Slide Set 4. for ENCM 369 Winter 2018 Section 01. Steve Norman, PhD, PEng

Slide Set 4 for ENCM 369 Winter 2018 Section 01 Steve Norman, PhD, PEng Electrical & Computer Engineering Schulich School of Engineering University of Calgary January 2018

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 2/63 Contents Encoding of MIPS branch and jump instructions Building and running executables starting from C source code Linking and Libraries Notes about some confusing words

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 3/63 Outline of Slide Set 4 Encoding of MIPS branch and jump instructions Building and running executables starting from C source code Linking and Libraries Notes about some confusing words

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 4/63 Encoding of MIPS branch and jump instructions (Textbook, Section 6.5.) Previous lectures and Lab 2 showed machine code for instructions like add, sub, addi, lw, sw. How is machine code organized for beq, bne, j, jal, jr?

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 5/63 Encoding of beq and bne A 6-bit opcode tells what kind of instruction it is. Two 5-bit fields specify which registers to compare. A 16-bit offset field indicates how many instructions to skip forward or back. Note: Unlike load and store offsets, a branch offset counts words, not bytes.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 6/63 The Simplest Model for How a Computer Works and MIPS branch instructions The model is: Perform Step 1, Step 2, Step 1, Step 2,..., forever. For MIPS, Step 1 is: Read the instruction the PC points to, then do PC = PC + 4. If the instruction turns out to be a branch, then Step 2 is: if (branch should be taken) PC = PC + 4 branch offset else do nothing Attention: By the time Step 2 starts, the PC = PC + 4 update in Step 1 has already taken place.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 7/63 beq encoding example What is the MIPS machine code for each of the beq instructions? L1: beq $t0, $t1, L2 # first branch lw $t2, ($t0) add $t3, $t3, $t2 addi $t0, $t0, 4 beq $zero, $zero, L1 # second branch L2: sw $t3, ($s0)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 8/63 Branch encoding and assemblers Assemblers can compute the offset between a branch instruction and its target instruction. (In the previous example, the target of first branch is the sw instruction.) So, humans can use labels as operands in beq and bne. So, humans avoid the pain of writing A.L. for first branch and second branch as and beq $t0, $t1, 4 beq $zero, $zero, -5 (Counting instructions would be painful. Updating the counts correctly when you insert or delete instructions would be more painful, and very hard to do perfectly.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 9/63 Machine code for j and jal opcode... address field bit number: 31 26 25 0... Both instructions use this format: 6-bit opcode (000010 for j, 000011 for jal); 26-bit address field giving part of the address of the target instruction.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 10/63 PC updates in j and jal A jump can be thought of as updating the PC register with something other than the usual PC+4. The PC update for MIPS j and jal is this: OLD PC: these 26 bits will change 0 0 bit number: 31... 28 27 4 bits get copied... 2 1 0 NEW PC: copy of address field from j or jal instruction 0 0 bit number: 31... 28 27... 2 1 0

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 11/63 Processing of j or jal: example Suppose instruction 0x0c10_0020 is located at address 0x0040_0064. What is the address of the next instruction to be run? Does $ra get updated, and if so, with what?

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 12/63 Encoding of j and jal, compared to beq or bne Attention: To encode a j or jal instruction, the address of the jump target must be known. This is unlike encoding beq or bne, in which it is enough to know how many instructions to skip forward or back the exact address of the branch target does not have to be known.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 13/63 Encoding of jr 31 26 25 21 20 16 15 11 10 6 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 Bits 31 26 and 5 0 together indicate that this is a jr instruction. The number of the GPR used is encoded in bits 25 21. So, what is the machine code for jr $ra? (By the way, bits 20 6 don t have any particular use in a jr instruction.) 0 0

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 14/63 Outline of Slide Set 4 Encoding of MIPS branch and jump instructions Building and running executables starting from C source code Linking and Libraries Notes about some confusing words

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 15/63 Building and running executables starting from C source code Here we consider real programming environments such as Linux, macos, and Windows, not simulated environments like MARS. Examples are focused on C, but are quite applicable to C++ as well. Java works quite differently we won t look at Java in ENCM 369. Textbook reference: Section 6.6. However, note this warning: Section 6.6 oversimplifies things. Contrary to the example on page 339, an assembler won t be able to decide on absolute addresses of procedures and global variables. (Don t worry if this warning doesn t make sense the first time you see it it should make more sense once we get to slide 41 or so.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 16/63 Building and running C programs: a note about the word compiler Definition One: A compiler is a program that translates high-level language to assembly language. Definition Two: A compiler is a package of programs and other files that can be used to develop software. Example: Apple s Xcode is the compiler used for development of iphone apps. (Such a package includes a compiler, in the Definition One sense of that term.) In ENCM 369, Definition One applies when we use the word compiler.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 17/63 Building and running C programs: tools and the toolchain Tools are programs that read input files and write output files. Preprocessor: tool to convert C code into rearranged or edited C code. Compiler: tool to translate C code into assembly language.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 18/63 Building and running C programs: tools and the toolchain, continued Assembler: tool to create an object file using assembly language input. (Wait, what does object file mean?) Linker: tool to combine object files and information from library files to make an executable file. (Wait, what are the meanings of library files and executable file?) Together, preprocessor, compiler, assembler, and linker make a chain of tools.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 19/63 Building and running C programs: many kinds of files C source code files:.c and.h files written by a programmer as part of a specific program. Library header files:.h files provided to give information about types and functions defined in the library. (Example file: stdio.h. Example types and functions: FILE, fopen, fprintf, printf.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 20/63 Building and running C programs: many kinds of files, continued Translation unit: File that is output from the preprocessor, which modifies C code according to directives like #include, #define, #ifdef, etc. When you use gcc on Linux, translation units (.i files) usually don t get saved permanently. The same is true with Cygwin64 on Windows and for translation units produced in most other C and C++ development systems. So you ve probably never noticed them.

Preprocessor inputs: C source files /* file foo.h */ int foo(int arg); #define BAR 42 /* file foo.c */ #include foo.h int quux = 77; int foo(int arg) { quux++; return arg + BAR; } Preprocessor output: a translation unit int foo(int arg); int quux = 77; int foo(int arg) { quux++; return arg + 42 ; } Note: Some translation unit contents omitted to keep the slide simple.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 22/63 Building and running C programs: many kinds of files, continued again Assembly language file: File that is output from the compiler. Remember, the compiler is the tool that translates C code into assembly language. With gcc on Linux or Cygwin, assembly language files (.s files), like translation units, usually don t get saved permanently, so you ve probably never noticed them either.

Compiler input: a translation unit int foo(int arg); int quux = 77; int foo(int arg) { quux++; return arg + 42 ; } Note: Some translation unit contents omitted to keep the slide simple. Compiler output: An assembly language file.data.globl quux quux:.word 77.text.globl foo foo: la $t0, quux lw $t1, ($t0) addi $t2, $t1, 1 sw $t2, ($t0) addi $v0, $a0, 42 jr $ra

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 24/63 Building and running C programs: many kinds of files, continued further Source code, library headers, translation units, and assembly language files are all text files sequences of ASCII codes, organized in lines, editable with a text editor. Files yet to be seen object files, library machine code files, executable files are binary files, full mostly of machine code for instructions and base two representations of numbers.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 25/63 Building and running C programs: many kinds of files, on and on... An object file is output from the assembler. Let s make a sketch of typical object file organization.

Assembler input: An assembly language file.data.globl quux quux:.word 77.text.globl foo foo: la $t0, quux lw $t1, ($t0) addi $t2, $t1, 1 sw $t2, ($t0) addi $v0, $a0, 42 jr $ra Assembler output: An object file header machine code for instructions of procedure foo 32-bit base two representation of 77 relocation info and symbol table text segment data segment

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 27/63 Building and running C programs: many kinds of files, second last slide... Library machine code files contain instructions and static data belonging to library procedures such as printf, fopen, fprintf, strcpy, and thousands more. These files occupy many megabytes or gigabytes in the file system of a modern OS. Chunks of machine code and data can be copied out of these files as needed when building executable files.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 28/63 Building and running C programs: many kinds of files, finally done! An executable file is created by the linker, the last tool in the chain. The linker combines one or more object files; information from the library. Suppose an executable is built from object files alpha.o and beta.o. Let s sketch the organization of the executable file.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 29/63 Linux, Cygwin64, and other platforms These slides were originally written under the assumption that the platform used to build and run programs would be Linux. Building and running programs with Cygwin64 on top of Microsoft Windows is very similar to doing the same things on Linux. So I ve edited all the slides in this slide set to refer to Cygwin64 instead of Linux. Please keep in mind that all of the ideas here apply not only to Cygwin64 but also to important OSes such as Linux, FreeBSD and OpenBSD. (The ideas also apply to macos, for the most part, for now, but might not work so well with future macos versions.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 30/63 Running an executable Suppose you have an executable file called a.exe in the current directory of your Cygwin64 terminal window. Suppose you type and enter the command./a.exe Cygwin64 copies the text and data segments from a.exe into main memory. (That s an oversimplified model a system called demand paging reduces the amount of copying needed to get a program started, but I don t want to get into the complex details of that here.) Cygwin64 starts the program by making the PC point to the start of the in-memory text segment.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 31/63 Running an executable, not just in a terminal window These days, users of desktops, laptops, tablets, and smartphones usually launch programs via some kind of Start menu, or by clicking or double-clicking or tapping a program icon or file icon. At the operating system level, that s the same as entering a command line: text and data segments get copied from an executable file to memory, then the PC gets pointed to the start of the in-memory text segment.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 32/63 Review of the toolchain: running gcc Example command: gcc aa.c bb.c Students might say, informally, that this is running the compiler. Better-educated students would say that this is running a bunch of programs that are tools in the toolchain. What sequence of programs gets run?

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 33/63 Outline of Slide Set 4 Encoding of MIPS branch and jump instructions Building and running executables starting from C source code Linking and Libraries Notes about some confusing words

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 34/63 Linking and Libraries Topics in the upcoming slides: What the linker does How symbol tables and relocation information in object files help the linker. Comparison of static linking with dynamic linking.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 35/63 Review: object files and executable files Contents of an object file: machine code (in text segment), initial values for static data (in data segment), symbol table, relocation info. An object file is NOT a runnable program! The linker makes an executable file by combining one or more object files with instructions and data from library files. An executable file IS a runnable program!

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 36/63 Symbol tables, relocation information, and linking Symbol tables and relocation information are sections within object files; the roles of these sections haven t yet been explained. These two sections play key roles in helping the linker to insert important pieces of addresses into instructions in executable files.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 37/63 The symbol table The symbol table is a part of an object file that lists.globl symbols from the A.L. file used by the assembler to generate the object file. In this context, symbol means the same thing as label. For each symbol coming from a text segment, the table gives the offset of that symbol relative to the start of the text segment in the object file. Similarly, for each symbol coming from a data segment, the table gives the offset of that symbol relative to the start of the data segment in the object file.

# A.L. example to help explain symbol tables and relocation info..text.globl foo foo: [... some instructions omitted to save space on slide... ] jal quux [... more instructions omitted to save space on slide... ] jal bar [... more instructions omitted to save space on slide... ] jr $ra.data.globl my_array my_array:.word 0x100, 0x200, 0x300.globl my_int my_int:.word 0x9999.text.globl bar bar: [... some instructions omitted to save space on slide... ] la $t0, my_int [... more instructions omitted to save space on slide... ] jr $ra

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 39/63 Attention, regarding the previous slide That is obviously not a complete A.L. program procedures main, quux and possibly other important things were not defined there. So to make an executable file, the linker will have to combine the object file that comes from that A.L. file with one or more other object files.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 40/63 Symbol table example For the A.L. file from two slides back, let s assume... foo has 18 instructions in total; jal quux is the 5th instruction in foo; jal bar is the 11th instruction in foo; la $t0, my_int generates lui and ori instructions that will be the 5th and 6th instructions of bar. Let s sketch out the text segment, data segment, and symbol table that will appear in the object file.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 41/63 Problems solved by relocation information, Part 1 Consider jal quux in our A.L. example. The function quux is not defined in the given A.L. file, so it must be defined in some other file. The assembler is supposed to encode the jal instruction in the text segment of the object file, but can t because the assembler doesn t know what address information to put into bits 25 0 of the machine code for the instruction.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 42/63 Problems solved by relocation information, Part 2 [This slide has been corrected to give the la pseudoinstruction a destination GPR.] Now consider jal bar and la $t0, my_int in our A.L. example. Does the assembler have enough information to completely encode the jal instruction, and the lui and ori instructions that will be needed for the la pseudoinstruction?

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 43/63 Solution to problems: role of assembler The assembler partially encodes jal (and j) instructions, but just puts zero bits in the address fields, and makes notes in the relocation info about what must be fixed. The assembler partially encodes lui and ori instructions, but just puts zero bits in the 16-bit constant fields, and makes notes in relocation info about what must be fixed. Let s sketch the relocation info for our A.L. example.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 44/63 Solution to problems: role of linker The linker must combine multiple text segments and multiple data segments to build a single text segment and a single data segment for the executable file. The linker unlike the assembler knows what base addresses (for example, 0x0040_0000 and 0x1001_0000) are expected by the operating system for.text and.data segments. So the linker can compute the addresses for all instructions and data items. The linker uses symbol tables and relocation information to insert correct pieces of addresses into instructions such as jal, j, lui, and ori.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 45/63 Example of a common error seen by gcc users A file called joe.c... void foo(void); int main(void) { foo(); return 0; } Will the command gcc joe.c succeed? Why or why not?

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 46/63 Error message from command gcc joe.c run on an a Cygwin64 system... /tmp/ccyl2a05.o:joe.c:(.text+0xe): undefined reference to foo /tmp/ccyl2a05.o:joe.c:(.text+0xe): relocation truncated to fit: R_X86_64_PC32 against undefined symbol foo collect2: error: ld returned 1 exit status

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 47/63 Explanation of a common gcc error What happened? Did the compiler have a problem? Did the assembler have a problem? Did the linker have a problem?

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 48/63 Static vs. Dynamic Linking Slides so far have presented the process for creating a kind of executable file called a statically-linked executable. Most executable files on current Mac, Windows, or Linux operating systems are a different kind, called dynamically-linked. (But it s still possible to create and run statically-linked executables on these systems.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 49/63 Contents of a statically-linked executable file (review from earlier slides) Executable file header (information about sizes and layouts of other segments). Text segment: start-up code; machine code from object files; machine code for all necessary library procedures. Data segment: initial values for static data from object files; initial values for static data belonging to library procedures.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 50/63 Contents of a dynamically-linked executable file Executable file header (information about sizes and layouts of other segments). Text segment: start-up code; machine code from object files; Data segment: initial values for static data from object files; Information about where to find library instructions and library data in the file system.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 51/63 Running a dynamically-linked executable Text and data segments get copied into memory from the executable file (as is the case with a statically-linked executable). It is the operating system s responsibility to make sure library machine code and data are in memory when needed. Often, library machine code is already in memory when a program starts, because some other running programs also need it this is useful sharing of memory by multiple running programs.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 52/63 Running a dynamically-linked executable: example C code Source code blastoff.c... #include <stdio.h> int main(void) { int count; for (count = 10; count > 0; count--) printf("%d... \n", count); printf("blastoff!\n"); return 0; } Command to build executable: gcc blastoff.c -o blastoff

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 53/63 Running a dynamically-linked executable: example, continued What is in the executable file called blastoff? What important and relevant information is NOT in the executable file called blastoff? What happens when blastoff is run? (Note: On Cygwin64, the name of the executable file would be blastoff.exe.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 54/63 Static linking: Advantages Compared to dynamic linking, static linking is easy to understand and implement. Installing software is relatively easy to manage it may require placing only one file, the executable, in the right place.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 55/63 Static linking: Disadvantages Executables are big. A collection of statically-linked executables contains many copies of the same library machine code and data. This wastes space in the file system. If many programs are running on a multi-tasking operating system, many copies of the same library machine code may be in memory. This wastes memory and hurts performance. Installed executables can t take advantage of library upgrades such as bug fixes and performance improvements.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 56/63 Dynamic linking: Advantages Essentially, all the main disadvantages of static linking are overcome: Executables are smaller, less total memory is needed to run multiple executables at the same time, executables can benefit from library upgrades.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 57/63 Dynamic linking: Disadvantages It s harder to understand and implement than static linking. Software installation can be complicated are all the right versions of library files for dynamic linking available to support all the executables? (Failures in this area on older versions of Microsoft Windows were called DLL hell but that was a long time ago.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 58/63 Quick Tour of C library, for Cygwin64 in Winter 2018 Cygwin64 is a complicated mess that mixes Linux-like stuff with Microsoft Windows, so a quick tour is impossible. (Previous years versions of these slides were able to say a few useful things about where important library files were located on Linux systems in ICT 320.)

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 59/63 Outline of Slide Set 4 Encoding of MIPS branch and jump instructions Building and running executables starting from C source code Linking and Libraries Notes about some confusing words

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 60/63 Notes about some confusing words Computer programming terminology and jargon have evolved (or have just accumulated chaotically) over the past several decades. Some of the choices that were made were not very helpful to students trying to learn about computers and programming!

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 61/63 Notes about some confusing words: text The text segment, where the instructions go in an object file or executable file, is not related to the concept of text as a sequence of character codes in ASCII or some other character set!

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 62/63 Notes about some confusing words: object The term object file has nothing to do with the concept of an object in object-oriented programming in languages such as C++, Java, Objective-C, Python, etc.

ENCM 369 Winter 2018 Section 01 Slide Set 4 slide 63/63 Notes about some confusing words: link The word link in the MIPS jump-and-link instruction means, leave a return address to allow return from a callee procedure. The word link in the term linker means, connect together procedures and data from one or more object files to make an executable program. So there are two different meanings for link.