Real mode assembly II: Difference between revisions

This is where things get fun. We're going to convert a value in al to two characters and print them out. The following code snippet from http://www.df.lth.se/~john_e/gems/gem003a.html shows how to convert the low nybble of al to an ASCII character.

 

cmp al,10

sbb al,69h

das

 

Simple, but doesn't do that much. After all, it only converts the low nybble and can do weird things occasionally. So we need to shift (and rotate for the low nybble). Here's how I do it:

 

shr al,4 ; Shift right four places to put upper nybble in lower spot

cmp al,10

mov ah,0Eh ; Print out upper nybble

int 10h

 

Sure, it works, but it's only printing out the upper nybble and the low nybble disappears! Oh noes! So we have to store a copy of al in a variable first:

 

mov [.temp],al

 

And restore it after to do the second:

 

mov al,[.temp]

 

Now we rotate and shift to do the conversion for the low nybble:

 

ror al,4

shr al,4

mov ah,0Eh

int 10h

 

===The code===

Now put that all together:

 

print_hex_byte:

mov [.temp],al

.temp db 0

 

And that function is now callable! Using what you've learned, you can also write '''your own''' call to print all of ax and not just al. Try it, it's not that hard!

[[NASM]] has some great macro abilities, including automating tasks such as writing your db statements for you. Simple, multi-line macros go something like this:

 

%macro name operands

; code code code

%endmacro

 

You can use %1, %2, etc. to represent the contents of arguments passed to your macro. You can even emulate BASIC statements with macros, like discussed [[printing to the screen without a db|here]].

Even simpler macros are the single-line macros, or %define statements. They create assemble-time statements to be inserted, but they are more like constants than functions.

 

%define D_SIGNATURE 0xCAFEF00D

%define D_INFO 0x29A3B83D

push D_INFO

jmp 0x2000:0

 

Now, that's just example code, it could mean anything, but it depends on what's at 0x2000:0, of course. But the principle is the same, %define is used to create constants and the like.