Info Guide #01

Программирование

Assembler - A short lesson in assembler for lamers.

    A brief lesson in assembler for lamers
  Alone Coder


            Preface.


   Until recently I believed that
light is no longer suschestuvuet people who do not
just can not write a computer game, and - terrible to say! - 
Can not distinguish the case from the port. My naive assumption 
rubbished recent visit to the Ryazan radio market in Gorky 
Park. 

   - Do you have any program for
Sinclair? - Said the young man came
to the table in the center of the site.

   I have already alerted the word :
maybe someone else does not know, but the computer on which you 
are viewing this text, called the ZX Spectrum, from the Latin 
word meaning, it seems <Rainbow>. I went

closer.

   - Sorry, I'm a local encoder. My name is
Dima Bystrov ...- I began awkwardly.

   Into conversation, we exchanged addresses and
goodbye, promising to visit each other.

   A couple of weeks, we met again;
each of my program expressly surprised my
new acquaintance, who probably have not seen
fresh software already for two years. I threw him
major utilities and wished soon
write something good.

   It took another two weeks ... He returned my
drives and brought the old glossy magazine
cover.

   The magazine was called <Home Computer>;
It was, in truth, pisyushny Journal
( - adjective from , which we term, coders, 
using instead IBM PC). Category <HELP!> Most interested me. 
This topic questions readers and their responses, the data 
certain Dr. HELP. When I came across These questions, I could 
not help laughing: 


.


   The next question, I specifically note.
Now you'll understand what I'm generally led
this conversation. Get ready ...

.


   Here is a description of how the laptop should be vacuumed 
and wiped with a cloth. But the most important authors of the 
journal is not able to understand: the reader BELIEVE the 
hardware can be spoiled, Did not press the wrong button!


   Dispel these myths and help readers eccentric computer 
magazines to feel themselves masters of the computer and this 
should be Page. 


               Chapter 1

   how to overcome fear and to write

    elementary program in assembler


   Firstly, I will give some basic concepts that we use and 
application of which you can not avoid in the future. 

   CODER - the man who wrote (do)
program in machine code.

   Machine code - instructions the processor Z80
in the computer, which reduce to simple computing operations. 
Program in machine code is always faster kakihlibo written in 
other languages ​​(BASIC, Pascal, C ,...). 

   ASSEMBLER (AFM) - a method of recording, MK on
paper (screen, disk, ..) in the form of letters and numbers.
And A. - a program that translates ACM
in MK

   Lamer - not a coder. Also, LA - who likes to spoil someone 
else's program. 

   Hacker - a good coder, which alters
someone else's program for the better (eg,
inserts eternal life, beautiful screen saver
packs the game, rarely - bugfix
in the program or add to it new opportunities).

   Memory - computer memory. In a narrower sense - random 
access memory (RAM, it can still be recorded).


   Byte - character / number from 0 to 255 / number
-128 to 127, and corresponds to -128
128, -127 corresponds to 129, etc. Of these,
is memory.

   BIT - one-eighth bayta.Soderzhit minimum of information: 
either 0 or 1. (Despite the endless repetition of meaningless 
phrases in the literature, I hasten to inform you that at least

information - it is a zero bit, and if you say that does not 
think so, then give more example of a smaller data: 0, with 
probability 1 / 3 or 1 with probability 2 / 3.) Numbered

in a byte from right to left with zero, and the sum
bits, multiplied by two in their degree of
numbers give the value of bytes.

   Assembler is the best ALASM.

   It has 2 modes - command and editing. The first set after
loading. Log in second - to collect EDIT
(Button E) name. For example: EDIT AAA.

   Immediately you enter the newly AAA.H text file, and you can 
change it. 

   Now you're sitting in front of a screen, and the head
Your reason was bad think. In
such cases, I write first and last
lines of future programmy.Vot they usually look like:

        ORG 25000

        RET

   The first indicates the location in memory where the program 
will work (from 24576 to 65535) and not a command processor (ie

does not take place). Last - exit
program (or subroutine, which is essentially the same thing) to 
where it had dared start (cause).


   Teams such as the directives ORG
Assembler. Another example of a directive:

        ENT

   She needs to know with any
place to start your teeming ORG'ami
program, since some Assemblers (MASM)
prefer the latter ORG, others (ALASM) first, and some - 
generally refuse to broadcast (assemble) the program without

ENT or ENT $.

   Lines for inserting commands in the middle of the text apart 
buttons SS + W (<>), and the shift buttons SS + Q (<=). First

this may seem inconvenient, but then you
find it very good way.

   Useful guidelines

        DEFB "text", # 3F, 5, "a"

   She puts in the place of memory where
would have placed the following command line program, any set 
of bytes (characters). Similar team DEFW, but it sticks Bytes 
two and can not stomach tekstov.Esche enigmatic team DEFS 
40,1,2,3 - which 40 times in a row shoved into the memory bytes 
1,2,3. Lattice # indicates a hexadecimal number

# 3F = 3 * 16 + F, where A, B, C, D, E, F, respectively
are 10,11,12,13,14,15.

   But I digress. That our program has done anything, although 
we still do not able, blatantly using some strange

program from the ROM (there are many interesting things):

        ORG 25000

        CALL 3582

        RET

   Now we call the program shift
the whole screen up on familiarity (SCROLL UP
CHR $) in the same way as our program
caused the system (Basic, ALASM, TR-DOS ,...).

   To start this plagiarism (hazing?)
and see how it works, exit edit mode to command mode. For
Click EXTEND; will be waiting command line editor. You can 
enter different commands: B - beginning of the text, E - end S 
- search for something, X - replacement, and much more I do not 
remember what, but you have to press Q - Quit. 

   Now the program is written: SAVE.
After this assembliruem: ASSEMBLE and run: RUN. Screen is moved 
up, cheers. 

   You can also call it from BASIC: QUIT
(Yield in BASIC), RANDOMIZE USR 25000.

   Return to ALASM - RANDOMIZE USR 23600.

   From built-in debugger StS: DEBUG
(Debugging), JUMP 25000. RANDOMIZE USR 23600.
Go to Edit - EDIT.

   For a complete mastery of the system you should read HELP'y 
to ALASM and StS.Dalneyshy text for compactness implies what 
you already know how to handle them. 


               Chapter 2

         everything little by little


   The Register is a small variable
which is not stored in memory and the processor. In the case of 
1 byte. The names of registers: 

  A
B C
D E
H L

   I have listed only the general purpose registers, just the 
same registers 24.6 last collected in pairs, called simply

<Register pairs> (reg pair, rp) and can
used as a double-byte number / address
(160K of memory simultaneously addressed 64
K, so the byte address is transmitted 2 bytes)
Register can be set to team

        LD E, 100
or LD BC, 40000
or LD A, H
but not LD HL, BC

   A label (label) is similar to the row number in
BASIC for islyucheniem so that it does not need
placed in each stroke.Metku can use expressions instead of the 
relevant chisla.Metka contains 7 letters long and starts with 
the letter. Example: 

        ORG 25000

        LD DE, TEXT

        LD BC, TEND-TEXT

        CALL # 203C; again the program from the ROM

        RET
TEXT DEFB "TEXT - a label, 13
TEND

   Printed text at TEXT = 25010
length TEND-TEXT = 17 bytes. All of these calculations produces 
ALASM during translation. 

   If you do not want the program to run only in a row, you can 
use go to label:


        ORG 25000

        LD BC, # 50F0; B = # 50 = 80, C = # F0 = 240

        JP 8933

   Appears on the screen point. JP 8933 passes control to the 
ROM, which of course, already contains a RET, so that we can

do not worry about the return. Go
no more than 127 bytes forward / backward is called a short or 
a relative address, and writes JR. 

   When you need to do something more
times (to perform the cycle), in particular, the command DJNZ 
address: 

        ORG 25000

        LD B, 80
CYCLE LD (BUF), BC; write; 8933 Port B

        LD C, B

        CALL 8933

        LD BC, (BUF); in BC old value

        DJNZ CYCLE

        RET
BUF DEFW 0

   Draw diagonal.DJNZ subtracts from B
unit, and if it does not zero, becomes
at that address. It is easy to calculate that
so the loop is executed exactly as many times
how much was in the B at the entrance to his (t.e.80).
Parentheses indicate that we have in mind
not a number, a memory cell with this number.
LD (BUF), BC BC writes at BUF.

   Use cells with labels for the storage registers is 
inconvenient, so contrived Team: PUSH rp (shove) and POP rp 
(push). The first stores rp in the memory area called the 
stack, the second takes out the current value of rp. Many 
meanings, put on the stack are removed from there in reverse 
order. There's also stored the address to which passes the 
command RET. Therefore the number of PUSH and POP should be the 
same. Example: 

        ORG 25000

        LD B, 80
CYCLE PUSH BC

        LD A, 80

        SUB B

        LD C, A

        CALL 8933

        POP BC

        DJNZ CYCLE

        RET

   Draw a diagonal inverse. Team
SUB r belongs to the category arifmeticheskologicheskih and 
subtracts from the A register r. Other well-l command: ADD A, 
r; add r to A ADD HL, rp; rp added to HL

ADC A, r; A = A + r + CY
ADC HL, rp; HL = HL + rp + CY
SBC A, r; A = A-r-CY
SBC HL, rp; HL = HL-rp-CY
DEC r / rp; reduce rp 1
INC r / rp; increase rp 1
AND r; bitmap A and r is written in A. Obnulyayutsya all the 
bits of A, which correspond zero bits r.

OR r; bitwise OR A r is written in A. In
unit address all of the bits A, corresponding to which bits r 
were equal to 1. XOR r; the same, but the bitwise XOR.

B each bit of A is written to 1 if bits
this number in A and r were different, 0 otherwise.
CP r; A comparison with the r
CPL; A = 255 A.Vse bits are inverted (01)
NEG; A =- A.
DAA; BCD correction; produce it immediately after the addition 
/ subtraction, if s / v'emye numbers are BCD. It is unlikely 
you will come in handy t.k.d e-numbers - a method of recording 
two digits in a byte, and Dr Dr # 34 is assumed to be 34.


   A few words about what is CY.On
is called a flag (sign bit) transfer
and is set to 1 if the result of addition exceeds 255 (65535 in 
case of 2-byte addition) or by subtracting the larger number of 
smaller, or in other special specified circumstances. Another 
flag is used Z, which equals 1 if the result of a-l operation 
is zero, and zero otherwise. For He is called a flag of zero. 
All Flags are in the flags register F, which, together with 
register A is yet another (fourth) rpAF, which, unfortunately, 
can not used in double-byte calculations.


   Comparison - the same deduction, but the result was not 
memorable, but only appropriately changing the flags: Z = 1 if 
A = r; Z = 0 if A <> r; CY = 1 if A <r; CY = 0 if AЄr.


   Why flags? Yes, and that at
it then get the condition branching program, calling the latter 
or out of it. Team conditional short transition looks like: JR 
cc, <label>, where cc - condition: Z - Z = 1 Because there are 
flags, that is NZ - Z = 0 and more rarely ispolzuC - CY = 1 
emye conditions used in the koNC - CY = 0 manda a long 
transition JP. 

   That is, if the condition is satisfied, it should make the 
transition, otherwise - no. 


               Chapter 3

   Several important practical tips


   The program becomes your only
when you were working on making it. By the way, we should not 
also be happy after writing any program, if the latter is up to 
you was written at least one person.


   There are two ways of writing programs at ACME: on paper, 
pen / pencil and then set on the computer and sitting directly 
behind / in front of ekranom.Pervy method is good that you can 
write programs away from your computer, and bad low 
productivity and increasing the number of errors; the second 
bypasses the shortcomings of the first and loses 
dostoinstva.Zhelatelno his own two. 

   Despite the fact that the Pentagon is 128-st, remember that 
not every computer - the Pentagon. Consequently, it is not 
necessary to use two screens, when quite alone, as well as

eat all the 71680 processor cycles, or to put the first team to 
break EI. 

   NEVER operate the on / off the drive to verify that the disk 
drive, or play through the music! In 5313 the domestic drive at 
the same time disrupted the mechanism of lowering the head on 
the drive and the drive can not write INFORMATION!


   Keep copies of your important programs on
their own and / or foreign disks. The latter sometimes
even safer. Do not have copies of stuff.
Keep track system.

   Save your time, e.g. Do not look
twice the same TV shows, do not go
at discos and drinking. Remember: the life of the
a man and a short, but remember about you
only if you have time to do a lot for it.
Learn to speak, print, walk and think quickly.

   Have patience; errors in programs
cases and in the masters of CodeBusters, but they
know how to fix them => and you nauchites.Sperva
complex structure of the program you will be able to
keep in mind, but after a short time
regular work on a computer the size of
program, typed at a time, will be unrestricted.

   Do not make a program of pieces!

   Tags recommended that programs should come up with this: the 
name of the subroutine + letter / figure, for example: MOVTXT0 
in semiconductors MOVTEXT; READKYQ in READKEY; can SYMN13 in 
SYM. 

   If someone tells you that a certain method
is always the best, do not believe him. For
example, consider moving pamyati.Medlenny LDIR indispensable in 
a text editor; LDI is very useful for vertical shear

screen and the lack of memory for sdvigalku;
POP HL: LD (...), HL is used in shifts,
multicolor and mapping; POP rp ... PUSH rp
well to accelerate mapping (sm.statyu in
the next issue); LD rp, ... PUSH rp hitherto
only used for drawing reticula, although in one fell swoop 
increases the width Multicolor 24 CHR $ + allows us to derive

screen in one interrupt (sm.statyu).

   Despite the fact that the switching of two
images gives a variety of graphic
modes, remember that this leads to flickering
screen, which may fit for trial,
but not for the program that the user
active use. You can not even remember:
Quod licet Iovi, non licet bovi, which means that <What could 
Jupiter, it is not allowed to bull>. Switching three screens do 
not use ever, because it entails not

flicker, but the blinking screen.

   Do not limit the complexity of the compiled
Your programs, justifying the fact that someone
is a very smart or he has more
free time than you have. No gods pots fired! I do not have free
time, but I wrote this journal!


          To be continued ..