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CS 752: Advanced Computer Architecture I (Fall 2015 Section 1 of 1)

Homework 2

Due 1pm, Monday, 9/21

You should do this assignment alone. No late assignments.


The purpose of this assignment is to help you become familiar with gem5's language for describing instruction sets. You will go through the ISA files in src/arch/x86/isa and understand how instructions are decoded and broken down into micro-ops which are ultimately executed. To get a better understanding, you will implement a missing x87 instruction (FSUBR). Note that x87 is a subset of the x86 ISA. This subset was originally added to provide the floating point support, but is not used much now. To test your implementation of the instruction, you will write a small program that will use this particular through inline assembly feature of GCC. The program then would be simulated using gem5.

As you might already know, the x86 instructions typically do a lot of work. While one can implement the functionality of each instruction individually, since a lot of work is common across many instructions, typically, each instruction is implemented as a combination of several smaller parts. The entire instruction is typically referred to as a macro-op, while the smaller parts are referred to as micro-ops. To implement an instruction in gem5, we first provide the ISA decoder with the information on the macro-op, then we provide an implementation of the macro-op in terms of micro-ops. Finally, we implement the micro-ops that are not already implemented. We will carry out these steps for the FSUBR instruction. Our implementation of FSUBR will mirror that of FSUB, whose implementation is already available in gem5.

1. There are many ways in which instructions are encoded in the x86 ISA. We would focus on the x87 subset. You can read more about instruction encoding in a manual provided by AMD. Let's go through the file one_byte_opcodes.isa to understand how gem5 decodes instructions from the x86 ISA. The file is written in a language designed specifically to express instruction sets. The contents of the file are ultimately converted to a C++ switch case. We first decode the top 5 bits of the opcode byte. There are 32 possible ways in which we can construct binary numbers using 5 bits. The switch case lists all the possible cases.

All x87 instructions begin with an opcode byte in the range 0xD8 to 0xDF. Therefore the topmost 5 bits always are 0x1B. For this case, we include the file x87.isa. Let's jump to that file. In this file, we start with decoding the bottom 3 bits. You can take a look at Table A-15 (page 443) in the manual mentioned above for the instructions represented by different cases for the bottom three bits. For example, FSUB and FSUBR are represented by opcodes 0xD8 and 0xDC, ie. the cases 0x0 and 0x4. To distinguish between the functionality provided by these different opcodes for the same instruction, you will have to understand the meaning of the ModRM field of the instruction. Read about it in the manual linked to. In the file x87.isa, you can check that we have FSUB appearing the cases statements for 0x0 and 0x4. You can also observe that FSUBR's implementation is missing.

As a first step, understand the difference between the two implementations for FSUBR instruction: one with opcode byte D8h and the one with opcode byte DCh. For this, you should read the description of FSUBR provided in the manual on x87 instructions.

2. Now, figure the three places in which fsubr appears in the file x87.isa. Replace the currently appearing statements with ones similar to those specified for FSUB in the same file. By mentioning something like Inst::FSUBR, you are asking for that instruction to be used, instead of the default one, which simply prints a warning that the instruction is not implemented.

3. Now, we need to provide an implementation of the FSUBR macro-op in terms of some micro-ops. Again we will mirror the implementation of the FSUB instruction. Go to the directory src/arch/x86/isa/insts/x87/arithmetic/. This directory holds the definition of different x87 arithmetic instructions in terms of micro-ops. Take a look at how the FSUB instruction has been implemented using micro-ops. FSUB1 and FSUB2 correspond to the two different opcodes that we mentioned before. For each type, we have to provide three different implementations: one that only uses registers, one that reads one of the operands from the memory using the address provided in the instruction and the last one uses the address of the instruction pointer to read the operand. The micro-ops used for the three implementations should be straight forward to understand.

They way gem5's instruction parser works requires us to define all the three implentations for the FSUBR instruction. In all you should have six separate code blocks for FSUBR, like those specified for FSUB.

4. Lastly, we need to provide an implementation of the micro-op subfp. You can check that the implementation is already available in the file: src/arch/x86/isa/micro-ops/fpop.isa. So, you would not need to do anything for this step.

5. Compile gem5 for x86 ISA to test that you did not make any mistakes in the implementation.

There are many aspects of gem5's ISA language that we have not discussed at all. Most of these aspects are not documented at all and one needs to figure them out by going over the code in relevant files.

6. Now, we will test the implementation of the FSUBR instruction. For this purpose, we will first write a short C program that reads a file with two floating point numbers, subtracts them and prints the output. To make sure that FSUBR is used for subtraction, we will explicitly use it using the inline assembly feature of GCC.

Assembly instructions are written inline with the rest of the code using the 'asm' code block. This code block contains two portions: the instruction portion, and the constraint portion. In instruction portion is a string containing the assembly instructions. The GNU C compiler does not check this string for correctness, so anything is allowed. The constraint portion specifies what GCC can or cannot do with the input and output operands, what registers or memory are affected by the instruction portion. There is documentation available from GCC and other sources. I recommend reading:

It is likely that most of you would have never used this feature of GCC. Since it is sort of hard to understand how to correctly all the constraints related to assembly instructions, we are providing an implementation with some explanation.

  float subtract(float in1, float in2)
    float ret = 0.0;
    asm ("fsubr %2, %0" : "=&t" (ret) : "%0" (in1), "u" (in2));
    return ret;

The aim of the function is to subtract the two floating point input values and return the result. To do so, we use FSUBR in the 'asm' code block. In our case the instruction string is "fsubr %2, %0". After that we specify constraints on the output operand, which we ask to be the variable ret. We then specify the two input operands: in1 and in2. The letters 't' and 'u' specify the top and the second to top register of the x87 stack.

Ok, getting back to our main purpose. Use the function provided above in a C program that takes as input a file name, reads two floating point numbers from the file, uses FSUBR to subtract the numbers and prints the result to stdout. Compile the program statically to generate a binary. You may also look at the assembly code generated by the compiler using the -S flag.

7. Now simulate this program using gem5. You will need to figure out how to supply command line arguments to programs in gem5. You can either use the example script supplied with gem5 in configs/examples/ or the script you created in Homework 1. Take a look at the file configs/examples/ and the file configs/common/ to figure out how input arguments are to be supplied. Although it is not necessary for this assignment, there is a new section of the tutorial on using the example configuration scripts:

What to Hand In

Turn in your assignment by sending an email message to Prof. David Wood <> and Nilay Vaish <> with the subject line: [CS752 Homework2].

1. The email should contain the name and ID numbers of the student submitting the assignment. The files below should be attached as a zip file to the email.

2. A file named fsubr.c which is used for testing the implementation of FSUBR.

3. A patch file containing the changes made to src/arch/x86/isa/insts/x87/arithmetic/ and src/arch/x86/isa/decoder/x87.isa. You can generate the patch using the mercurial command 'hg diff src/arch/x86/isa > /tmp/changes.patch'.

4. stats.txt file for two different simulations of the program: one using the TimingSimpleCPU and the other using the MinorCPU.

5. A short report (200 words) on your experience with the language / method used by gem5 for implementing different ISAs.

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