What BASIC does with your memory

Useful when a program gets big, when you want somewhere safe to POKE, or when you're wondering where the 30,718 bytes went.

The map

Everything BASIC owns sits between address 2048 and address 32768.

From	To	What's there
2048	↓	Your program, growing upwards as you add lines
	↓	Variables, added as they're first used
	↓	Arrays, added as they're DIMmed
	↑	Strings, growing downwards from the top
	32768	the ceiling

Program, variables and arrays grow up from the bottom. Strings grow down from the top. ?OUT OF MEMORY is what you get when the two meet.

PRINT FRE(0) is the gap between them. On a machine with nothing typed in yet:

PRINT FRE(0)

 30718

OK

Your program and its numbers grow up from 2048; strings grow down from 32768. FRE(0) is the gap in the middle.

What things cost

A number, in a variable or an array element	5 bytes
A string	1 byte per character, plus a few for the bookkeeping
A line of program	roughly its typed length, minus a byte for every keyword

That last one is why DIM X(999) costs 5,000 bytes and why long variable names are free at runtime but not while you're typing them in.

Why strings pause the machine

Every time you build a new string the old one is abandoned where it sat. When the space runs low, BASIC stops and shuffles the live strings back together — "garbage collection", and on a program juggling a lot of text you can watch it happen as a pause of a second or so. It is not broken. It is tidying.

Somewhere safe to POKE

Address 2560 is above the bottom of the program area, which makes it fine as scratch space for a short program and a bad idea for a long one — your own program lines will grow up into it.

For something bigger and genuinely out of the way, use banked RAM: the window at 32768 is not where BASIC keeps anything, and BANK gives you a fresh one whenever you want.

Reserved words

Your variable names can't contain any of these, anywhere in them — which is why SCORE fails (it contains OR) and TOTAL doesn't:

ABSANDASCATNBANKBLOADBRKBSAVECHR$CLRCLSCOLORCONTCOSDATADATEDEFDELDIMDIRDISKELSEENDEXPFNFORFORMATFREGOSUBGOTOHEXIFINKEYINPUTINTJOYLEFT$LENLETLISTLOADLOCATELOGMAXMEMMID$MINNEWNEXTNOTNVRAMONORPAUSEPEEKPOKEPOSPRINTREADREMRESTORERETURNRIGHT$RNDRUNSAVESETDATESETTIMESGNSINSOUNDSPCSQRSTEPSTOPSTR$SYSTABTANTHENTIMETOVALVOLWAIT

The two-letter ones are the dangerous ones: TO, ON, IF, OR, FN.

Why a keyword can hide inside a name

When you press Enter, BASIC immediately replaces every keyword it can find with a single byte — PRINT becomes one byte, GOTO becomes one byte. That's what makes a 40-line program fit in a few hundred bytes, and it's what LIST undoes to show you the text again.

The catch is that it does this by scanning for keywords before it has any idea that the letters it's looking at are meant to be a variable name. SCORE gets crunched into SC, the byte for OR, E — which isn't a name any more, so the line won't parse.

Text files and program files

A program on the memory card can be either of two things.

A tokenized program is what SAVE writes: the crunched bytes exactly as they sit in memory. LOAD reads it straight back. It's compact, it loads instantly, and it isn't readable by anything else.

A text file is a listing as plain characters, one line after another. That's what you get if you write your program on a laptop in an editor, and it's what you send when you paste a listing into a terminal on the serial port. The ACE reads it by pretending you typed it.

Cross-development tools convert between the two. Two rules travel with them:

• Load a tokenized program with LOAD, not by pasting it. The bytes are not text and the machine will not enjoy them.
• Don't edit the first line of a converted program. A program built to be started with RUN from a machine-code stub carries a SYS on its first line pointing at an address. Retype that line and the address moves.

Where the rest of it is

That's BASIC's half of the map. The other 32K — the ROM that BASIC itself lives in, the character set, the Monitor and the eight slots where the cards appear — belongs to the machine rather than to BASIC, and you reach it with PEEK.