COM SCI 111
Lecture 2: Abstractions and Bootstrapping

Current Events

In a recent article, "Adviser Guides Obama Into the Google Age" , which was published by the New York Times on January 3, 2015, a former Google executive Megan Smith tries to resolve a culture clash of the Obama Administration as she works as the CTO (Chief Technology Officer). She attempts to solve the government's technology problems and upgrade still-used, ancient technology such as floppy disks.

Problem

Suppose that you're a paranoid person that wishes to keep secret data safe. How do you keep the data secure?

Solution

Write a simple toy program (wc) that can be trusted.
The program must be secure, so we have to write our own program to ensure that. We're so paranoid to the point that we cannot trust the Linux kernel or any Ubuntu distribution.
In addition, we cannot trust any operating system out there. However, what type of hardware is going to be run on when the following is run?

wc foo.txt

We assume that the machine has the following characteristics.

• White box desktop
• Intel Core i3-4160 processor: 3 MB L3 cache, 3.6 GHz
• 4 GiB RAM, dual channel DDR3 SDRAM 1600 MHz
• 1 TB hard drive, SATA, 7200 rpm
• Intel HD integrated graphics
• Disk controller

1. Plug the desktop into the wall.
2. Push the power button.
3. A 213406-word document appears on the screen.

ROM (Read-Only Memory)

Instead, we will put the program on hard drive. The instruction pointer will point to an instruction to read the first 40 KiB, containing the program, on disk.

Chain Loading

#/bin/sh
foo = bar

# Loads a different program
exec sed "s/foo/$foo/g"

BIOS (Basic Input/Output System)

MBR (Master Boot Record)

Disks and Disk Controller

The disk controller indirectly connects the disk. It has a cache that remembers data written onto disk. The CPU sends disk commands to the disk controller in the form of x86 instructions.
inb - takes in an address as an argument, grabs the byte from the disk controller, and stuffs it into that register.
outb - takes in an address, grabs the byte from the disk controller, and outputs a specified number of bytes.

The BIOS reads sector into 0x7c00.

The following are the steps to read a sector from disk.

1. Wait for it to be ready; look at 0x1f7, which is the status register.
2. Store the number of sectors in 0x1f2.
3. Store the sector offset - (0x1f3 - 6).
4. Write into 0x1f7 and store a bit pattern to perform READ.
5. Read 0x1f7 to wait for it to be ready. (15 ms)
6. Get the results from the disk controller's cache into RAM.

Actual Code

void main(void){ // boot loader
    for (int i = 0; i < 80; i++) // 40 KiB = 80 sectors
        read_ide_sector(i + 100, 0x20000 + i * 52);
    goto *0x20000;
}

void read_ide_sector(int s, int a){
    wait_for_ready();
    outb(0x1f2, 1);
    for (int i = 0; i < 4; i++)
        outb(0x1f3 + i, (s >> (8 * i)) & 0xff);
    outb(0x1f7, 0x20);
    wait_for_ready();
    insl(0x1f0, a, 128); // Copies data 4 bytes at a time
}

void wait_for_ready(void){
    while ((inb(0x1f7) & 0xc0) != 0x40)
        continue;
}

Main program for wc

void main(void){
    long long int nwords = 0;
    bool inword = false;
    int s = 90000;
    int len;
    do{
        char buf[513];
        buf[512] = 0;
        read_ide_sector(s++, (int) buf);
        len = strlen(buf);
        nwords += cws(buf, len, &inword);
    }while (len == 512);
    display_ans(nwords);
}

int cws(char *buf, int bufsize, bool *inword){
    int w = 0;
    for (int i = 0; i < bufsize; i++){
        bool alpha = isalpha((unsigned char) buf[i]);
        w += alpha & !*inword;
        *inword = alpha;
    }
    return w;
}

void display_ans(long long int nwords){
    char* screen = 0xB8000 + 200;
    do{
        screen[0] = (nwords % 10) + 0;
        screen[1] = 7; // grey on black
        screen -= 2;
    }while ((nwords /= 10) != 0);
}