Problems of Computer Systems (continued):

4.) Complexity

• This bites us harder than it bites elsewhere
• Aircraft carrier most complicated software decades ago
• Now even our phones are much more complicated

                                               Moore's law (transistors)                                         Kryder's law (harddrive) : allows us to put more complicated data structures in disk

Computer-ception:

Building computers with computers:

• Univac I (had lots of error checking.... really slow!) was designed by hand
• Univac II, however, used Univac I for its design

Say we want a small program with these ideas in mind:

• We don't want an OS
  • Called a "Bare Metal" or "Standalone" program
• We are paranoid
  • Don't want others to see code. The machine is completely offline
    • This explains why we don't want an OS. We don't trust the OS
• We want to count the number of words in a doc.
• Program starts with plugging in the machine to power and ends with plugging the machine out

Quick disk review:

http://en.wikipedia.org/wiki/Cylinder-head-sector

Disk Controller and how it works:

To Tell disk controller a command:

1. Wait for it to beocme ready
2. Store # of sectors you want to read into
3. Store which sector you want to read
4. Store read command into register
5. Wait for it to be ready again
6. Slurp data from disk controller cache through CPU into RAM

The Machine:

• x86 Intel i3-2130
• 3MiB cache, 3.4 GHz
• 4 GiB RAM, Dual Channel, DDR3 SDRAM 1600 MHz
• 1 TB HDD, SATA, 7200 RPM
• Intel HD integrated graphic
• No network connection
• Air gap security

Problems with Bootstrapping:

Trying to get some code that starts rest of program:

• Lets say we have a panel that let's us:
  • Specify a bit value (1 switch)
  • Specify which bit (3 switches)
    • Get's difficult with 32 bits.

Instead, we use a BIOS on a ROM:

Pros of a BIOS:

• Survives a power outage
  • Since RAM gets cleared when there is no more power
• Hard to change once it's set

Set program counter to the location in ROM and run the BIOS

So, we could make our own BIOS that runs the word count program or...

Some features of BIOS

1. Self tests
2. Looks for devices

• Sends requrest over the bus

Looks for special pattern in 1st sector indicating the device is bootable

512 bytes per sector (classic boot)

1st sector - Master Boot Record (MBR) runs some code

• Looks at last 2 bytes: 0x55 and 0xAA to detect if bootable
• Rest of 64 bytes describes up to 4 partitions

The other 64 bytes are for the partition descriptor:

16 bytes include:

• Status (bootable?)
• Start sector (4 bytes)
• Size (in sectors) (4 bytes)
• Type (DOS, Linux, etc)

We could use boot program to run an actual program... but doesn't give us enough flexibility.

So, we use a Volume Boot Record (VBR)

• Can be OS-specific 
• MBR-OS-agnostic

Place size of VBR in BOOT program: 

• We are replacing the VBR with Wordcount

How do we write the program?

In C: MBR.c -> cc -> mbr.o -> ld -> mbr.executable (512 bytes)

Assume wordcount program is 10KiB, so we need to read 20 sectors

static void wait_for_ready(void){
	while(inb(ox1f7) && 0xc0 != 0x40)
	continue;
}

int main(void){
	for(int i = 0; i < 20; i++){
		read_sector(i+1, 0x10000 + i*512);
	}
	goto 0x10000
}

static void read_sector(int s, char* a, int num_sec){
	wait_for_read();

	outb(0x1f2, num_sec)
	for(int i = 0x1f6; i > 0x1f3; i--){
		outb(i, s&oxff);
		s >> = 8;
	}

	outb(0x1f7, 0x20)
	wait_for_ready();
	insl(0x1f0,a,128);
}

so we have loaded the wordcount program at memory 0x10000 wc.c:

void main(void){
	unsigned long nw = 0
	int sec_num = 21;
	bool inw = false;

	for(;;){
		char buf[512];
		read_sector(s, buf);
		for(int i = 0; i < 512; i++){
			if(buf[i] == '\0'){
			finish(nw);
			for(;;); //must unplug machine to turn off
			}
		bool inw1 = (buf[i] | 'a'-'A') - 'a' < 26
		nw += inwl & ~inw;
		inw = inw1;
		}
	}
}

void finish(int n){
	short *p = (short)* 0xb8014;
	do {
		*p-- = '0' + n % 10;
		n /= 10
	} while(n);
}