CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL
MODE) 22
; when to stop printing characters.
GOODBYE_MSG:
db ’Goodbye!’, 0; Padding and magic number.
times 510-($-$$) db 0
dw 0xaa55For good marks, make sure the function is careful when modifying registers and that
you fully comment the code to demonstrate your understanding.
3.4.9 Summary
Still, it feels that we have not come very far. That’s okay, and that’s quite normal, given
the primitive environment that we have been working in. If you have understood all up
until here, then we are well on our way.
3.5 Nurse, Fetch me my Steth-o-scope
So far we have managed to get the computer to print out characters and strings that we
have loaded into memory, but soon we will be trying to load some data from the disk,
so it will be very helpful if we can display the hexadecimal values stored at aribitrary
memory addresses, to confirm if we have indeed managed to load anything. Remember,
we do not have the luxury of a nice development GUI, complete with a debugger that will
let us carefully step though and inspect our code, and the best feedback the computer
can give us when we make a mistake is visibly to do nothing at all, so we need to look
after ourselves.
We have already written a routine to print out a string of characters, so we will
now extend that idea into a hexadecimal printing routine --- a routine certainly to be
cherished in this unforgiving, low-level world.
Let’s think carefully about how we will do this, starting by considering how we’d like
to use the routine. In a high-level language, we’d like something like this:printhex(0x1fb6),
which would result in the string’0x1fb6’being printed on the screen. We have already
seen, in Section XXX, how functions can be called in assembly and how we can use
registers as parameters, so let’s use thedxregister as a parameter to hold the value we
wish our printhex function to print:
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:
...
...
ret