Subliminal Extacy #02
01 января 1995

Basics Of Machine Code - Part 2

<b>Basics Of Machine Code</b> - Part 2
                 Basics Of Machine Code - Part 2                
                 -------------------------------                
                       By L.A. Of Extacy-3                      
                       -------------------                      
Before I start, let me say that if you haven't read Part 1 (In  
S.E.#1) then you must read that before reading this.  Otherwise 
none of this article will make any sense at all.                
                                                                
Down 2 business then.  Did you try the source I gave you last   
ish?  I hope you liked it.  You may think that alot of it was   
crap and pointless, but if you don't understand or know very    
basic stuff like that then you ain't got a chance at learning to
code!!                                                          
                                                                
This issue I thought we'd look into loops and a few other       
instructions.                                                   
                                                                
First up we will look at a 'LOOP'ing command known as 'DJNZ'.   
Now DJNZ works on the 'B' register.  Let's make some source and 
then I'll explain later.                                        
                                                                
         LD B,69                                                
POOCAKE  DJNZ POOCAKE                                           
         RET                                                    
                                                                
Okay, LD B,69 sets our counter up at 69.  DJNZ POOCAKE - this is
the very clever part.  DJNZ ruffly means Decrement B and jump if
B is Not Zero.  So as we 'LOAD'ed B with 69 it will keep looping
until B = 0, but don't forget everytime DJNZ is executed, B is  
decremented!!  So after 69 times that DJNZ has been executed B  
will equal 0.  So the DJNZ command will then carry on with next 
instruction, which in our case is RET.  So to recap, We load B  
with 69, DJNZ loops to itself 69 times and then RETurns.        
                                                                
Understand?  I hope so!!  Let's try something a bit more complex
now!!                                                           
                                                                
         LD A,0                                                 
         LD HL,16384                                            
         LD B,255                                               
POOCAKE  LD (HL),A                                              
         INC A                                                  
         HALT                                                   
         DJNZ POOCAKE                                           
         RET                                                    
                                                                
Get that!!  You should beable to understand the above program   
above (if you have read all my tutorials so far!).  But just    
incase you are a bit unsure of what is happening then here goes 
for an explaination.                                            
                                                                
We LoaD A with 0.  Then we LoaD HL with 16384 (start of the     
display file or Screen$).  Then B (our counter) is LoaDed with  
255 (as B is an 8 bit register so it's max value is 255).  Then 
we LoaD the contents of HL with A, which basically means we poke
HL (16384) with A (at the minute it's 0). A is then INCreased  
by 1.  Then the Z80 is told to HALT causing the Speccy to pause 
for 1/50th of a second!!  Then it's told to DJNZ back to        
POOCAKE.  So remember that DJNZ DECrements B and JumPs if B is  
NOT ZERO!!   So if B isn't zero then it will go back to         
LD (HL),A Now this goes through this cycle 255 times because of 
us setting B to 255. After that there is a RET. Now because we  
are doing it 255 times you may think '255. Corr that'll take    
it's time!' but try it out, it will hardly take any time at all 
hence I've put a HALT in it. If it still goes too fast for you  
then add in more HALTs after the one I've put in!!              
                                                                
Another way to loop is JP which means JumP.  This does'nt depend
on B (like DJNZ does) or any other register.  It will JumP to an
address which you define.  It's a bit like RANDOMIZE USR from   
basic but different.  Because when you RANDOMIZE USR a RETurn   
address is left on the STACK (I'll explain that at a later      
date!!) so that the Speccy knows just where to return to when it
finally reaches a RET.  But when you use JP no return address is
left on the STACK.  So If you do a RAN. USR from basic to a bit 
of code that JumPed to a bit of code which ended with a RET then
when that RET is reached, the Speccy will return to BASIC.  It  
won't JumP back to where it JumPed from (like a CALL does (Oh   
shit I haven't explained CALLs) Shit).                          
                                                                
It had to happen.  I knew I'd start rambling on and end up going
on about instructions you may never had heard of.               
                                                                
CALL is a very simple command.  It's basically the M/C version  
of the RAN. USR command. (Or GO SUB in basic.ED) But when you   
CALL an address if it reaches a RET the Speccy will RETurn to   
the next instruction after the CALL. So say we had a bit of Code
that went sommit like the following:-                           
                                                                
         CALL POOCAKE                                           
POOCAKE  LD HL,16384                                            
         LD A,69                                                
         RET                                                    
                                                                
What do you expect will happen here?  Well CALL POOCAKE will    
go to POOCAKE and execute the chain of instructions until it    
reaches a RET.  When the RET is reached and executed the Speccy 
will RETurn to the next instuction after the CALL.  Which is the
LD HL,16384 command.  The Speccy then continues to execute the  
commands until it reaches a RET.  Then it will RETurn back to   
basic in our case.  If the CALL was changed into a JP then the  
Speccy would JP to POOCAKE, execute all the commands until it   
reaches a RET. Then instead of RETurning to the next instruction
after the JP it will RETurn to BASIC if you RAN. USR from Basic 
which how all code is run.  You load up your code and then      
RANDOMIZE USR an address which sets the ball rolling.           
                                                                
I hope all this has made sense.  If you getting confused by this
tutorial then read it through again, a section at a time until  
it does make sense!!                                             



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