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V/+INDIRE

This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

Tape/disk: Home » Personal collection » Acorn ADFS disks » Electron_User_Group » EUG_53.ADF
Filename: V/+INDIRE
Read OK: ✔
File size: 185E bytes
Load address: 56204556
Exec address: 4E492B2E
Duplicates

There are 2 duplicate copies of this file in the archive:

File contents
             FASTER PROGRAMMING USING INDIRECTION OPERATORS
                          By C. E. H. Francis

IF you have tried writing anything but the most simple Basic game, the 
chances are that you will have tried using arrays. This causes two
problems: arrays use up a lot of memory, and they are relatively slow.
Fortunately, both problems can be solved without resorting to machine
code by the use of the so-called INDIRECTION OPERATORS provided by BBC
Basic. The full description of indirection operators lies in the part of
the manual that most of us don't read (pages 129, 130 and the section on
assembly language programming), but in fact they are remarkably easy to
use.
   An indirection operator allows a Basic program to use any memory
location directly; on many micros the actions involved are known as 
'peeks' and 'pokes'. The simplest way to use indirection operators 
applies to integer arrays when it is known that all the numbers to be
stored are positive integers less than 256. For example, two arrays 
could be used to store the X and Y co-ordinates of user defined char-
acters within a game, with one for the X values, and the other for the
Y values. In this case, the two arrays can be replaced directly. 
Consider the following short program, which loops round setting an array
to a specific value:
      10 DIM A%(999)
      20 TIME=0
      30 FOR I%=0 TO 999:A%(I%)=1:NEXT
      40 PRINT TIME

In Mode 6, this took 1.38 seconds on my Electron. Now replace A%(999) by
A% 999 and A%?I% (? is pronounced as 'query'). With this replacement, 
the loop takes 1.02 seconds: a saving of 25%. This can be a very major
saving in a game in which you may access a number of arrays hundreds of
times whenever the main loop of the program is executed.
   DIM A% 999 assigns 1000 bytes of memory from A% to A%+999. To find 
out where in memory this might be, type PRINT A% after the above program
has been run. A%?I% gives the contents of memory location A%+I%, but
this could also be accessed with ?(A%+I%). This gives a clue to an even
greater saving. Replace line 30 above with:
      30 FOR I%=A% TO A%+999:?I%=1:NEXT

This accomplishes exactly the same thing, but the program now runs in 
0.92 seconds, a total saving of around 33%.
   This is all very well, but what if you want to use an array which 
takes values requiring more than one byte of memory (i.e. integers 
greater than 255)? The savings in time are no less dramatic using the !
(pronounced 'pling') indirection operator. The Electron stores integers
in four consecutive bytes, and !I% gives the integer stored in locations
I%, I%+1, I%+2 and I%+3. For an integer array with 1000 members, 4000 
(1000*4) memory locations must be reserved, and the step size should be
4. Replace lines 10 and 30 with:
      10 DIM A% 3999
      30 FOR I%=A% TO A%+3999 STEP4:!I%=1:NEXT

This is barely slower, taking 0.94 seconds. In addition, any integer
value can be placed in the location I% by using !I%=<value>.
   String arrays can be replaced using the $ (pronounced 'string') 
indirection operator, and the savings are even greater. Try using
      10 DIM A$(999)
      30 FOR I%=0 TO 999:A$(I%)="TEST":NEXT

This took 1.48 seconds on my Electron. When using indirection operators
the string "TEST" requires five memory locations, one for each letter,
and one for a <RETURN> to indicate the end of the string. To see how the
$ indirection operator works, try altering lines 10 and 30 to:
      10 DIM A% 9999
      30 FOR I%=A% TO A%+4999 STEP 5:$I%="TEST":NEXT

This saves over half a second, taking 0.96 seconds. Virtually no time is
lost by setting aside more memory, and using a larger step size, for 
example DIM A% 9999 and STEP 10. The maximum length of a string which 
can be stored like this is one less than the step size used. Due to the
way in which string arrays are allocated memory on the Electron, the
savings in memory are quite significant.
   Incidentally, when indirection operators are used like this some of
the Basic string operators can be simulated very easily. In the above
examples, PRINT ?A% gives the ASCII code for T, which PRINT $(A%+1) 
gives EST, simulating the operation of the RIGHT$ function. The Electron
simply reads the string from wherever it is told to start, and stops 
when it reaches a <RETURN> (ASCII value 13). LEFT$ can also be simulated
by placing a <RETURN> in a particular memory location. For example 
$(A%+2)="" which puts a <RETURN> character at A%+2, followed by PRINT 
$A% will give TE. Strings can be combined by placing the first letter of
one string in the location occupied by the <RETURN> for another string.
For example, type S(A%+2)="STING". PRINT $A% now gives TESTING.
   Finally, it is worth examining the savings which can be made on using
two (or more) dimensional arrays. These are even slower than one 
dimensional arrays. For example, type in the lines listed here:
      10 DIM A%(9,99)
      30 FOR K%=0 TO 9:FOR J%=0 TO 99:A%(K%,J%)=1:NEXT

When run, the program now takes 2.27 seconds. Indirection operators do 
not allow the use of more than one dimension, so the two dimensional
array above must be replaced by the one dimensional set of memory 
locations DIM A% 999. Then K% and J% have to be replaced by I% running
from A% to A%+999. This brings the time down to 0.92 seconds, but you do
have to be careful with the arithmetic. In this example, the actual
location of the element A%(K%,j%) is found as follows:
      I%=A%+100*K%+J%

So you must remember that increasing I% by one corresponds to increasing
J% by one, unless the value of J% is 99, when it corresponds to increas-
ing K% by 1 and resetting J% to 0. Increasing I% by 100 corresponds to
increasing K% by 1 and leaving J% alone. The situation is essentially
similar in the case of the ! and $ indirection operators, but then you
also have to be careful about the sizes. By the way, don't succumb to
the temptation of letting the micro do the arithmetic for you by using 
an equation such as the one above inside the FOR-NEXT loop - you will
very likely lose all the time saved!
                                                     C. Francis, EUG #53
                                         'Stolen' from ELBUG Vol 1 No 10

00000000  20 20 20 20 20 20 20 20  20 20 20 20 20 46 41 53  |             FAS|
00000010  54 45 52 20 50 52 4f 47  52 41 4d 4d 49 4e 47 20  |TER PROGRAMMING |
00000020  55 53 49 4e 47 20 49 4e  44 49 52 45 43 54 49 4f  |USING INDIRECTIO|
00000030  4e 20 4f 50 45 52 41 54  4f 52 53 0d 20 20 20 20  |N OPERATORS.    |
00000040  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00000050  20 20 20 20 20 20 42 79  20 43 2e 20 45 2e 20 48  |      By C. E. H|
00000060  2e 20 46 72 61 6e 63 69  73 0d 0d 49 46 20 79 6f  |. Francis..IF yo|
00000070  75 20 68 61 76 65 20 74  72 69 65 64 20 77 72 69  |u have tried wri|
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00000090  74 20 74 68 65 20 6d 6f  73 74 20 73 69 6d 70 6c  |t the most simpl|
000000a0  65 20 42 61 73 69 63 20  67 61 6d 65 2c 20 74 68  |e Basic game, th|
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00000100  61 72 72 61 79 73 20 75  73 65 20 75 70 20 61 20  |arrays use up a |
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00000200  69 65 73 20 69 6e 20 74  68 65 20 70 61 72 74 20  |ies in the part |
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00000280  69 6e 20 66 61 63 74 20  74 68 65 79 20 61 72 65  |in fact they are|
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00000300  6f 6e 20 6d 61 6e 79 20  6d 69 63 72 6f 73 20 74  |on many micros t|
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00000580  34 30 20 50 52 49 4e 54  20 54 49 4d 45 0d 0d 49  |40 PRINT TIME..I|
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00000850  3d 31 3a 4e 45 58 54 0d  0d 54 68 69 73 20 61 63  |=1:NEXT..This ac|
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00000a40  64 20 49 25 2b 33 2e 20  46 6f 72 20 61 6e 20 69  |d I%+3. For an i|
00000a50  6e 74 65 67 65 72 20 61  72 72 61 79 20 77 69 74  |nteger array wit|
00000a60  68 20 31 30 30 30 20 6d  65 6d 62 65 72 73 2c 20  |h 1000 members, |
00000a70  34 30 30 30 20 0d 28 31  30 30 30 2a 34 29 20 6d  |4000 .(1000*4) m|
00000a80  65 6d 6f 72 79 20 6c 6f  63 61 74 69 6f 6e 73 20  |emory locations |
00000a90  6d 75 73 74 20 62 65 20  72 65 73 65 72 76 65 64  |must be reserved|
00000aa0  2c 20 61 6e 64 20 74 68  65 20 73 74 65 70 20 73  |, and the step s|
00000ab0  69 7a 65 20 73 68 6f 75  6c 64 20 62 65 0d 34 2e  |ize should be.4.|
00000ac0  20 52 65 70 6c 61 63 65  20 6c 69 6e 65 73 20 31  | Replace lines 1|
00000ad0  30 20 61 6e 64 20 33 30  20 77 69 74 68 3a 0d 20  |0 and 30 with:. |
00000ae0  20 20 20 20 20 31 30 20  44 49 4d 20 41 25 20 33  |     10 DIM A% 3|
00000af0  39 39 39 0d 20 20 20 20  20 20 33 30 20 46 4f 52  |999.      30 FOR|
00000b00  20 49 25 3d 41 25 20 54  4f 20 41 25 2b 33 39 39  | I%=A% TO A%+399|
00000b10  39 20 53 54 45 50 34 3a  21 49 25 3d 31 3a 4e 45  |9 STEP4:!I%=1:NE|
00000b20  58 54 0d 0d 54 68 69 73  20 69 73 20 62 61 72 65  |XT..This is bare|
00000b30  6c 79 20 73 6c 6f 77 65  72 2c 20 74 61 6b 69 6e  |ly slower, takin|
00000b40  67 20 30 2e 39 34 20 73  65 63 6f 6e 64 73 2e 20  |g 0.94 seconds. |
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000017d0  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
*
00001800  20 43 2e 20 46 72 61 6e  63 69 73 2c 20 45 55 47  | C. Francis, EUG|
00001810  20 23 35 33 0d 20 20 20  20 20 20 20 20 20 20 20  | #53.           |
00001820  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00001830  20 20 20 20 20 20 20 20  20 20 20 20 20 20 27 53  |              'S|
00001840  74 6f 6c 65 6e 27 20 66  72 6f 6d 20 45 4c 42 55  |tolen' from ELBU|
00001850  47 20 56 6f 6c 20 31 20  4e 6f 20 31 30 0d        |G Vol 1 No 10.|
0000185e
V/+INDIRE.m0
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V/+INDIRE.m2
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