Writing a Simple Operating System — from Scratch

CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL

MODE) 23

Since we are printing a string to the screen, we might as well re-use our earlier printing
function to do the actual printing part, then our main task is to look at how we can build
that string from the value in our parameter,dx. We definitely don’t want to confuse
matters more than we need to when working in assembly, so let’s consider the following
trick to get us started with this function. If we define the complete hexadecimal string as
a sort of template variable in our code, as we defined our earlier “Hello, World” messages,
we can simply get the string printing function to print it, then the task of ourprinthex
routine is to alter the components of that template string to reflect the hexadecimal
value as ASCII codes:

mov dx, 0x1fb6 ; store the value to print in dx

call print_hex ; call the function

; prints the value of DX as hex.

print_hex:

; TODO: manipulate chars at HEX_OUT to reflect DX

mov bx, HEX_OUT ; print the string pointed to

call print_string ; by BX

ret

; global variables

HEX_OUT: db ’0x0000 ’,0

3.5.1 Question 5 (Advanced)

Complete the implementation of theprinthexfunction. You may find the CPU instruc-
tionsandandshrto be useful, which you can find information about on the Internet.
Make sure to fully explain your code with comments, in your own words.

3.6 Reading the Disk

We have now been introduced to BIOS, and have had a little play in the computer’s
low-level environment, but we have a little problem that poses to get in the way of our
plan to write an operating system: BIOS loaded our boot code from the first sector of
the disk, but that isallit loaded; what if our operating system code is larger --- and I’m
guessing it will be --- than 512 bytes.
Operating systems usually don’t fit into a single (512 byte) sector, so one of the first
things they must do is bootstrap the rest of their code from the disk into memory and
then begin executing that code. Luckily, as was hinted at earlier, BIOS provides routines
that allow us to manipulate data on the drives.

3.6.1 Extended Memory Access Using Segments

When the CPU runs in its intial 16-bit real mode, the maximum size of the registers is 16
bits, which means that the highest address we can reference in an instruction is0xffff,
which amounts by today’s standards to a measily 64 KB (65536 bytes). Now, perhaps
the likes of our intended simple operating system would not be affected by this limit,