CSC 101: Lab Manual#9 Machine Language and the CPU (largely based on the work of Prof. William Turkett) Lab due date: 5:00pm, day after lab session

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1 CSC 101: Lab Manual#9 Machine Language and the CPU (largely based on the work of Prof. William Turkett) Lab due date: 5:00pm, day after lab session Purpose: The purpose of this lab is to gain additional hands-on experience in working with different number systems, to gain insight into how a CPU executes low-level (machine and assembly) language instructions, and to gain insight into how CPU instructions are represented on the CPU. Lab Instructions If you had trouble with the software in the pre-lab (no window popped up), please let one of the instructors know early in the lab session. Hexadecimal Representations: As was discussed in the pre-lab, ultimately everything in a computer is represented in binary notation (strings of 0s and 1s). However, for humans this notation can sometimes get unwieldy for example, reading 32 binary digits (a common size used for representing information in a computer) can make your eyes go googly: 32 bits (binary digits): Accordingly, it is often easier to read and work with numbers in hexadecimal, the number system that is based on powers of 16 (instead of powers of 10 like we are used to or powers of 2 like binary). Continuing with some of the examples from the pre-lab, here is the setup for a 3-digit hexadecimal number. 256s place 16s place 1s place Powers: Remember that we are able to write into each position any numbers from 0 up to our (number system base-1): In binary (base 2) we can write 0s and 1s (base of 2-1) in the positions. In decimal (base 10), we can write the values 0 through 9 (base of 10-1) in the positions. In hexadecimal (base 16), we can write the values 0 through 15 (base of 16-1). However, we will write the numbers 10, 11, 12, 13, 14, and 15 as A, B, C, D, E, and F respectively so that they only take up one physical position. Given that we have a 3-digit hexadecimal number, the smallest number we can then represent is using a 0 in each position, leading to 0* *16 + 0* , while the largest number we can represent is using a F (equivalent to 15) in each position, leading to F*256 + F*16 + F*1 15* * * Why do we want to use hexadecimal in place of long binary numbers? Remember that there is a nice relationship between hexadecimal and binary. You should remember from previous math classes and the prelab that 2 4 is the number 16 - that means one hexadecimal digit is the same as four binary digits. So, our long binary number from above: 32 bits (binary digits): can be broken apart into eight different 4-bit chunks: which in hexadecimal is the value: 5 0 C 5 D 2 D 8

2 which is a lot more compact and easier on the eyes. A table with these 4-binary digit to 1-hexadecimal digit translations was shown in lecture and is included at the back of this lab. Answer Questions 1 and 2 on the Lab Report document. Low-level (Assembly and Machine) Language Instructions: As discussed in lecture, low level computer instructions are very simple instructions which are understood and executed by a computer. An example of such an operation is addition. A computer might be instructed to perform the addition operation on two values; let s say the numbers 8 and 16. If we were to write this process down, we would commonly write: However, for a computer it would more likely be written as ADD 8 16 That is, the operator (ADD for addition) is written first, followed by the two operands (the values to be added). As another example, computing 5-2 would be written for the computer as: SUB 5 2 The operators that are available to be used provide the functionality for the computer, and thus make up the set of potential instructions. The above two examples suggest ADD (addition) and SUB (subtraction) as potential low level instructions, but there are others as well. Remember that the internals of a computer break down similar to the picture shown at right. If there is information stored in RAM (the memory of your computer), sometimes it may need to be brought to the CPU over the BUS to be worked on, and other times the CPU may need to write information back out to the RAM memory. This suggests it could be useful to have instructions such as LOAD (get information from RAM onto the CPU) and STORE (save information from the CPU to RAM). In addition to addition and subtraction, we could expect our low-level language to support math operations such as multiplication (MUL); division (DIV); the AND, OR, NOT, and XOR Boolean operations we saw in lab #3; and comparison operations (LESS THAN, GREATER THAN, EQUAL TO). While there are more operations that modern low-level languages support than just these, the list above should give you an idea of the basics. All of these instructions would be written in the operator operand operand format, as shown in the examples below: ADD 8 16 equivalent to 8+16, answer is 24 SUB 4 2 equivalent to 4-2, answer is 2 GREATERTHAN equivalent to 12 > 10, answer is true AND true false answer is false (remember the AND truth table from Lab #5?) LOAD load information from memory location 1024 onto the CPU at a location 12

3 Answer Question 3 on the Lab Report document. All information understood by the computer, including the instructions, has to eventually be encoded in binary. That means we have to be able to encode the operator and both operands listed above all as binary numbers. So, taking one example from above: SUB 4 2 might be encoded as OPERATOR OPERAND1 OPERAND in binary (which in decimal is the 4 and 2 should make sense as they are the operands; the first 1 is an arbitrary number I decided to associate with the SUB subtraction operator). Let s assume that all of our instructions are to be encoded in a total of 12 binary digits (like the one above). We could divide up the bits to describe the instruction in a number of ways: 4 bits for the operator 4 bits for operand1 4 bits for operand2 6 bits for the operator 3 bits for operand1 3 bits for operand2 2 bits for the operator 5 bits for operand1 5 bits for operand2 any other combination that adds up to 12 total bits. The number of binary digits we allow for the operator constrains the number of different instructions we can encode, as each different available binary number would be associated with an operator (ie ADD would be assigned the binary version of the number 0, SUB would be assigned the binary version of the number 1, MUL would be assigned the binary version of the number 2, and so on). Answer Question 4 on the Lab Report document. Question 5: Let s assume that a STORE instruction (one of our example instructions on the previous page) has the following format: STORE VALUE LOCATION operator operand1 operand2 Answer the question 5 on the Lab Report document. Compilation As you discussed in lecture, computer software is initially written in high level languages languages whose instructions are similar to English and are human-readable. A tool (either a compiler or an interpreter) converts the English-like instructions to machine language instructions to be executed by the CPU. Answer Question 6 on the Lab Report document.

4 CPU Simulator: Navigate to the following page in your web-browser: This should open a web-page that contains a CPU simulator, similar to what is shown below: This simulator was demonstrated in the video you were requested to watch before lab. Using the New Prgm button will open a text entry window similar to the one at right. In that window, enter the following program: lod-c 0 sto 12 lod 12 inc inc sto 12 jmp 2 and hit the Translate button on that window. If you typed everything correctly, it should close and your program will show up in the positions labeled 0-6 in the right of the large window. If you made an error, then it should recognize that as well.

5 Question 7 (7 points): When the program was translated successfully (we can think of this as compilation of the program), what are the 7 encoded instructions shown in memory positions 0-6? Make sure the drop-down box above the memory box says Integers. RECORD YOUR ANSWERS ON THE LAB9 REPORT. This program works as shown below: lod-c 0 // load the number 0 onto the CPU sto 12 // store that number (0) out to position 12 in memory lod 12 // load the value in memory position 12 onto the CPU inc // increment (add 1 to) that value(ie 0 has one added to become 1) inc // increment (add 1 to) that value (ie 1 has one added to become 2) sto 12 // store the resulting value in memory at location 12 jmp 2 // go back to instruction 2 (lod 12) and start over We now want to execute this program. Read the rest of this paragraph, and then repeatedly click the Step button, which will start the fetch-execute cycle. Watch the PC (Program Counter) field, the field which says what the next instruction to execute is. Every time it changes (which may be after multiple hits of the Step button), record the following information as your answer to Question 8. After recording the values above, you should have seen the memory in position 12 change values from 0 to 2. Now, press the Set PC=0 button, which will prepare to restart our program from the beginning. The questions on the next page should be answered based on what you are about to see, so you may want to go ahead and read the questions and then come back to this page to read the final instruction of what to press in the simulator. If you have read the questions on the next page and are back here, now instead of pressing Step, press Run (which automates execution of the program) and watch Memory position 12 (the values counting up from 0). Answer Question 9 on the Lab Report document. CPU Speed: As you saw in the pre-lab, CPUs maintain an internal clock that ticks many times a second. For my laptop, the CPU ticked at a frequency of 2.4 GHz, or 2.4 billion (2,400,000,000) ticks in a second. We will assume that the computer can execute 1 machine language instruction per tick, so that s 2.4 billion machine language instructions a second! Answer Question 10 on the Lab Report document. Note: Nothing needs to be uploaded into Sakai for this lab.