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Subliminal Extacy
#01
01 января 1995 |
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Basics of Machine Code
Basics of Machine Code
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By L.A.
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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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