CPSC 121: Models of Computation Lab #9: A Working Computer

Transcription

1 CPSC 121: Models of Computation Lab #9: A Working Computer Objectives In this lab, we revisit the Y86 processor, which you saw components of in labs 4 and 6. Recall that in lab 4, you learned about the ALU and in lab 6, you looked at the RAM. Our goal with this lab is for you to appreciate that a computer is a complex sequential circuit that you now have the tools and knowledge to analyse. This lab will also expose you to machine code which are instructions executed directly by the computer s central processing unit (CPU). Our goal here is for you to realize that an appropriate string of binary numbers actually can be used to program a circuit like this one. A version of this processor will appear in more detail in CPSC 313 enjoy! 1 Prelab Download and print out the file playcpu.pdf from the course website (it might make your work easier if you print out the document single-sided). Notice that in the first four pages, the first two columns for the larger tables are filled in for you. Each of these four pages refers to a particular stage in the paper computer. The names of these stages are given at the top of each page: Memory and Registers, Fetch and Decode, Execute and ALU and Decide Branch. TODO (pre-lab): Starting with the Fetch/Decode stage and following the instructions given at each stage, correctly fill in the next column for each of the four stages. Note that all values are in hexadecimal. You should have three columns filled in when you are finished: two of which were done for you as examples and one that you have filled in yourself. If you are having trouble with the prelab, a step-by-step example is given in Appendix B and a FAQ is given in Appendix C 2 A Paper Computer Team up with another group to form a team of four. The four of you will be running the rest of the program you started in the prelab, with each of you taking on the role of a different part of the computer. Halfway through the exercise, you will switch roles as follows (the table is shaded to indicate where you should switch): Memory and Registers becomes Fetch and Decode; Fetch and Decode becomes Execute; Execute becomes ALU and Decide; ALU and Decide becomes Memory and Registers. Memory/Registers: Your job is to handle two types of computer memory: its RAM (just called memory ) and its small component of fast memory (called registers ) Fetch/Decode: Your job is to take instructions out of memory and parse them for the computer to use Execute: Your job is to execute instructions using the ALU and then tell the computer what instruction to execute next ALU: Your job is to do the arithmetic and logic operations for the Execute stage TODO: Pick your roles and run the computer program on paper. Check with your TAs regularly to make sure you haven t made a mistake. What does the program do? 1

2 3 A Mystery Program 1. Download the files Y86-cpu.circ and simple-loop.img from the course website and open up Y86-cpu.circ in Logisim 2. Find the 16MB RAM module and click on it using the poke tool. Double click on the magnifying glass in the middle to open the module. 3. Find the RAM module and right-click on it. Select Load image and load in simple-loop.img. Your screen should now look like this: 4. Now return to the main screen by pressing Control-Left arrow. Note: There is a bar in the bottom left hand corner of your Logisim screen that allows you to zoom in and out of the circuit. 5. To clock the computer, press Control-T. Note: Two clock cycles equal one line of instruction. If you reset the clock using Reset Simulation or Ctrl-R, you will have to reload the image into the RAM module. Go to the Simulate menu and select Logging to log the following values over time: Program Counter, icd, ifn, bch, vale, eax, ecx and edx. For each of the eight values, click Change Radix twice so they are all set to 16. Press Ctrl-K to begin clocking your computer, which will then fill in the table for these values. (To stop clocking the computer, press Ctrl-K again.) TODO: Look at the column of the Program Counter in the table. Can you specify where the numbers repeats? This is a loop in the program. 2

3 TODO: Look at the table again. What are these eight values (Program Counter, icd, ifn, bch, vale, eax, ecx) doing over time? What do you see? Explain what each value s role is in the program. Note: %eax, %ecx, %edx, %ebx, %esp, %ebp, %esi, and %edi are the register values. The other inputs and outputs to the modules you will be interested in looking at are named similarly to the paper computer from the first part of lab. TODO: Pick one of the following modules in Logisim and look at in detail: Decode, Branch? (inside Execute), or PC Update. What is this module doing? What are its inputs and outputs? How does it relate to the paper computer we simulated at the start of class? 4 Further Analysis TODO (further analysis): Why have we broken up the computer into different stages? What are each of the following stages doing when the current instruction is being processed by a different stage? 5 End of Lab Survey TODO: To help us improve these labs both this term and for future offerings, complete the survey at TODO: In addition to the End of Lab Survey, you will receive one bonus mark for participating in 3

4 the End of Term lab curriculum survey, available at: 6 Challenge Problem TODO (challenge): Look back on the paper computer you completed at the start of lab and write out an instruction (in hexadecimal) that takes the value from register 5, subtracts it from the value in register 6 and stores the resulting value into register 6. A Marking scheme All labs are out of ten marks, with two marks for prelabs, and eight marks for in-lab work. In more detail: Two marks - Prelab questions Five marks - In-lab questions (two marks for the paper computer, one mark for finding the line where the program loops, one mark for logging the eight values and one mark for looking at a module in detail) Two marks - Further analysis question One mark - End of lab survey, with one bonus mark for the other survey TAs may at their discretion award one bonus mark, such as for completing the challenge problem. (In this lab, it is possible to get 12 out of 10.) 4

5 B Paper Computer: Step-by-step Example Let us step through the process of filling in the second column, which was done for you as an example. 1. Following the prelab instructions, we start at the Fetch and Decode Stage. In the reference sheet (the last page of playcpu.pdf), we re told, in step one, to fetch an instruction from memory at the address PC. Looking at the bottom table in Fetch and Decode, we see that in the second column, PC is To fetch the instruction at address PC = 6, we go to the Memory and Registers stage and look at the Memory table at the top. In this table, the top row contains the memory addresses and the bottom row holds the actual data. Since we want the instruction at address 6, we locate 6 in the top row and see that the byte of data stored there is This date byte, 30, tells us the icd and ifn values we need. The first hex digit, 3, is the icd and the second hex digit, 0, is the ifn. These values are filled in for you in column 2 in Fetch/Decode. 4. Once we have the icd and ifn values, we can know how many more bytes to fetch for our instruction. This is detailed at the top of the Fetch/Decode stage. In short: The length of the instruction depends on your icd value! When icd = 3, you fetch 12 hex digits When icd = 6, you fetch 4 hex digits (there is no data for valc so you can just write n/a or leave it blank) When icd = 7, you fetch 10 hex digits (there s no data for ra and rb so you can just write n/a or leave them blank) In this case, since icd = 3, we fetch 12 hex digits, starting from PC. Thus, our full instruction is 30 F Now, following the instructions in Fetch/Decode, since icd = 3, the third hex digit, F, is our ra value and the fourth hex digit, 1 is our rb value. The rest of the digits, , or 5, make up the valc value. 6. To get our valp value, we look at the smaller table in Fetch/Decode. Since icd = 3, our valp value is PC + 6 = = C. (Remember that all values are in hexdecimal). This table also tells us srca = F, srcb = F and dste = rb = We are now ready to move to the Execute stage. Following the instructions in Execute, we first copy over values from Fetch/Decode. Then, to get vala and valb, we look at srca and srcb. As stated in the instructions, srca and srcb are register addresses. This means we need to go to the Memory and Registers stage (page one) and look at the Registers table at the bottom. In the Registers table, locate the register address in the leftmost column and get the data from the most recent column. In our case, since we re filling in column 2, the most recent column is column 1. For the prelab, since you re filling in column 3, the most recent column is column 2. In our case, srca and srcb are both F, but looking at the Registers table, we see there is no address F in the leftmost column! When this happens, we just write 0 for both vala and valb in Execute. If srca and srcb are not both F, see FAQ # 4 below. 8. Now, as per instruction 3 in Execute, we need to send some values over to the ALU/Decide stage to get vale and bch. 9. In ALU/Decide, fill in the values that were sent over from Execute. Follow the three steps to get vale and bch. In our case, since icd = 3, Step 1 gives us alua = valc = 5 and alub = 0. Then, Step 2 gives us vale = alub + alua = 5 and Step 3 tells us bch = 0. For the instruction in Step 2, see FAQ # 3 below for an example. 10. Having bch from ALU/Decide, we can get the value of nextpc in Execute. This value will usually be valp with one exception: when icd = 7 and bch = 1. Since icd = 3, in our case, we write down valp = C for nextpc and then write the value of nextpc to the PC row in Fetch/Decode. This is the C value we filled in for you in column Lastly, write the value of vale to the register with address dste. This means we go back to the Memory and Registers stage and find dste in the leftmost column of the Registers table, as we did for Step 7, with srca and srcb. In our case, dste = 1 so we locate address 1 in the leftmost column and write down vale = 5 into that row in column 2. All of the values in the other rows stay the same, since we didn t write any new values to them. 12. We have now completed a full clock cycle. To fetch the next instruction, we start over again at Fetch/Decode and see that the PC value is now C. Begin again at Step 1. 5

6 C Paper Computer: Frequently Asked Questions 1. How many columns do we need to fill in for the prelab? The prelab states to go through one complete clock cycle, which corresponds to one column in the tables. Remember that a clock cycle goes through all four stages (the first four pages in playcpu.pdf are the four stages) so you need to fill in the corresponding column in all four tables to complete one clock cycle. We did two clock cycles for you so two columns in each of the four tables are filled in. 2. How many bits are in a hexadecimal digit? How many bits are in a byte? A hexadecimal digit is 4 bits and a byte is 8 bits. Thus, one byte corresponds to 2 hexadecimal digits. 3. What is the instruction in the ALU and Decide Branch stage? This is a logical bitwise AND. For example, if you wanted to AND together 13 and 7, you would first write both of those numbers in binary and then AND the corresponding bits: 13 7 = = (1 0)(1 1)(0 1)(1 1) = 0101 = How do we get vala and valb from srca and srcb? When you re given a register address (srca or srcb) and need the value stored in that register, go to the Registers table (on the Memory/Register page), locate the address in the leftmost column and get the data from the corresponding row in the most recent column. The most recent column is the column previous to the one you re currently filling in. For example, when filling in column 3 for the prelab, the most recent column is the second column we ve filled in for you. If srcb = 1, we look at row 1 in column 2 and see that the value we want is 5. If the address is F, just write down a 0 for the value. 5. What do we write for valc when icd = 6? What do we write for ra and rb when icd = 7? The length of the instruction depends on your icd value! You won t always fetch the same number of bytes of data. When icd = 3, you fetch 12 hex digits When icd = 6, you fetch 4 hex digits (there is no data for valc so you can just write n/a or leave it blank) When icd = 7, you fetch 10 hex digits (there s no data for ra and rb so you can just write n/a or leave them blank) 6. How do we add 6 to C? Doesn t this exceed F in hexadecimal? Yes, in this case, you will have a carry over digit of 1. Your result would be two hex digits long. 7. What do we do if the icd value is not 3, 6 or 7? This won t happen for the examples you ll be doing in the lab. 6