Spectrofon #19 31 марта 1996

Программирование

System - "quick chart" for novice programmers.

(C) Bob Skath


      Since B R A C T I G R A F K A



   Any programmer there are situations when he was like and 
knows assembler and still can not write anything worthwhile, 
and only look with envy six SMOOTH running along the curb and 
sidewalk scrollers have a newfound anything MAX IWAMOTO and 
compares with its squalid begushkoy size 3 * 3 pixels, which is 
feverishly cringe every time in the throes of emerging new 
symbol. A the thing is that the programming of such effects 
(like almost all dynamic toys) is built on the principle of so- 
called "fast" graphics. 



   Here's how, for example, the program looks
Programmer (My goodness, what I write), who had just mastered 
the work with sprites, and whose head is occupied by structured 
programming: 


        DI; While the all clear

        LD HL, # 4000; It is also clear

        LD DE, SPRITE; DE - address of the sprite in memory

        LD BC, # 0504, B - the size of X, C - the size of Y
LOOP1 PUSH BC; save BC

        PUSH HL; keep current address on the screen
LOOP2 LD A, (DE); take a byte sprite

        LD (HL), A; put it on the screen

        INC HL; increase the address on the screen

        INC DE; increase the address on the sprite

        DJNZ LOOP2; while X <> 0

        POP HL; restore the address on the screen

        POP BC; restore the size counter

        CALL NXTLIN; define the following line in the screen

        DEC C; the size of Y is over?

        JR NZ, LOOP1; if not then repeat the cycle.

        ....

NXTLIN INC H; routine determination of subsequent
        LD A, H; yuschey line in the video. At the inlet

        AND 7; in HL - address of the screen. At the exit

        RET NZ; desired address again in the HL.

        LD A, L

        ADD A, # 20

        LD L, A

        RET C

        LD A, H

        SUB 8

        LD H, A

        RET


   Well ... Quite decent  program to 
display static images , but the trouble starts when this 
same programmer, inspired by previous success, is trying to 
remake this very program from for the withdrawal of Moving 
sprites and yes even Larger, in terms of register pair HL to 
the entrance, which would display images in different parts of 
the screen. Here is where the fun begins ... 


   The image begins to distort,
blink and do everything to make the pseudo-programmer biting 
elbows of fury. 


   So I decided to help a bit sim
people in order to alleviate their suffering to their
series of articles on this topic. The whole principle
fast graphics is that the image is not built directly on the 
computer screen, and the so-called virtual screen that is 
somewhere in the depths of memory machines. Picture first 
construct all - for the sprite sprite overlap in

the correct sequence subroutine
similar to the above, with the difference that the virtual 
buffer line Raster go one after another, and then as one 
gigantic sprite is displayed. 


   Here's how I see, half of the readers
of this article with a cry: "Wooooow, I poooonyayayal !!!!" 
famously shed a car and quickly downloaded TASM and SPRITE 
EDITOR. 

   Well, nothing a couple of minutes they are satisfied that 
everything is not so simple and clear, as seems at first. The 
whole point is that the picture must have time to rebuild no 
more than one interval between interrupts, otherwise it will be 
a beard, and all work go awry. And it's not so easy,

This proves the simple calculation of execution time the above 
program. Perhaps some of the readers who have hung their noses 
and think about the impermanence of all things, but nothing - 
just remember ROBOCOP'a or there WEC LE MAN. There are in fact 
nothing jerks?? That means you can?? Of course you can ! Only 
need to consider several points: 

1. Usually not required to rebuild the entire
screen, grabs a redrawing its
part and the rest - static information (
as the number of lives, the name and MEGADEMO
etc. )


2. Need to rebuild discard any silly ideas about the structural 
approach and nested loops (that you do not sort

database fields 172627373)

3. Need to think a little. And yet!


   As an example, I will give a procedure
which builds familiarity picture size 26 * 128 pixels at the 
top of the screen. 


        DI; Interrupts necessarily DI!!!

        LD C, # 48; token move to another one-third

                                  Screen

        LD HL, SCR +28 +1; address output to the screen, taking 
into account 

                                  O right-left

        LD (ADDR1 +1), HL; Set screen address

        LD IX, PICT + # 11; address of the virtual screen

                                  Memory based read-out cross
                                  fray

        CALL ROUT1; Drawing the first third of the screen

        LD C, # 50; Marker transition in the last third

                                  Screen


        LD HL, SCR1 +28 +1; address output to the screen in the 
second 

                                  third

        LD (ADDR1 +1), HL; Set screen address
ROUT1 LD B, 64; number of lines drawn for

                                  one-pass procedure
LOOP2 LD (SPSAVE), SP; save SP for the correct

                                  Released

        LD SP, IX; Current Address Read

        POP AF; Chi

        POP DE; that

        POP HL; it

        EXX;

        POP BC; car

        POP DE; Ting

        POP HL; ku
ADDR1 LD SP, 0000 It has been established to address

                                  recording images to the screen

        PUSH HL; We write

        PUSH DE; raw

        PUSH BC; Vai

        EXX;

        PUSH HL; car

        PUSH DE; Ting

        PUSH AF; ku

        LD (ADDR2 +1), SP; Save the current address of record

        LD SP, # FFF2; Set the register-

        ADD IX, SP; couple IX to the next byte

        LD SP, IX; virtual screen

        POP AF; Chi

        POP DE; that

        POP HL; it

        EXX;

        EX AF, AF ';

        POP AF; car

        POP BC; Ting

        POP DE; ku

        POP HL;
ADDR2 LD SP, 0000 It has been established to address

                                  recording screen

        PUSH HL; Over

        PUSH DE; pi

        PUSH BC; raw

        PUSH AF; Islands

        EXX; it

        EX AF, AF ';

        PUSH HL; car

        PUSH DE; Ting

        PUSH AF; ku

        LD HL, # 011A; calculate the address of the follow-

        ADD HL, SP; line in the same third of the screen

        LD A, H; not check out whether

        CP C; we are in another third of the screen

        JR C, LOOP1; or other familiarity

        LD DE, # F820; If so, produce

        ADD HL, DE; correction
LOOP1 LD (ADDR1 +1), HL; Save the current address for

                                  recording screen

        LD SP, 46; next line virtual

        ADD IX, SP; buffer

        LD SP, IX;

        DJNZ LOOP2; If the cycle has not been exhausted, 
repeated                                   We say

SAVESP LD SP, 000; Restore stack

        RET; Return


   Well?? At first glance, cumbersome,
but it works fine. Oh, and it is clear
why such a performance? Correctly,
because at one time read and write
two bytes.



   Attention is drawn to the fact that
to accelerate the work is missing here
routine determination of successive rows, and output in each 
third is separate. 


   Thus it is possible to display
buffers of any size (but the multiple
on X two, I wonder why?) with a fairly decent speed for 
animation or revive your toys.



  Well ... Enough for today ... The next time I'll talk about 
how fast imprint sprites in a virtual buffer

mask and without. And more ... But more about that next time.


      Bob Skath. 1996. Latvia. Daugavpils.