Why not an OS?

Hypothetical task: write a program that

• Count words in a file
• Without an OS
• The file starts at sector 50,000

Assume x86-64 CPU with disk drive, RAM, motherboard, display

Assume you have functions for:

• Putting number to a display
• Reading from the disk
• Start running? Boot just happens

Assume the file:

• Is ASCII with 1 byte per char
• Is terminated with null byte
• Maximal count of regex [A-Za-z]+

Access the file using the IDE interface

• Partitioned into 512 byte sectors
• read_ide_sector(long long sector_num, char buf[SECTOR_LENGTH])

Memory Mapped I/O

CPU
 |
------------------------------------- Bus
      |
     RAM         Device Register
     0x0-0x7fff  0x8000

Device Register:
|         | 7 bit color, 8 bit char |   ...2000 characters    
0xB800    ^
          16 bits/character

Display:
            80
--------------------------
|                        |
|                        |
|                        | 25
|                        |
|                        |
--------------------------