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01 января 1995 |
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Basics Of Machine Code - Part 2
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By L.A. Of Extacy-3
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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 1/5Oth 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!!
Other articles:
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