Discussion Week 5TA: Kyle Dewey

OverviewHW 3.10 and 6.2 reviewBinary formatsSystem call execution in NACHOSMemory management in NACHOSI/O in NACHOS

Homework 3.10Identify the values of pid at lines A, B, C, and D. Assume that the actual pids of the parent and child are 2600 and 2603, respectively.

Homework 6.2The Cigarette-Smokers ProblemAgentMatch SmokerPaper SmokerTobacco SmokerTable(holds two of three items)

Problem SpecificsAgent places two itemsSmoker with remaining item grabs the two and smokesThe process repeats

Java Implementation

Binary Formats

NOFFNACHOS Object File FormatMagic Number (0xBADFAD)Code (Text)Initialized Data (Data)Uninitialized Data (BSS)

Why Bother?CPU sees only a stream of instructionsAll gets loaded into memory anyway

AdvantageTells OS roughly how portions will be usedOptimizations possibleShare (reentrant) code and constant dataPrevent execution of non-code regions

System Calls Revisited

System Call Executionread()User SpaceKernel Spaceread()readFromDisk()read()

User -> KernelProblem: kernel space and user space are enforced by hardwareHardware must be informed of jump

Solution?Instruction to specify the levelBy necessity, it is privilegedNeed kernel space to tell the system were in kernel space - catch 22

Existing MachineryInterrupts are serviced by the kernelGenerated from other devices, often I/OPreempt all else and enter the kernelThe routines that service interrupts are called interrupt service routines - ISRs

InterruptsHard DriveISR 1ISR 2ISR 3Int 1MemoryCPU

Using InterruptsTrigger a software interruptKernel mode entered synchronouslyParameters can be passed in registers, in a specific memory location, etc.Note that the actual mechanism and lingo is hardware dependent

MIPS System CallsMIPS has the syscall instructionProcessor throws a system call exception, triggering the OS system call service routineBy convention, the syscall ID is in $v0, and arguments are passed in $a0 and $a1

MIPS System CallsAssume we want the system call with ID 5This call takes no argumentsaddi $v0, $zero, 5syscallCPUSyscall ISRSyscall 5 Handler

code/userprog/exception.cccode/userprog/syscall.hcode/test/start.s

Memory Management

Project #2 MemoryPhysical = virtual (until Project #3)Must using pagingNeed to allocate and free pages as requested

NACHOS MemoryDoes not have much128 byte pages32 pages total8 pages for each process stack + data + codeSimple bitmap is sufficient to record what is and is not used

Contiguous MemorySince physical = virtual, served memory requests must be contiguousI.e. if a process requests 5 pages, they must be contiguous*Could* do compaction, but this is a terrible idea

Fork() ExampleUsed - P1FreeFreeFreeUsed - P2Used - P1FreeFreeUsed - P2Used - P3P1 fork()s P3Memory by pageMemory by page

Exit() ExampleUsed - P1FreeFreeUsed - P2Used - P3P2 exit()sMemory by pageMemory by pageUsed - P1FreeFreeUsed - P3Free

Getting PagesMemory is available through:machine->mainMemoryMerely array of 1 byte charactersNeed to split into pages on your own

Memory and ConcurrencyMultiple processes may request pages at the same timeOnly one may get any given pageSynchronization primitives from Project #1 will have to be usedMake sure they work correctly!

I/O Syscalls

NACHOS DiskDo not need to worry about this until Project 3I/O syscalls for Project 2 utilize Linuxs existing syscalls for file I/O directly

I/O SyscallsActually implement Read() and Write(), NOT readAt() and writeAt()readAt() and writeAt()s provided implementations are sufficient to implement Read() and Write()

Files and ConcurrencyProcess A prints Hello world!Process B prints Goodbye cruel world!Hello woGoodbye crld!ruel world!

Files and ConcurrencyDetermining what needs to be locked may be difficultMay have separate things that need lockingMay need multiple locks for distinct resourcesConcurrent reads are OK, but not concurrent writes

Open File SemanticsSemantics of Fork() are that child processes inherit open filesRead() and Write() can only manipulate open filesIf a process will not close its files upon Exit(), then the OS must do so

Open FilesWhich files are opened must be recorded in the PCBThis allows for all aforementioned behaviorsAlso allows for an offset for subsequent Read() and Write() requests

ConsoleRead() and Write() may also manipulate the consoleConsole is not opened or closedConstants specifying console usage are in syscall.h

CaveatsThe given code is really getting buggyProvided code is also getting really ugly

How-To ImplementProject #2 has a step-by-step implementation guide at http://www.cs.ucsb.edu/~cs170/projects/homework_2guide.htmlPlease read carefully

-BSS: Historically Block Started by Symbol, psuedo-op in an antique assembler. Backronymed to Better Save Space - variables that dont currently have a value, but may in the future.-Reentrant code does not modify itself. Most code has this property; the system fights you tooth and nail if you try to break it-OpenBSD will not allow regions of memory to be executed that were not marked as code. This prevents many buffer overflow attacks which write to non-code regions and try to execute those regions.-Note that the Int 1 is just meant to signal that ISR 1 should be used. This is all very hardware-dependent, but the general model of interrupt->CPU->ISR remains the same.-MIPS has the syscall instruction, which causes the CPU to throw a special exception that is reserved for system calls-x86 has the int instruction, which triggers a software interrupt-They do the same thing, but the terminology differs-Why is it a terrible idea?-Expensive-We have virtual memory to avoid this - making people do compaction would teach bad practice-Student pointed out a memory leak in bitmap.cc-I wont go over it myself because it requires a lot of browsing through code