Info Guide #01

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Programming - On the derivation of the screen for one interrupt.

  On the derivation of the screen for the interruption
  Alone Coder


   The first issue of PC Magazine
ZX-Power was opubikovana article, whose author, calling in aid 
of mathematics, convincingly refuted programming legend of that 
someone allegedly threw a screen (without attributes t.e.6144 
bytes) per one interrupt Pentagon (71680 CPU cycles).


   The author was right and wrong simultaneously.
Rights, it was that in 1996 none of the
Programmers do not write the above effekta.Ne rights, he was in 
the fact that its bias on the impossibility of implementing the 
above-mentioned effect, he missed a (One) way to display the 
screen, namely a method LD-PUSH. POP-PUSH method requires 
frequent modification of the stack pointer SP and therefore not

provides the desired speed, the method also modifies the stack 
LDPUSH least twice. 

   That's what I say:

      LD SP, ...

      LD DE, xx

      PUSH DE

      LD DE, yy

      PUSH DE

      ...

   You can tell me that in this
article dealt with the moving screen, and my
method displays only those data which are specified directly in 
the program output. This is wrong: LD also reads the data, but

lying only in a certain area of ​​memory. This shortcoming 
redeem three advantages: exemption registers allows us to 
modify SP calculations, the most recent offers the ability to 
move data receiver, which makes the method POP-PUSH; uniformity 
of operations in the program simplifies the implementation. 

   Show you how to use the method, in particular, for smooth 
scrolling upwards. PROG; beginning of the program output


      DOWN SP


      LD DE, nn 16 times 192 times

      PUSH DE


      JP PROG

   Under DOWN SP means protsedurka,
which calculates the address of a string lying
one below this in the HL, and puts it
in SP.Prosteyshy example of such protsedurki:

      SET 7, H

      LD E, (HL)

      INC L

      LD D, (HL)

      EX DE, HL

      LD SP, HL

   It uses a table, lying in the address # C000Ў # D800 and 
takes 35 clock cycles. At first, I did and used, but because of 
its slowness, I had to donate two outermost columns of the 
screen, so the program no longer satisfies the given condition. 
If you change it to this model, the rate will be quite meet the 
required: 

      SET 7, H

      LD SP, HL

      POP HL

      LD SP, HL

   This version uses a similar table, but eat for 30 
taktov.Finalny bar in this procedure:


      INC H

      RRCA

      JR NC, $ +6

      SET 7, H

      LD SP, HL

      POP HL

     LD SP, HL

   Now uses only 28 3 / 8 stroke
(A is set in a register one bit, so
central part is called a time out of 8).
If you replace the JR to JP, a total of 28 cycles.

   The program is closed on itself. How did it
cause?

   The plant in the main program (Manager) counter, which 
indicates some line of 192, which displays the program.

Make an breakpoint, ie, JP USE, and
jump into a program to bypass the modification procedure SP 
(which I just mentioned). It looks like this: BEGIN LD (HL), 195


      INC HL

      LD DE, USE

      LD (HL), E

      INC HL

      LD (HL), D

      PUSH HL

      LD DE, 6

      ADD HL, DE

      LD (USE +1), SP

      PUSH HL

      LD HL, # 4020; end of the 1-th row of the screen

      LD A, 128

      RET
USE LD SP, 0

      POP HL

      LD (HL), code jr nc

      DEC HL

      LD (HL), code rrca

      DEC HL

      LD (HL), the code inc h

      LD DE, 4 * 16 9

      ADD HL, DE

      BIT 7, (HL)

      JR Z, $ +5

      LD HL, PROG; if you've reached the end

      JR BEGIN

   To insert a HALT, must have another
one meter (eg, HL '), which would indicate a 96 rows below the 
first, but instead breakpoint'a put JP (IX), where IX = IMER

IMER LD SP, 0

      EXX

      HALT

      LD (HL), code rrca

      DEC L

      LD (HL), the code inc h

      PUSH HL

      EXX

      RET

   How to display the letters? According to one line: in
that line, which is released after changing the counter N 1, 
the parameters of the LD must be write the corresponding line 
bukv.Ne me you write about it: you yourself know how

this is done, and if you do not know, so more interesting to 
discover it for themselves. 

   The program, which depicts the effect of
shift upwards with a yardstick of letters, there is a
Appendix. It's called PUSHROLL.H.


   PS: one of the demonstrations FUNTOP '98
 Displays the screen in an interrupt, but
it seems to be some other effect,
because it is the epilogue to the shift occurs
As always, the classic - for two