Nambafa
Graphics tutorials
Downloads: TUT19.zip
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│ W E L C O M E │
│ To the VGA Trainer Program │ │
│ By │ │
│ DENTHOR of ASPHYXIA │ │ │
╘═══════════════════════════════╛ │ │
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--==[ PART 19 ]==--
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■ Contents
- Introduction
- Assembler - the short version
- Fire Routines
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■ Introduction
Hi there. As promised in Tut 18, this trainer is on assembler. For those
people who already know assembler quite well, this tut is also on the flame
effect.
Okay, here is the total list of ways to get my trainers :
http://goth.vironix.co.za/~denthor (WWW)
ftp.eng.ufl.edu pub/msdos/demos/code/graph/tutor (FTP)
denthor@beastie.cs.und.ac.za Subject : request-list (EMAIL)
As well as the BBS numbers shown at the end ... I will add a few ftp sits
to that list (x2ftp.oulu.fi etc.)
Tut 20? How about 3d shading, hidden surface removal etc? Mail me :)
If you would like to contact me, or the team, there are many ways you
can do it : 1) Write a message to Grant Smith/Denthor/Asphyxia in private mail
on the ASPHYXIA BBS.
2) Write to : Grant Smith
P.O.Box 270 Kloof
3640
Natal
South Africa
3) Call me (Grant Smith) at (031) 73 2129 (leave a message if you
call during varsity). Call +27-31-73-2129 if you call
from outside South Africa. (It's YOUR phone bill ;-))
4) Write to denthor@beastie.cs.und.ac.za in E-Mail.
5) Write to asphyxia@beastie.cs.und.ac.za to get to all of
us at once.
NB : If you are a representative of a company or BBS, and want ASPHYXIA
to do you a demo, leave mail to me; we can discuss it.
NNB : If you have done/attempted a demo, SEND IT TO ME! We are feeling
quite lonely and want to meet/help out/exchange code with other demo
groups. What do you have to lose? Leave a message here and we can work
out how to transfer it. We really want to hear from you!
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■ Assembler - the short version
Okay, there are many assembler trainers out there, many of which are
probably better then this one. I will focus on the areas of assembler that
I find important ... if you want more, go buy a book (go for the Michael
Abrash ones), or scour the 'net for others.
First, let us start off with the basic set up of an assembler program.
DOSSEG
This tells your assembler program to order your segments in the same manner
that high level languages do.
.MODEL <MODEL>
<MODEL> can be : Tiny Code + Data < 64k (Can be made a COM file)
Small Code < 64k Data < 64k
Medium Code > 64k Data < 64k
Compact Code < 64k Data > 64k
Large Code > 64k Data > 64k
Huge Arrays > 64k
.286
Enable 286 instructions ... can be .386 ; .386P etc.
.STACK <size>
<size> will be the size of your stack. I usually use 200h
.DATA
Tells the program that the data is about to follow. (Everything after this
will be placed in the data segment)
.CODE
Tells the program that the code is about to follow. (Everything after this
will be placed in the code segment)
START :
Tells the program that this is where the code begins.
END START
Tells the program that this is where the code ends.
To compile and run an assembler file, we run
tasm bob
tlink bob
I personally use tasm, you will have to find out how your assembler works.
Now, if we ran the above file as follows :
DOSSEG
.MODEL SMALL
.286
.STACK 200h
.DATA
.CODE
START
END START
You would think that is would just exit to dos immediately, right? Wrong.
You have to specifically give dos back control, by doing the following :
START
mov ax,4c00h
int 21h
END START
Now if you compiled it, it would run and do nothing.
Okay, let us kick off with registers.
Firstly : A bit is a value that is either 1 or 0
This is obviously quite limited, but if we start counting in them, we can
get larger numbers. Counting with ones and zeros is known as binary, and we
call it base 2. Counting in normal decimal is known as base 10, and
counting in hexidecimal is known as base 16.
Base 2 (Binary) Base 10 (Decimal) Base 16 (Hexidecimal)
0 0 0
1 1 1
10 2 2
11 3 3
100 4 4
101 5 5
110 6 6
111 7 7
1000 8 8
1001 9 9
1010 10 A
1011 11 B
1100 12 C
1101 13 D
1110 14 E
1111 15 F
As you can see, you need four bits to count up to 15, and we call this a
nibble. With eight bits, we can count up to 255, and we call this a byte.
With sixteen bits, we can count up to 65535, and we call this a word. With
thirty two bits, we can count up to lots, and we call this a double word. :)
A quick note : Converting from binary to hex is actually quite easy. You
break up the binary into groups of four bits, starting on the right, and
convers these groups of four to hex.
1010 0010 1111 0001
= A 2 F 1
Converting to decimal is a bit more difficult. What you do, is you multiply
each number by it's base to the power of it's index ...
A2F1 hex
= (A*16^3) + (2*16^2) + (F*16^1) + (1*16^0)
= (10*4096) + (2*256) + (15*16) + (1)
= 40960 + 512 + 240 + 1
= 41713 decimal
The same system can be used for binary.
To convert from decimal to another base, you divide the decimal value by the
desired base, keeping a note of the remainders, and then reading the results
backwards.
16 | 41713
16 | 2607 r 1 (41713 / 16 = 2607 r 1)
16 | 162 r F (2607 / 16 = 162 r 15)
16 | 10 r 2 (162 / 16 = 10 r 2)
| 0 r A (10 / 16 = 0 r 10)
Read the remainders bacckwards, our number is : A2F1 hex. Again, the same
method can be used for binary.
The reason why hex is popular is obvious ... using bits, it is impossible
to get a reasonable base 10 (decimal) system going, and binary get's unwieldly
at high values. Don't worry too much though : most assemblers (like Tasm)
will convert all your decimal values to hex for you.
You have four general purpose registers : AX, BX, CX and DX
Think of them as variables that you will always have. On a 286, these
registers are 16 bytes long, or one word.
As you know, a word consists of two bytes, and in assembler you can access
these bytes individualy. They are separated into high bytes and low bytes per
word.
High Byte | Low Byte
0000 0000 | 0000 0000 bits
[--------Word-------]
The method of access is easy. The high byte of AX is AH, and the low byte is
AL ... you can also access BH, BL, CH, CL, DH and DL.
A 386 has extended registers : EAX, EBX, ECX, EDX ... you can access the
lower word normally (as AX, with bytes AH and AL), but you cannot access the
high word directly ... you must ror EAX,16 (rotate the binary value through
16 bits), after which the high word and low word swap ... do it again to
return them. Acessing EAX as a whole is no problem ... mov eax,10 ;
add eax,ebx ... these are all vaild.
Next come segments. As you have probably heard, computer memory is divided
into various 64k segments (note : 64k = 65536 bytes, sound familiar?) A
segment register points to which segment you are looking at. An offset
register points to how far into that segment you are looking. One way
of looking at it is like looking at a 2d array ... the segments are your
columns and your offsets are your rows. Segments and offsets are displayed
as Segment:Offset ... so $a000:50 would mean the fiftieth byte in segment
$a000.
The segment registers are ES, DS, SS and CS. A 386 also has FS an GS.
These values are words (0-65535), and you cannot access the high or low bytes
separately. CS points to you your code segment, and usually if you touch this
your program will explode. SS points to your stack segment, again, this
baby is dangerous. DS points to your data segment, and can be altered, if
you put it back after you use it, and don't use any global variables while
it is altered. ES is your extra segment, and you can do what you want with
it.
The offset registers are DI, SI, IP, SP, BP. Offset registers are generally
asscociated with specific segment registers, as follows :
ES:DI DS:SI CS:IP SS:SP ... On a 286, BX can be used instead of the above
offset registers, and on a 386, any register may be used. DS:BX is therefore
valid.
If you create a global variable (let's say bob), when you access that
variable, the compiler will actually look for it in the data segment.
This means that the statement :
ax = bob
could be
ax = ds:[15]
A quick note : A value may be signed or unsigned. An unsigned word has a
range from 0 to 65535. A signed word is called an integer and has a range
-32768 to 32767. With a signed value, if the leftmost bit is equal to 1,
the value is in the negative.
Next, let us have a look at the stack. Let us say that you want to save the
value in ax, use ax to do other things, then restore it to it's origional
value afterwards. This is done by utilising the stack. Have a look at the
following code :
mov ax, 50 ; ax is equal to 50
push ax ; push ax onto the stack
mov ax, 27 ; ax is equal to 27
pop ax ; pop ax off the stack
At this point, ax is equal to 50.
Remember we defined the stack to be 200h further up? This is part of the
reason we have it. When you push a value onto the stack, that value is
recorded on the stack heap (referenced by SS:SP, SP is incremented) When you
pop a value off the stack, the value is placed into the variable you are
poping it back in to, SP is decremented and so forth. Note that the computer
does not care what you pop the value back in to ...
mov ax, 50
push ax
pop bx
Would set the values of both ax and bx to 50. (there are faster ways of doing
this, pushing and poping are fairly fast though)
push ax
push bx
pop ax
pop bx
would swap the values of ax and bx. As you can see, to pop the values back
in to the origional variables, you must pop them back in the opposite
direction to which you pushed them.
push ax
push bx
push cx
pop cx
pop bx
pop ax
would result in no change for any of the registers.
When a procedure is called, all the parameters for that procedure are pushed
onto the stack. These can actually be read right off the stack, if you want
to.
As you have already seen, the mov command moves a value...
mov <dest>, <source>
Note that dest and source must be the same number of bits long...
mov ax, dl
would not work, and neither would
mov cl,bx
However, mov cx,dx
mov ax,50
mov es,ax
are all valid.
Shl I have explained before, it is where all the bits in a register are
shifted one to the left and a zero added on to the right. This is the
eqivalent of multiplying the value by two. Shr works in the opposite
direction.
Rol does the same, except that the bit that is removed from the left is
replaced on the right hand side. Ror works in the opposite direction.
div <value> divides the value in ax by value and returns the result in
al if value is a byte, placing the remainder in ah. If value is a word,
the double word DX:AX is divided by value, the result being placed in ax
and the remainder in dx. Note that this only works for unsigned values.
idiv <value> does the same as above, but for signed variables.
mul <value> If value is a byte, al is multiplied by value and the result
is stored in ax. If value is a word, ax is multiplied by value and the
result is stored in the double word DX:AX
imul <value> does the same as above, but for signed variables.
The j* commands are fairly simple : if a condition is met, jump to a certain
lable.
jz <label> Jump if zero
ja <label> Jump above (unsigned)
jg <label> Jump greater (signed)
and so forth.
An example ...
cmp ax,50 ; Compare ax to 50
je @Equal ; If they are equal, jump to label @equal
call MyProc Runs procedure MyProc and then returns to the next line of code.
Procedures are declared as follows :
MyProc proc near
ret ; Must be here to return from where it was called
MyProc endp
Variables are also easy :
bob db 50
creates a variable bob, a byte, with an initial value of 50.
bob2 dw 50
creates a variable bob2, a word, with an initial value of 50.
bob3 db 1,2,3,4,5,65,23
creates bob3, an array of 7 bytes.
bob4 db 100 dup (?)
creates bob4, an array of 100 bytes, with no starting value.
Go back and look at tut 7 for a whole lot more assembler commands, and get
some sort of reference guide to help you out with others. I personally use
the Norton Guides help file to program assembler.
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■ Fire Routines
To demonstrate how to write an assembler program, we will write a fire
routine in 100
Downloads: TUT19.zip