Summary for Midterm:  it’s open book, open notes, can bring posted solutions and your own hw.

Note practice midterm, solutions linked to class web page.

Is there a theme connecting all the things we’ve covered so far?  I think so, as follows.  We have concentrated on the transitions, or handoffs, between the three major players, the user code, kernel code, and hardware.  A major concern is user-kernel separation, needed to keep the system well-understood and secure.  This separation is implemented by being very careful about transitions.  The user code is kept “bottled up” in its virtual machine.  System calls communicate a small amount of information (the syscall # and arguments, return value) across the user-kernel boundary and back.

Hardware-user code relationship

• The only “device registers” that user code is allowed to access are the CPU general registers.  The UART registers, the disk controller registers, etc., are hidden away, invisible to the user code.  The user code may only use them via system calls.
• The user code runs only in the user mode of the CPU (except in our homework projects, where we have simplified things), so it may only use the non-privileged instructions of the CPU.  The system call instruction itself is (of course!) a non-privileged instruction.
• The user code can see the IF bit in EFLAGS, but it can’t change it.  All interrupts are handled by the kernel.  They can (and usually do) happen during user code execution, but are invisible to the user code (unless it is doing very high precision timing.)
• All the user program can see is a flat address space, with code, data, stack, and DLLs, and the CPU.  In the user code are the system call instructions.
• The user code can use multiple CPUs via threads, setup and managed via system calls.

Hardware-kernel code relationship

• The kernel runs in the “kernel mode” of the CPU, and thus can do all CPU instructions.
• The device registers are accessible to the kernel, and the kernel contains “device drivers” that are software modules devoted to managing a certain kind of hardware.
• The kernel handles all interrupts, so all interrupt handlers are kernel code.
• The kernel uses memory for code, data, much like other programs, but has many stacks, one for each thread.
• The kernel can do the cli, sti instructions (or equivalent on other CPUs), and so IF=1 part of the time, IF=0 other times as the kernel is running.  Putting IF=0 is a kind of kernel mutex usable for uniprocessors.

User-Kernel relationship

• The whole user virtual machine is set up so that the user code can be trusted to use the CPU for hundreds of instructions at a time without intervention of the kernel.  This is key to performance.
• The three major ways to go from user code to kernel execution is by system calls, page faults, and interrupts.  Other traps are possible too, such as illegal-instruction traps. All these cause the execution of the CPU’s interrupt/trap cycle, with its access to the IDT (or similar table for another CPU) to specify what kernel function to execute.
• System call instructions are part of the hardware assistance needed to allow user code to safely call on the kernel.

Hardware/Software terms that sometimes cause confusion on exams

instruction:  like mov, push, in, out, etc., belong to “instruction set”, used in assembly language

register:  array of hardware bits, part of CPU, memory, i/o devices, holds a value in its bits

CPU register:  like eax, ecx, esp, etc. for x86.

device register:  like UART’s transmit register, receiver reg, LSR

port or i/o port: 16-bit number providing an address for various device registers, in x86 architecture.  Ex: 0x2f8 for COM2’s transmit register.

memory address: 32-bit number of a byte of memory, in separate space from i/o ports.

interrupt vector: the address of the interrupt handler, held in IDT[nn], and specifying the entry point of the assembler interrupt handler

command: string used to tell a program what to do, ex: “ls” is a shell command

system call: two meanings, system API call such as write(fd, buf, nbytes), certain instruction execution (int, ta)

user stack: one for each thread, holds execution state of user code.

kernel stack: one for each thread, holds execution state of current system call execution, or is empty

interrupt stack: we assume this is built on top of the kernel stack of the thread executing at the time of the int.

Midterm Reading: midterm is open books, open notes, handouts, open solutions—yours or mine or both.

Tan., Ch. 1, but light on history

Ch 2 to pg. 123, plus 132-146 (not explicitly covered in class, but most of the essential concepts have been covered)

Ch 5 to pg. 294. 

Ch. 10 (UNIX) to pg. 704, then sec. 10.7, UNIX Security

Ch. 11 (Windows): 11.2 Win32 API, skip registry, 11.3 to pg. 779, 11.4 to pg. 802 (processes, threads), 11.8 Windows Security

Lecture notes through today.

hw1 ideas:

• interrupt programming: PIC, IDT, int. handler, etc.
• interrupt-driven i/o: read-ahead, write-behind, also vs. programmed i/o
• use of cli/sti for mutex, need for it with queue accessed two ways
• use of kprintf for debugging (itself using programmed i/o with interrupts off)
• device independent i/o system, and how to do it in C (array of structs with function pointers)
• not really an “OS”, just a library of useful things
• also, first part, system calls in Solaris

hw2 ideas:

• mechanism of system calls: put args and syscall# in registers, special instruction, use of IDT
• trap vector, system call trap handler: as, C parts, like int handler, but runs with IF=1 to start
• startup module for user program
• library envelope routine for system calls
• first OS: should be able to classify each piece of code as user or kernel

hw3—not yet!