at will be given as part of the example / problem statement. When the Control Unit fetches an instruction, it gets the value from memory and places it in the Instruction Register (IR). The memory fetch gets one byte, so the IR is one byte. Here is how it is used:

Each instruction has an Operation Code, which says what instruction the computer will actually execute, and an address in the 32-byte RAM, the Referenced Memory Location or RML.

• How many bits do we need to hold the addresses from 0 to 31?
• How many bits are left over for the Operation Code?
• How many different Operation Codes can we have?

There is another number contained in the Program Counter (PC). When the Control unit goes to fetch an instruction, it fetches it from the memory address in the PC.

• How many bits does the PC have to have to be able to address all 32 bytes of memory?

So the Control Unit has three numbers that it keeps track of:

• The PC, or address where the instruction is fetched from
• The IR where the fetched instruction is stored and decoded
  • 3-bit Operation Code
  • 5 bit Referenced Memory Location (RML)

Next, the Arithmetic and Logic Unit, or ALU. This has the logic gates that do the arithmetic. It contains the Accumulator (AC). This is location in which the CPU does all of its arithmetic.

Instruction Set:
A CPU's Instruction Set is the collection of all of the instructions it can execute or carry out. Each CPU has it's own instruction set. Here is the instruction set for the Paper and Pencil Computer.