BY DF0: / NFA

   This  is ( Hopefully ) going to be the first in a series of tutorials
on  how  to  code  in  ARM  Machine  code  on  the  Archimedes series of
computers.Despite  their many faults,these machines offer a real kicking
processor,and as it is RISC based,it is a piece of piss to code on it,as
well  as  being  PRETTy  damn  fast.Note :- I have currently written two
articles  on  this  subject....I  dont know if they are both going to be
included  in  the  next  issue,so  bear  in mind any references to 'Last
months article' mean this one....

   I  will  assume  that  you do not have a dedicated assembler for this
machine,so  we  will  use  the  BASIC  assembler.The  Archimedes  has an
assembler  bulit  into  BASIC,so  you can start coding straight away.The
only  thing  you  need to do is to create a skeleton program to assemble
and run your code.Type this program in and save it :-

-----------------------------------------------------------------------

*|  ARM Machine code tutorial program (C) 1994 by DF0: of NFA
*|  Use to slot your ARM code into...

Reserved_space = 1024

DIM code Reserved_Space
For Pass=0 TO 3 STEP 3
P%=code
[
OPT Pass

]
NEXT Pass
CALL code.

-----------------------------------------------------------------------


  I  will  now  go  over  the  program  line  by line.The Reserved_Space
variable holds the amount of memory you want to free for your program.In
this case it is 1024 bytes,or 1 Kilobyte.

  The  next  line actually frees the code by using a specialised form of
the DIM statment.

  The  P%  variable  is  an  internal variable,and is the pointer to the
start of the code.

  Any  instructions  between  the [ and ] signs are treated as ARM code.
The  ] character tells the compiler that you now want to start executing
BASIC instructions again.

  The  OPT  Pass  bit  is  the code that tells the compiler to check for
errors,etc.  in the 2-Pass compiler.I will go into this next time.

  CALL  actually  runs your code from the P% location where your code is
stored.

  Now  we can try an example bit of ARM code.  Type this code in between
the [ and ] characters.

; Simple string printing example by DF0: of NFA 1994

SWI      "OS_NewLine"
ADR      R0,Message_String
SWI      "OS_Write0"
SWI      "OS_NewLine"
MOV      PC,R14
.Message_String
EQUS     "Hi there! This is a string being printed using SWI calls!"
EQUB     &0

If  you  run  this,the  message  in the EQUS statment will appear on the
screen.

                    Basic stuff you need to know :-
                    \/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

The  Arc  has  32-bit registers from R0 to R13 that you can use to store
whatever  you  want  in  them.Register  R15  is  sometimes called the PC
register,and  is  the  program  counter.The  R14  register  is  the Link
register.

The  MOV  PC,R14  command  is  the  equivalent  to  a  RTS call in 68000
code.Without it,the machine will crash.

Anything with a . in front of it is a label.

           &04e = Hexadecimal                   [ & suffix ]

           %011 = Binary                        [ % suffix ]

           #543 = Decimal.                      [ # suffix ]

Basic rundown of the commands covered so far .....

ADR <register>,<label>  = puts the actual address of a location into
                          the specified register.

SWI <Routine>           = Calls a routine built into the OS.These
                          routines are given names and numbers.
                          For instance,both SWI "OS_NewLine" and
                          SWI &00003 do the same thing,enter an enter
                          character to the screen.

MOV <destination>,<src> = Moves contents of <src> register into the    
                          <destination> register. Example :-
                          MOV R0,R1 moves R1 into R0.

EQUS                    = Reserves a string

EQUD                    = Reserves a double word ( 4 bytes )

EQUW                    = Reserves a word ( 2 bytes )

EQUB                    = Reserves a byte

-----------------------------------------------------------------------

Well,  I  hope  this file has given you a ( brief ) insight into the RAM
chip  and  how  to  code  on  it.Next  issue,Ill deal with data shifting
commands,and some more SWIs.

DF0: of NFA logging.....