Lecture 2 Scribe Notes

Abstractions and Bootstrapping

What would happen if we had no Operating System?

Example Problem: English professor needs to count the words in a file.

Constraints:

1. Paranoia – Use only our own code*

2. We want it to consume few resources

3. Simplest User Interface (UI) – power on to run

*Except for firmware. We trust Dell has supplied quality code.

Machine Specs:

• Dell Desktop x86

• 1 GB RAM

• 200 GB SATA Disk Drive

File Format:

• ASCII text file

• Each line terminated by a newline byte

• A word is composed from a sequence of letters, a-z and/or A-Z

• Data is contiguous on disk

Possible Future Extensions:

• Word Processor

• Editor

• Printing

Output Format:

Power on to use. Immediately display word count.

Problem: RAM is initialized to 0's after power on. How does our program get loaded onto RAM to get run?

What we want:

Solution: Use Programmable Read Only Memory (PROM) because after a program is loaded to PROM once, it will remain there after power off.

So what could we do with PROM?

1. Load our program to PROM

• • Would work, but if we ever want to change the code, we'd have to take apart the hardware of PROM to reprogram it

2. Use Dell's firmware on PROM, which is a program that will eventually load our program

Bootstrapping/BIOS

“Pull oneself over a fence by one's bootstraps.” - Teddy Roosevelt

Program in PROM needs to get our program off the disk and into main memory. This program is the Basic I/O System (BIOS).

BIOS:

1. Tests system (hardware checks).

2. Looks for devices.

3. Find one that is bootable. Usually, this is the Master Boot Record (MBR), identified by the 2 bytes, 0xaa55.

• The MBR is some code located on a sector of the disk, followed by a partition table

• Each entry of Partition Table (4 for historical reasons)

  • Status of partition (e.g. bootable?)

  • Start (index of starting sector)

  • Size (# of sectors)

  • Type (e.g. swap portion, Windows boot, etc.)

• • The MBR is operating system agnostic, meaning it works no matter what operating system you are booting.

  • Due to sector size constraints (512 bytes), the MBR only contains a small program (too small to do anything useful), so its job is to find another bootable partition

  • After selecting a bootable partition from the partition table, it reads the first sector of that partition, which is the Volume Boot Record (VBR)

    • Set the IP to the start of VBR

  • The VBR is operating system-specific

4. Read MBR to address 0x7c00 of main memory.

5. Set instruction pointer (IP) to 0x7c00.

Programmed I/O

• Special I/O x86 instructions

• Read function

  • Wait for disk controller to become ready by checking register 0x1f7

  • Store # of sectors into register 0x1f2

  • Store sector offset into registers 0x1f3-6

  • Store READ into 0x1f7

  • Wait for ready from disk controller

  • Get results into CPU

  • Store into RAM

void read_sector (int s, char* a) {

// s: sector #

// a: RAM address

while((inb(0x1f7) & 0xc0) != 0x40) { // 0xc0: 11000000 0x40: 01000000

continue;

}

wait_for_ready();

outb(0x1f2, 1);

outb(0x1f3, s & 0xff);

outb(0x1f4, (s>>8) & 0xff);

outb(0x1f5, (s>>16) & 0xff);

outb(0x1f6, (s>>24) & 0xff);

outb(0x1f7, 0x20);

wait_for_ready();

insl(0x1f0, a, 128); // insl reads 128 32-bit words from register 0x1f0 to a

}

And finally, our Word Count Program:

int main (void) {

// Word Count Program

int words = 0;

bool inword = false;

int s = 0x100000;

for ( ; ; s++) {

char buf [512];

read_sector(s, buf);

for (int j = 0; j < 512; j++) {

if (buf [j] == '\0') {

words += inword;

writeout (words);

return;

}

bool thisalpha = isalpha ( (unsigned char) buf [j]);

words += inword & ~thisalpha; // This yields 1 when we cross a word boundary

}

}

}

void writeout (int n) {

// Print to screen

char* screen = 0xb8000 + (25*80*2/2);

do {

*--screen = (n % 10) + '\0';

*--screen = 7; // grey on black

} while ( (n /= 10) != 0);

}