Computer OrganizationCS224

Fall 2012

Lesson 12

Synchronization Two processors or threads sharing an area of memory

P1 writes, then P2 reads Data race if P1 and P2 don’t synchronize

- Result depends of order of accesses !!

Software synchronization routines needed Lock and unlock, etc. To implement mutual exclusion to shared memory

Hardware support required Atomic read-and-modify memory operation No other access to the location allowed between the read and write Used for e.g. atomic swap of register ↔ memory Could be a single instruction (as in some processors) Or an atomic pair of instructions (as in MIPS and others)

§2.11 Parallelism

and Instructions: Synchronization

Atomic Exchange Support

Atomic exchange (atomic swap) – interchanges a value in a register for a value in memory atomically, i.e., as one operation (instruction)

Implementing an atomic exchange would require both a memory read and a memory write in a single, uninterruptable instruction. An alternative is to have a pair of specially configured instructions

ll $t1, 0($s1) #load linked

sc $t0, 0($s1) #store conditional

Atomic Exchange with ll and sc

If the contents of the memory location specified by the ll are changed before the sc to the same address occurs, the sc fails (returns a zero)

try: add $t0, $zero, $s4 #$t0=$s4 (exchange value)ll $t1, 0($s1) #load memory value to

$t1sc $t0, 0($s1) #try to store exchange

#value to memory, if fail #$t0 will be 0

beq $t0, $zero, try #try again on failureadd $s4, $zero, $t1 #load value in $s4

If the value in memory between the ll and the sc instructions changes, then sc returns a 0 in $t0 causing the code sequence to try again.

The C Code Translation Hierarchy

C program

compiler

assembly code

assembler

object code library routines

executable

linker

loader

memory

machine code

§2.12 Translating and S

tarting a Program

Assembler Pseudoinstructions

Most assembler instructions represent machine instructions one-to-one

Pseudoinstructions: figments of the assembler’s imagination

move $t0, $t1 → add $t0, $zero, $t1

blt $t0, $t1, L → slt $at, $t0, $t1bne $at, $zero, L

$at (register 1): assembler temporary

Producing an Object Code Module

Assembler (or compiler) translates program into machine instructions, makes an object module

Provides information for building a complete program from the pieces

Header: described contents of object module Text segment: translated instructions Static data segment: data allocated for the life of the

program Relocation info: for contents that depend on absolute

location of loaded program Symbol table: global definitions and external refs Debug info: for associating with source code

Linking Object Modules

Produces an executable image (.exe file)1.Merges segments

2.Resolve labels (determine their addresses)

3.Patch location-dependent and external refs

Could leave location dependencies for fixing later with a relocating loader

But with virtual memory, no need to do this Program can be loaded into absolute location in virtual memory

space

Loading a Program

Load from image file on disk into memory1.Read header to determine segment sizes for text and data

2.Create virtual address space (large enough for text and data)

3.Copy text and initialized data into memory

4.Copy parameters for main program (if any) onto stack

5. Initialize registers (including $sp, $fp, $gp)

6. Jump to startup routine

- Copies arguments to $a0, … and calls main

- When main returns, do exit syscall

Dynamic Linking

Only link/load library procedure when it is called Requires procedure code to be relocatable Avoids image bloat caused by static linking of all

(transitively) referenced libraries Automatically picks up new library versions

Lazy Linkage

Indirection table

Stub: loads routine ID,jumps to linker/loader

Linker/loader code

Dynamicallymapped code

Starting Java Applications

Simple portable instruction set for

the JVM

Interprets bytecodes

Compiles bytecodes of “hot” methods

into native code for host

machine