1. Topic2d High-Level languages and System Software (Toolchain)
Introduction to Computer Systems Engineering
(CPEG 323)
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2. Reading List Slides: Topic2d Operating System and Compiler Books
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3. Tool Chain
toolchain
A collection of system softwares used to develop for a particular hardware target
If you designed a new processor, what is the basic system software tool set you need?
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4. A Typical Toolchain and its Translation Hierarchy
C program
Compiler
Assembly language program
Assembler
Object: Machine language module
Object: Library routine (machine language)
Linker
Utility tools
Executable: Machine language program
Debugger
Loader
Memory
A Typical Toolchain and its Translation Hierarchy
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5. Tool Chain Two good examples SimpleScalar GNUPro www.simplescalar.com
(Search GNUPro)
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6. Tool Chain Basic Set: • Compilers: C, C++, Fortran, and etc.
• Binary utilities: assembler, linker, objdump, ar, nm
• Debugger
• Simulator (functional / cycle-accurate)
• Others: performance monitor (VTune of Intel)
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7. Tool Chain gcc –v –O0 –o foo foo.c (old Step 0: cpp) foo.i
Step 1: cc compiler
foo.s
Step 2: as assembler
foo.o
Step 3: ld linker
foo
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8. Tool Chain - Preprocessor
Functionality
Header files
Definitions
Conditional compilation
Pragma(Preprocessor Directives )
Delete the comments
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9. Tool Chain - Compiler
Transform a program from high level language to assembly language (or machine language)
Optimizations
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10. Parameter Passing
Caller save. The calling procedure (caller) is responsible for saving and restoring any registers that must be preserved across the call. The called procedure (callee) can then modify any register without constraint.
Callee save. The callee is responsible for saving and restoring any registers that it might use. The caller uses registers without worrying about restoring them after a call.
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11. Saving Registers
If you call a function, whatever you have in $s0 to $s7 is guaranteed to be there when the function gets back to you
But registers $t0 - $t9 are fair game to be reused by the function
What are the alternatives?
- Save nothing?
- Save everything?
Why caller/callee save ?
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12. Why caller save / callee save?
If all caller save ?
Even callee doesn’t kill any of the saved registers – waste of cycles and memory resource
If all callee save ?
Callee has to save all the register (which will be used by callee), even caller doesn’t use them
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13. Caller save Function A Function B How to save ? - save to stack
Add $10, $11,$12
Save $10, $12, $13
Jal B
Restore $10, $12, $13
Sub $11, $2, $12
Mul $12, $10, $13
Add $2, $4, $5
Br $31
How to save ? - save to stack
sw $10, 20(sp)
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14. Callee Save Function A Function B Add $10, $11,$12 Save $10, $11, $12
Jal B
Sub $11, $2, $12
Mul $12, $10, $13
Save $10, $11, $12
(if they are used in B)
Lw $10, 4(sp)
Add $11, $8, $9
Sub $12, $11, $10
Mul $2, $12, $11
Restore $10, $11, $12
Br $31
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15. Caller Save / Callee Save
When to use caller save register ?
TEMPORARY VARIABLE
Also called Scratch Register
When to use callee save register ?
GLOBAL VARIABLE
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16. Tool Chain - Assembler
Transform assembly code into binary (machine code)
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17. Tool Chain - Linker Linking – resolve symbols
Relocation – assign memory address
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18. Tool Chain - loader Cannot see by user Done by Shell and OS kernel
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19. Tool Chain – Library Libc/Libm 2/27/2019
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20. Tool Chain Objdump See the memory layout and sections Symbol table
Disassembly code
Relocation information
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21. Creating an Executable File(User view)
C program
Compiler
Assembly language program
Assembler
Object: Machine language module
Object: Library routine (machine language)
Linker
Executable: Machine language program
Loader
Memory
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22. Compiling/Assembling a Program
Code converted from high-level to machine language (binary)
Each source file converted to a separate “object file”
in Unix, object files have extension .o
This is not executable (yet)!
Intended to be combined with other modules, not stand-alone
May not have everything we need (e.g., a main () function)
Functions not assigned specific locations in memory
Each function given a “relocation table”
- This table tells exactly which addresses need to be resolved
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23. Relocation Table Suppose function f() in module a.c calls g() in b.c
a.c should declare g with extern (directly or in .h file)
Relocation table in a.o says something like: ”the jal at the 52nd instruction in f calls g, but I don’t know where g is.”
Relocation table in b.o says something like: “I have a function g, which starts at location 628 in my file.”
g unresolved even if local (why?)
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24. Linking a program
Linker combines one or more object files into a proper executable
Linker determines which functions needed, discards the rest
All needed functions put one after the other in text segment
Linker resolves all labels; for instance
Function f() in a.o calls g() in b.o
Linker knows where it put g(), so it fixes the jal in f()
Linker includes extra code:
Initialization code before call to main()
“Cleanup” code after main() returns
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25. Loading
A program that links without an error can be run. Before being run, the program resides in a file on secondary storage, such as a disk. On Unix system, the operating system kernel brings a program into memory and starts running.
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26. Libraries
Libraries contain functions intended to be shared & reused, e.g.,
C library: printf(), malloc(), strcmp(), sin(), cos()
STL (Standard Template Library) in C++
Big software projects may make their own libraries
Static libraries (* .a in Unix) made part of the executable by linker
Dynamic libraries (* .so in Unix, * .dll in Windows) combined at runtime
Executable still has relocation table of unresolved function calls
Loader does the final resolution when you execute the program
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27. Dynamic vs. Static Library
Dynamic library: processes share one copy of the code
Static library: each process has its own copy of the code
Why?
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28. Dynamic Shared Library ?
Also called dynamic linked library
Program A
Program B
Call printf
Call printf
Printf:
DSO(
dynamic shared
object )Table
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29. Static Library Program B Program A Call printf Printf: Call printf
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30. Comparison Dynamic Static Less memory space Less disk space
Most of the case: Slower
Static
More memory size
More disk size
Most of the case: faster
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31. Debugger Instruction level debugger Source level debugger
Major techniques
Ptrace (POSIX API. on Linux/Unix system)
Embedded or raw machine
Software trap
Single step mode
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32. Debugger
“Source-level” debugger lets you step through your source code
Requires extra information attached to executable
Location and type of every function and variable
First instruction address corresponding to each line of source
Usually requires extra switches to compiler and linker, e.g., -g
Two popular graphical debuggers are ddd and xxgdb (on ECE/CIS machines)
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33. Run a Executable File (OS View)
The operating system performs the following steps:
1. Reads the executable file’s header to determine the size of the text and data segments.
2. “Establish” a new address space (e.g. via the creation of a new page table) for the program. This address space is large enough to hold the text and data segments, along with a stack segment
3. Copies instructions and data from the executable file into the new address space
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34. 4. Copies arguments passed to the program onto the stack.
(cont’d)
4. Copies arguments passed to the program onto the stack.
5. Initializes the machine registers. In general, most registers are cleared but the stack pointer must be assigned the address of the first free stack location
6. Jumps to a start-up routine that copies the program’s arguments from the stack to registers and calls the program’s main routine. If the main routine returns, the start-up routine terminates the program with the exit system call.
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35. Typical Layout of an Executable File
stack
Dynamic date
Reserved
Text
Static data
$sp fff ffff
hex
$gp pc
(From Patterson and Hennessy, p. 152; COPYRIGHT 1988 MORGAN KAUFMANN PUBLISHERS, INC. ALL RRIGHTS RESERVED)
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36. The Role of the OS Kernel
The OS “kernel” performs the following essential functions:
Manages resources (memory, disks, I/O) – mostly via “drivers”
Switches between users (in a multi-user system such as copland)
Provides convenient functions for applications to access resources
Protects users from one another
Provides essential “glue”, e.g., support for loaders
For this to work efficiently, the CPU must have some support for the kernel.
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37. Processor Support for the OS kernel
Most processors have at least 2 distinct levels or “modes”:
“Supervisor” (or “privileged” or “kernel”) mode
“User” level (including “root” or “administrator”)
Lower levels can’t do some things, e.g., access the disk drive
CPU boots in kernel mode; drops to user mode to run user code
Early micros (such as 8086) lacked such modes
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38. Traps
So once we’re in user mode, how do we get back to the privileged mode? Through “traps” – exceptional or unusual conditions requiring intervention by the kernel:
Hardware error (divide by 0 or access to illegal memory address)
Hardware “interrupt” (Ethernet card got data; mouse clicked)
Clock signal telling multi-user OS to switch to another user
“Software trap” when user code requests something from kernel
PC reaches value stored in special “breakpoint” register
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39. Trap Handlers
When a trap occurs, the CPU:
Sets bits in special “status” reg., indicating the cause of the trap
Switches to privileged mode
Jumps to a “trap handler” (installed at boot time) at fixed location
Handler reads status bits and takes appropriate action
Return address saved, like jal instruction
* When kernel is done, a special instruction return to the user code, dropping into user mode automatically
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40. Software Traps
To do just about anything on the system involving shared resources (such as write to a file), the user code must ask the kernel to do it!
User code gets access to the kernel through “trap” instructions
“System calls” provided for operations such as writing files
A function call to a system call converted to a software trap
Args passed in the usual way (e.g., $a0-$a3 in MIPS)
In MIPS, use the “syscall” instruction
No operands in assemble-language instruction
Specify which system call you want by putting a value in $v0
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41. System Calls POSIX standard defines system calls and their numbers
For instance, call no. 4 is the write() function:
#include <unistd.h>
ssize_t write(int fildes,
cost void *buf, size_t nbyte);
Every open file is identified by a unique “file descriptor” (int)
This function writes nbyte bytes, starting at address buf, to the file
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42. Example: Call to printf()
User code a.c calls printf (“Answer is %d\n”, i);
printf() declared as an extern function in stdio.h
Compiler generates a.o with printf unresolved in relocation table
Data segment of a.o has string “Answer is %dl_” (NUL at end)
14 bytes, with local label (e.g., L314) in relocation table
Reference resolved when linked with libc (C library):
By linker if statically (e.g., -Bstatic in Sun CC)
B y loader if dynamically
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43. Calling printf()
Program sets args to printf (L314 and i)’ does jal printf
printf (still in user mode) does the following:
Creates new stack frame (as any non-leaf function should)
Processes args; makes new string “Answer is 42l” in heap
Creates args to write() function:
Constant 1 in $a0 (file descriptor 1 is stdout)
Address of heap string in $a1
Constant 13 in $a2
- Puts constant 4 in $v0 and does a syscall instruction
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44. Processing the Trap The CPU executes the syscall (trap) instruction:
Switches to privileged mode
Sets bits in status regs indicating trap caused by syscall
Jumps to trap handler
Trap handler checks status bits; sees trap came from syscall
Checks call # in $v0; fetches 4th entry in function table and jumps
System call transfers 13 bytes to low-level driver
- Driver writes them to graphics display (if normal stdout)
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45. Toolchain Review Caller save /Callee save register
Caller save register. The registers that the calling procedure (caller) is responsible for saving and restoring across the call. The called procedure (callee) can then modify the registers without constraint.
Callee save register. The registers that the callee is responsible for saving and restoring if it might use. The caller uses the registers without worrying about restoring them after a call.
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46. Program Translates a.c a.o compile a.s assembly a.o- relocation table
Int I; …
Printf(“Answer is %d”, i)
a.o
compile
a.s
323: Parameter pass
444: Jal reloc add.<printf>
assembly …
.text
Parameter pass
Jal printf
.data
a.o- relocation table
Ref: <printf>
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47. Program Translates(cont.)
printf.o
printf.o- relocation table
Executable file
555:
Create stack
Process args
$a0 <- file ID
$a1 <- adds. Of heap
$a2 <- length of string
$v0 <- 4 (write)
Syscall
ret
__main:
jal main
jal exit
Def: <printf>
Start-up
routine
L323: Parameter pass
L444: Jal L555
a.o
L555:
Create stack
Process args
$a0 <- 1
$a1 <- adds. Of heap
$a2 <- 13
$v0 <- 4
Syscall
ret
323: Parameter pass
444: Jal reloc add.<printf>
ret
linker
a.o- relocation table
Ref: <printf>
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48. Program Translates (Cont.)
Executable file
user mode
privileged mode
__main:
jal main
jal exit
main:
Set status register
Jal trap(4) handler
(software trap)
323: Parameter pass
444: J reloc 555
trap(4) handler
what trap -- syscall
$v0 ?
Jal 4th function driver
555:
Create stack
Process args
$a0 <- 1
$a1 <- adds. Of heap
$a2 <- 13
$v0 <- 4
Syscall
ret.
4th function driver
Transfer 13 bytes to graphic display
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