Basic Programming Tips

One of the greatest computers ever created is the one that I am using now to write this essay. Of course, it is our favorite baby, the MOD-100. It has a number of features that I have been used to for the past several years on larger commercial computers, i.e. a real-time interrupt driven clock, the interrupt structure itself, a typeahead buffer, a device structured I/O, etc.

I could rhapsodize a great deal more, but all of us know what a great machine this is. Withall, there are a number of shortcomings that exist as a result of creating a totally portable machine. The most aggravating of these is the shortage of memory. For the last five years, I have been programming on a DEC PDP-11/03, which also suffers the same deficiency. As a consequence, I deve-loped a number of strategies to cope with too much program in too little memory. In the process I also discovered methods of speeding things up a bit.

Finally, I have uncovered a number of "features" unique to our own favorite that can be useful in programming this machine.

Speed & Conservation of Memory

Every KEYWORD, quoted literal, punctutation mark, occupies a byte in memory. Every line number uses two bytes for the number and a "Null" byte that terminates each line. Each space takes a byte. Variables use a two-byte pointer to the area in memory that the variable is stored in. In addition, each type of variable uses different amounts of storage space. Integers -require 2 bytes, reals - four bytes and double precision - eight bytes. Strings take one byte for each character and they are allocated dynamically. This is all background for the following space & time saving tips:

All variables should be typed at the beginning of each program. Use DEFINT, DEFSNG, etc. Even string variables should be typed because the "$" used with each such variable takes a byte of storage.

Use integers wherever possible, especially in counting loops. The time savings is dramatic. Integers are 2 1/2 times faster than the default of double-precision. Reals are only 25% faster than D.P.

Eliminate the variable argument on the NEXT portion of FOR/NEXT loops.

Here is a real kicker - besides the saving of one byte of storage for each such occurence, integers are 60% slower, reals are 50% slower and D.P. is 80% slower when you specify the variable than when you simply specify NEXT.

Eliminate final quote marks on all literals that appear at the ends of lines. A byte is saved for each.

Another surprise that violates all that I knew of previous Microsoft Basics is that semi-colons are NOT routinely needed as variable punctuation. They only seem to be required after PRINT USING or if you want to keep the cursor from advancing to a new line or in an INPUT statement. All other uses are gratuitous and can be left out to save one byte per occurence.

Use as many multiple statements per line as you can since for each line number eliminated, you have a net savings of two bytes (the three needed for each separate line less the one byte used for each ":".

Eliminate ALL spaces. Your archival copy should have them for intelligibility, but your running copy will save one byte for each space that is removed.

Put all initialization code at the end of your program and do a GOTO or a GOSUB. Little used routines at the end make for a dramatic improv-ment in speed since MSBasic must search from the beginning of the file for each line referenced.

Put all time critical code at the beginning of the program for the same reason as 8: above.

Initialize all variables before you start the main body of code. This creates a stable, linear table of variables that are accessed more rapidly.

DO NOT use the supposed shortcut of raising a number to the .5 power to save some time over the SQR routine. Inaccuracies in the 11th and 12th decimal place might tend to screw up any further calculations in which you might use this result.

Assign literals that are used more than once in a program to a vari-able. Do this in your initialization code. Almost one byte per character is saved for each such duplication eliminated.

Use SPACE$(X) to assign X number of spaces rather than STRING$(X,32).

Another surprise: use REM instead of "'". This save two bytes per occurence. Of course, you should remove all REM statements of both types from your running copy.

I hope that these random ramblings of mine will prove useful and will provoke other users to add to this document to the benefit of us all.

One of the serious deficiencies of the built-in Basic interpreter is the lack of user-defined functions. I hope the RS will include them in any disk extended basic that they come up with, when they come up with one, when and if they come up with disk drives and an operating system. At the same time, perhaps they will implement the type of extended RAM addressing ala the NEC MEWS. Radio Shack, are you listening? With the DEFFN a great many subroutines could be turned into callable functions A Trace feature as well as Random file I/O would also be welcome. Let's hope!