Subliminal Extacy #01
01 января 1995

Basics of Machine Code

Basics of Machine Code
                     Basics of Machine Code                     
                     ----------------------
                             By L.A.                            
                             -------
Yep!  That's right!  L.A. of Extacy-3 is typing an article about
Machine Code!!  Hold on!  L.A. isn't a coder!!  That's where you
are wrong!!  I've always been into coding since E-3 has started.
It's just Bogie has always been better at Coding than me.  Hence
he is the groups main coder.                                    
                                                                
Before I start with all the Technical waffle I'll say that to   
follow this article you must:-                                  
                 Have a good knowledge of Basic,                
    Have a small understanding of what a Byte and a Bit are,    
    and have an idea how your speccies memory is mapped out!    
                                                                
Got all those things, then we'll begin!                         
                                                                
Machine Code is a low-level language.  What's that mean?  It    
means it is as low as you can get to just entering bytes into   
memory (Which you'll understand better after reading this shit!)
Also it is harder to understand than a high-level language like 
Basic.                                                          
                                                                
Like Basic, Machine Code (M/C for short) is made up of commands 
but in M/C they are called 'Mnemonics' (No that is spelt        
correctly!) pronounced 'New-Mon-Ics'.  These are split into 2   
sections, the Op-Code and the Operand.  If you pick up your +2, 
+2a or +3 maunal and look up Chapter 8 Part 27 (Grey +2 manual) 
or Part 28 (Black +2a or +3 manual), you'll see the following:  
                                                                
CODE    CHARACTER    HEX    Z80 ASSEMBLER  -After CBh -After EDh
                                                                
This is a good part to mark (maybe fold the corner of the page) 
because after you get a small amount of knowledge on Z80 M/C,   
you'll find yourself constantly going back to these pages!!!    
                                                                
Right Look up CODE 62.  Forget about the character bit and HEX  
bit, they're pointless at the moment.  Look at the Z80 ASSEMBLER
bit cos this the bit we're interested in.  It should say -      
                                                                
                             LD A,N                             
                                                                
Now as you know, Z80 mnemonics are split into 2 sections, the   
Op-Code and the Operand.  The Op-Code part of LD A,N is the LD A
part the Operand is N.  Now N is an eight-bit number which YOU  
define!!  So let's define it.  I'll use 69.  So the instruction 
now becomes 'LD A,69'.  That was easy, wasn't it?  Okay, we now 
should understand what a Z80 nmemonic is and consists of.       
                                                                
But what does 'LD A,69' acually do? Well it LOADs the register A
with the value 69.  Wooow!!  But A is short for Accumulator.    
Which is the place in your Z80 where all calculations,          
comparisons, etc. take place.  So to re-cap, LD = LOAD a        
register or address and A = Accumulator.                        
                                                                
Okay, if you think you've got the hang of that then I explain   
registers.                                                      
                                                                
Registers are basically areas where your Z80 stores numbers and 
addresses.  Here is a list of the main ones.                    
                                                                
                             H    L                             
                             D    E                             
                             B    C                             
                             A    F                             
                               IX                               
                               IY                               
                                                                
Registers can either hold an 8 bit number or a 16 bit number.   
16 bit numbers are only stored in Register Pairs like above, HL,
DE, BC, AF, IX, and IY.  8 Bit numbers are stored in the        
following registers A, B, C, D, E, F, H, and L.                 
                                                                
As you may, or may not know is, that an 8 bit numbers max. value
is 255 and a 16 bit numbers max. value is 65535.  Also you      
should know that the address of the diplay file is 16384.       
                                                                
Now we know that, let's try our first program.                  
                                                                
          LD HL,16384  ; LOAD HL with the value 16384           
          LD A,69      ; LOAD A with the value 69               
          LD (HL),A    ; LOAD the contents of HL with A         
          RET          ; RETurn                                 
                                                                
Load up your assembler, ORG to an address which won't land slap 
bang on your assembler, type it in, Assemble it, Go into BASIC  
and type RANDOMIZE USR 'address' where 'Address' is the address 
which you ORGed your code to.  If you're using Tornado, then add
the following lines at the start of your code.                  
                                                                
          ORG 40000                                             
          DUMP 40000                                            
                                                                
And then assemble it, return to BASIC and type RANDOMIZE USR    
40000.                                                          
                                                                
This piece of code should quite easy for a beginner to          
understand.  Let me explain it indepth.  The first line -       
'LD HL,16384' will load the register pair 'HL' with the value   
16384.  16384 is the address of the first byte of the display   
file.  The second line - 'LD A,69' will load the Accumulator    
with the value of 69.  The third line - 'LD (HL),A' is the magic
bit which will put the value of A into the address of HL.  In   
our case it will put the value 69 into the address 16384 (The   
display file).  The forth line - 'RET' merely returns to the    
place where the speccy was before that routine was called, which
was in BASIC.  Understand, we called from basic with the        
RANDOMIZE USR command, thus when the speccy is told to RETurn if
everything is OK the speccy will return back to basic.  This    
small routine is a Machine Code version of the BASIC command,   
POKE.                                                           
                                                                
Now I'll cover a new command - 'INC'.  INC is short for         
Increment which means add 1.  INC can have all registers after  
it except IX and IY. Now if we include this to our program we   
can see it's effect.                                            
                                                                
          LD HL,16384                                           
          LD A,69                                               
          LD (HL),A                                             
          INC A                                                 
          LD (HL),A                                             
          RET                                                   
                                                                
Now this will POKE 16384,69 then increase 'A' by 1 to make 'A'  
equal 70, then POKE 16384,70.  But this will happen so quickly  
that you may only just beable to see it happen!!                
                                                                
So we'll try something a bit more complex.                      
                                                                
          LD HL,16384                                           
          LD A,0                                                
POOCAKE   LD (HL),A                                             
          INC A                                                 
          HALT                                                  
          JP POOCAKE                                            
                                                                
WARNING..  If you try to assemble and run this, it will NEVER   
return back to basic.  Now let me explain how it works.         
LD HL,16384 - LD A,0 - LD (HL),A - you know what all of them do.
Notice the word POOCAKE to the left of LD (HL),A , this is known
as a LABEL.  INC A will increment A every time it is executed.  
HALT pauses your speccy for one 5Oth of a second so INCs effect 
is more visible.  JP POOCAKE will jump back to POOCAKE and      
execute the LD (HL),A - INC A - HALT - JP POOCAKE cycle.  Now   
you can see why it won't return back to basic.  Type it in and  
assemble it at the same address as last time.  Run it.  HaHa    
HaHa Ha Ha, Your speccy is constantly POKEing 16384 by an       
increasing number!!!  You may think that A will just keep       
getting bigger and bigger and bigger etc.  But because it's an 8
bit number, it means it will get to 255 and then the next time  
it's INCed it will become 0 because if you can remember we said 
an 8 bit had a max value of 255!!  So it can't go above 255 and 
when INCed turns to 0 and starts again!!!                       
                                                                
If you don't under stand the above program, here is the basic   
conversion.                                                     
                                                                
10 LET HL=16384 : REM LD HL,16384                               
20 LET A=0 : REM LD A,0                                         
30 POKE HL,A : REM LD (HL),A                                    
40 LET A=A+1 : IF A=256 THEN LET A=0 : REM INC A                
50 PAUSE 1 : REM HALT                                           
60 GOTO 30 : JP POOCAKE                                         
                                                                
Okay, that's the best I can convert it to BASIC, NOTE I've put a
check for 'A' getting greater than 255, because if you try      
POKEing an address in basic, with a value greater than 255,     
you'll get an error message back!!                              
                                                                
That's all for now.  I hope you have enjoyed this small         
introduction for M/C.  Get S.E.#2 for my continuation.  And     
Don't Forget that the word 'With' is a bad word to end an       
article with.                                                    



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