CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL
MODE) 19
We can see from the assembly example that there is something going on behind the
scenes that is relating thecmpinstruction to thejeinstruction it proceeds. This is an
example of where the CPU’s specialflagsregister is used to capture the outcome of
thecmpinstruction, so that a subsequent conditional jump instruction can determine
whether or not to jump to the specified address.
The following jump instructions are available, based on an earliercmp x, yinstruc-
tion:
je target ; jump if equal (i.e. x == y)
jne target ; jump if not equal (i.e. x != y)
jl target ; jump if less than (i.e. x < y)
jle target ; jump if less than or equal (i.e. x <= y)
jg target ; jump if greater than (i.e. x > y)
jge target ; jump if greater than or equal (i.e. x >= y)Question 3
It’s always useful to plan your conditional code in terms of a higher level language, then
replace it with the assembly instructions. Have a go at converting this pseudo assembly
code into full assembly code, usingcmpand appropriate jump instructions. Test it with
different values ofbx. Fully comment your code, in your own words.
mov bx, 30if (bx <= 4) {
mov al, ’A’
} else if (bx < 40) {
mov al, ’B’
} else {
mov al, ’C’
}mov ah, 0x0e ; int =10/ah=0x0e -> BIOS tele -type output
int 0x10 ; print the character in aljmp $; Padding and magic number.
times 510-($-$$) db 0
dw 0xaa553.4.6 Calling Functions
In high-level languages, we break big problems down into functions, which essentially
are general purpose routines (e.g. print a message, write to a file, etc.) that we use
over and over again throughout our program, usually changing parameters that we pass
to the function to change the outcome in some way. At the CPU level a function is
nothing more than a jump to the address of a useful routine then a jump back again to
the instruction immediately following the first jump.
We can kind of simulate a function call like this:
...
...
mov al, ’H’ ; Store ’H’ in al so our function will print it.