1/10/2016\course\cpeg323-07F\Topic2c-323.ppt1 Topic2c High-Level languages and System Software (Toolchain) Introduction to Computer Systems Engineering

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Topic2c High-Level languages and

System Software(Toolchain)

Introduction to Computer Systems Engineering

(CPEG 323)


Reading List

• Slides: Topic2c

• Operating System and Compiler Books


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?


C program

Compiler

Assembly language program

Assembler

Object: Machine language module Object: Library routine (machine language)

Linker

Executable: Machine language program

Loader

Memory

A Typical Toolchain and its Translation Hierarchy

ToolchainToolchain

DebuggerUtility tools


Tool Chain

Two good examples SimpleScalar

www.simplescalar.com GNUPro

www.intel.com (Search GNUPro)


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)


Tool Chaingcc –v –O0 –o foo foo.c

(old Step 0: cpp)

foo.i

Step 1: cc1 - compiler

foo.s

Step 2: as - assembler

foo.o

Step 3: ld - linker

foo


Tool Chain - Preprocessor

Functionality Header files Definitions Conditional compilation Pragma(Preprocessor Directives ) Delete the comments


Tool Chain - Compiler

Transform a program from high level language to assembly language (or machine language)Optimizations


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.


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 ?


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


Caller save

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

Function A Function B

How to save ? - save to stack sw $10, 20(sp)


Callee Save

Add $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

Function A Function B


Caller Save / Callee Save

When to use caller save register ? TEMPORARY VARIABLE Also called Scratch Register

When to use callee save register ? GLOBAL VARIABLE


Tool Chain - Assembler

Transform assembly code into binary (machine code)


Tool Chain - Linker

Linking – resolve symbolsRelocation – assign memory address


Tool Chain - loader

Cannot see by userDone by Shell and OS kernel


Tool Chain – Library

Libc/Libm


Tool Chain

Objdump See the memory layout and sections Symbol table Disassembly code Relocation information


C program

Compiler

Assembly language program

Assembler

Object: Machine language module Object: Library routine (machine language)

Linker

Executable: Machine language program

Loader

Memory

Creating an Executable File(User view)


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


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.”


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


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.


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


Dynamic vs. Static Library

Dynamic library: processes share one copy of the code Static library: each process has its own copy of the codeWhy?


Dynamic Shared Library ?

Also called dynamic linked library

Call printfCall printf Call printfCall printf

Program A Program B

Printf: Printf: DSO(dynamic shared

object )Table

DSO(dynamic shared

object )Table


Static Library

Call printfCall printf

Call printfCall printf

Program A Program B

Printf: Printf:

Printf: Printf:


Comparison

Dynamic Less memory space Less disk space Most of the case: Slower

Static More memory size More disk size Most of the case: faster


Debugger

Instruction level debuggerSource level debuggerMajor techniques Ptrace (POSIX API. on Linux/Unix

system) Embedded or raw machine

Software trap Single step mode


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)


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

Run a Executable File (OS View)


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.

(cont’d)


Typical Layout of an Executable File

stack

Dynamic data

Reserved

Text

Static data

$sp 7fff ffffhex

$gp 1000 8000hex

1000 8000 hex

pc 0040 0000 hex

(From Patterson and Hennessy, p. 152; COPYRIGHT 1988 MORGAN KAUFMANN PUBLISHERS, INC. ALL RRIGHTS RESERVED)


The Role of the OS KernelThe 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.


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


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


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


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


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


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


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


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)


Toolchain Review

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.

Caller save /Callee save register


Program Translates (Summary)

a.cInt I;…Printf(“Answer is %d”, i)…

compilea.s

.text…

Parameter pass

Jal printf

…

.data

assembly

a.o

323: Parameter pass

444: Jal reloc add.<printf>

Ref: <printf> 444

a.o- relocation table


Program Translates(cont.)

a.o323: Parameter pass

444: Jal reloc add.<printf>

ret

555:

Create stackProcess args$a0 <- file ID$a1 <- adds. Of heap$a2 <- length of string$v0 <- 4 (write)Syscallret

printf.o

Ref: <printf> 444

Def: <printf> 555

a.o- relocation table

printf.o- relocation table

L323: Parameter pass

L444: Jal L555

L555:

Create stackProcess args$a0 <- 1$a1 <- adds. Of heap$a2 <- 13$v0 <- 4Syscallret

linker

Executable file

Start-up routine

__main: jal main jal exit


Program Translates (Cont.)

323: Parameter pass

444: J reloc 555555:

Create stackProcess args$a0 <- 1$a1 <- adds. Of heap$a2 <- 13$v0 <- 4Syscallret.

Executable file

(software trap)

user mode privileged mode

Set status register

Jal trap(4) handler

trap(4) handlerwhat trap -- syscall

$v0 ? ------- 4

Jal 4th function driver

Transfer 13 bytes to graphic display

4th function driver

__main: jal main jal exit

main:

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1/10/2016\course\cpeg323-07F\Topic2c-323.ppt1 Topic2c High-Level languages and System Software (Toolchain) Introduction to Computer Systems Engineering