Energy #04 03 мая 1997

Программирование

Optimization - Optimizing procedures.

              OPTIMIZATION



                             (C) Demon / LGN


              OPTIMIZE!


                           Greed fraera

                                  ruin.

                        Folk wisdom.

 > D: (D - a DEMON) Recently, I realized that the word 
"Optimization" can hide not only benefit ... This article

is a response to a letter published in the ZX-Review 97/1-2 
(p.47), written certain AmiS. Tell him the word.


 > A: (A - a AmiS) Many have already written about
that it is time to put an end to raw and untested procedures.

 > D: Oh! Unlike you people think!

 > A: They themselves do in their papers are not valid from my 
point of view, the miscalculations. Here are some examples.


   Our task: to write the shortest procedure for cleaning the 
screen. 

 > D: And who is it staged?

 > A: Many beginners do things like:


        LD HL, # 4000

        LD DE, # 4001

        LD BC, # 1AFF

        LD (HL), L

        LDIR


    After sitting for some time can be reduced by this 
procedure: 


        LD HL, # 5B00

        DEC HL

        XOR A

        LD (HL), A

        OR H

        JR NZ, $ -6

 > D: I apologize that this nonsense stopped, but
I want to stand up for these most "beginners". If AmiS more for 
everything and could be considered, then the prices would be he 
was not. First program occupies 12 bytes and works

145,184 cycles. The procedure is a genius - 9 bytes
(Ur-r-r-a!! He won 3 bytes), but the time
performance ... 944,686 cycles ... Tidy
Tsiferki ... True, this work of genius
delirium will speed MORTAL KOMBAT on
ES1841 (namely, 6.5 times slower ...).
It is worth to think about the sense of this optimization. But 
AmiS'u achieved little and it does not ceases to surprise us ...


 > A: Look at this example. He
Born in a few hours of persistent assault.

 > D: Even BAYSYAK (who writes on
BASIC'e) could not slow down much.

 > A: But now you can safely use
this idea in their programs:

    Challenge: "turn" byte (ie, exchange
Bits 0-7, 1-6, 2-5, 3-4). Was 76543210
(% 01100001), was 01,234,567 (% 1,000,110)


        LD A, # 80

        ADD HL, HL

        RRA

        JR NC, $ -2

        LD H, A


    Bytes arrives at H, it is and leaves.

 > D: First give the procedure that I (and not just me!):


        RR n

        RLA

        RR n

        RLA

        RR n

        RLA

        RR n

        RLA

        RR n

        RLA

        RR n

        RLA

        RR n

        RLA

        RR n

        RLA

        LD n, A, where n - any register


    Procedure AmiS'a: takes 7 bytes, running 134 cycles. 
Limitations: Input B only H.


    The second procedure: occupies 25 bytes,
running 100 cycles. Restrictions: None.

    In size the author has saved 18 bytes, but
in the rate of losing 34 cycles.


    If I have anyone not yet convinced of the correctness of my 
words, then let them turn their eyes on MEGADEMY and many other 
programs that "well, oh-oh-oh-a" cool effects. In them, no one 
saves memory, as care about how to make it look

beautiful, but it did not take less space (which no one is 
never enough). 


    Also alarming words AmiS'a promised in the next room 
ZX-Review present to the "beginners" bend

best coders "perfected." Let's hope that he does not
want to optimize!