Programming - The procedure for calculating the next and previous line of the screen. Optimization.
(C) EB Golyakov
(Spencer Winset, Diamond group, Moscow)
500:95 / 462.8 @ ZXNet
2:5020 / 2065.608 @ Fidonet
Procedure for calculating the next and previous lines of
the screen.
Optimization.
This publication is the result of optimization algorithms
calculating the next and previous screen line and the desire to
convey A new method to as many programmers because
so far I have not met with similar modifications in the current
programs or competitive works with various computer
Festivals. The author is counting on the reader's knowledge
assembler Z80 [2], and also address device memory view Spectrum
[3,4] or compatible computers.
*
The original algorithm has been considered previously by the
author [1] so I do not think it necessary to pay attention to
the description of the principles of his work, limited to only
a slight historical narratives. Since 1982, this method of
calculating the memory address the on-screen and not widely
used, due to uncertainty or unpopularity of Subprogramme ROM on
the Spectrum.
In Memory ROM (ZX Spectrum 48/128 (c) Sinclair Research Ltd)
function raspolagaetsyasya in addresses 3769-3784 in the
in-line form. (Note edition: in-line - a structural term,
implying embeddedness functions in the body of the program in a
certain place, excluding its challenge to the team CALL).
Below is a comparison of the original (A) with advanced
and still widely used procedure DOWN_HL (B)
authorship of which could not be established.
A. (Logic_HL) BA (DOWN_HL)
HEX OP CODE string HEX OP CODE
_____ ________________ _____ ________________
24 INC H INC H January 1924
7C LD A, H 2 7C LD A, H
E607 AND # 3 July E607 AND # 07
200A JR NZ, BREAK 4 200A JR NZ, BREAK
7D LD A, L 5 7D LD A, L
C620 ADD A, # 20 June C620 ADD A, # 20
6F LD L, A 7 6F LD L, A
3F CCF August 3804 JR C, BREAK
9F SBC A, A 9 7C LD A, H
E6F8 AND # F8 10 D608 SUB # 08
84 ADD A, H November 1967 LD H, A
67 LD H, A 1912 BREAK ...
BREAK ...
_____________________ _____________________
16b - 27/60t 16b - 27/49/59t
It is seen that for the same length in the sample (B) was
isolated overflow event register L, signifying the transition
to another third of the screen (line 8), and the execution time
can reduced from 60 to 49 cycles, this can be considered a
success, but presence on the screen only two of these lines
significantly reduces the the probability of cost savings. Just
shorten the calculation transition to the next position on a
clock, it is not essential, although and is valid for every
eighth line of the screen (except for two mentioned above).
We can say that just one stroke decided the fate of the first
algorithm, which was simply forgotten, in spite of all
ornateness logical structures; hence the name Logic.
Analyzing pulikatsiyu [1], it was found that the CCF team
(Complement Carry Flag), amending the state carry flag CY
the opposite (Line 8), whose application here
means that the action taken earlier (Line 6) have affected
flag is exactly the opposite. However, simple addition can be
replaced by subtracting the number of completed and CY flag will
respectively inverted automatically. Mentioned in
Article VI Roshchina [5]. Replacing the command ADD A, # 20 on
SUB # E0, we find that the arithmetic ekvivalentny, and the
team CCF no longer needed. Below is the result - a new
procedure DOWN_HL +, the associated with the previously
optimized DOWN_HL:
BA (DOWN_HL) V. (DOWN_HL +)
HEX OP CODE LINE HEX OP CODE
_____ ________________ ______ _______________
24 INC H INC H January 1924
7C LD A, H 02 7C LD A, H
E607 AND # July 2003 E607 AND # 07
200A JR NZ, BREAK April 2009 JR NZ, BREAK
7D LD A, L 05 7D LD A, L
C620 ADD A, # 20 June C620 SUB # E0
6F LD L, A 07 6F LD L, A
3804 JR C, BREAK 08 9F SBC A, A
7C LD A, H 2009 E6F8 AND # F8
D608 SUB # 10 August 1984 ADD A, H
67 LD H, A November 1967 LD H, A
BREAK ... BREAK ...
_____________________ _____________________
16b - 27/49/59t 15b - 27/56t
Of course, success here can be regarded as reducing the code
length 1 byte for such a relatively small size of bytes - it is
more than 6%. Also achieved gains in speed for 3 cycles for
every eighth line of the screen. Comparing the execution speed
rascheta (in cycles) for all lines of the screen, assuming the
first line given, we obtain:
Procedure (A) 27 * 167 +60 * 24 = 5949
(B) 27 * 167 +49 * 2 +59 * 22 = 5905 (-44)
(B) 27 * 167 56 * 24 = 5853 (-96)
Speed advantage of the procedure (A) was as follows: for
(B) 44 cycles, and for (B) 96 cycles, which is 2.2 times
greater. Thus, both onscreen sprites 50 to 40 lines, derived
ekonoiya not 450, but already 1000 cycles.
*
The following are procedures for calculation of the overlying
line Popular UP_HL (F) and similarly optimized UP_HL + (D), with
identical performance optimimizatsii described above:
G. (UP_HL) D. (UP_HL +)
HEX OP CODE LINE HEX OP CODE
_____ ________________ ______ _______________
7C LD A, H 01 7C LD A, H
25 DEC H DEC H February 1925
E607 AND # July 2003 E607 AND # 07
200A JR NZ, BREAK April 2009 JR NZ, BREAK
7D LD A, L 05 7D LD A, L
D620 SUB # 20 June C6E0 ADD A, # E0
6F LD L, A 07 6F LD L, A
3804 JR C, BREAK 08 9F SBC A, A
7C LD A, H 09 E608 AND # 08
D608 SUB # 10 August 1984 ADD A, H
67 LD H, A November 1967 LD H, A
BREAK ... BREAK ...
_____________________ _____________________
16b - 27/49/59t 15b - 27/56t
The author hopes that the material described will be useful
as creators 256/512 byte intro, and the authors of large-scale
game or demo projects, where the first place there is no
number of effects, umeschennyh in polkilobayta and Beauty
visualization, multiplied by the elegance and poignancy of code.
Literature
________________________________________________________________
1. Golyakov EB Support operations in the derivation of the
sprite in
area of the display memory. - "Nicron", 1998, N119.
2. Personal computer "ZX-SPECTRUM". Programming in
machine code and assembly language: in 3 hours-M., Inforkom,
1993.
3. Graphics ZX SPECTRUM. -M., VA PRINT, 1994. s.168-171.
4. Hardman, J., E. Hyuzon top 40 procedures. - "ZX-REVIEW, 1992,
N1, 2.
5. Roshchin I. More about the programming of arithmetic
operations.
- Hams. Your computer ", 2000, N12, 2001, N1-4.
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Programming - The procedure for calculating the next and previous line of the screen. Optimization.
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