Porting between dialects

What every port between these BASICs involves, whichever two machines you pick. For the differences between one machine and another, see the porting guide.

Seven things account for most of the work.

Restructuring. If the target allows only one statement per line, every : becomes a new line, which renumbers everything after it — the Renumber file feature fixes that. Without ELSE, each IF … THEN … ELSE becomes a test and its inverse, and a dialect that requires LET rejects a bare X=1.

Variable names. Where only the first two characters are significant, SCORE and SCALE are one variable, and the program misbehaves rather than fails. Where names are a single letter, long names must be remapped by hand. On the machines that ignore spaces outside strings, a name containing a reserved word is a syntax error — SCORE contains OR. The emulator's variable watcher shows what the program actually ends up with.

The characters themselves. No machine here covers printable ASCII in full, and several fall well short — a ZX81 has no !, #, & or @, and a Commodore has no \ or ^. A character the target has no glyph for cannot appear anywhere: not in a string, not in a comment, not in a variable name. The porting guide lists the ones your program actually uses.

Anything numeric. An integer-only machine has no fractions at all: division truncates and every fractional calculation needs rescaling. The exponent operator is spelled **, ^ or ↑ depending on the machine — on the Atom it belongs to the floating-point half of the language and an integer expression rejects it — and every machine here folds it left to right, so 2^3^2 is 64. Integer division and remainder are DIV and MOD on the BBC, \ and MOD on the Amstrad, % for remainder alone on the Atom, and nothing at all on the Sinclair and Microsoft machines, where they become INT(a/b) and a-b*INT(a/b).

Logic and comparisons. This is the one that does not fail — it computes a different answer and says nothing. On most of these machines AND, OR and NOT combine their operands bit by bit; on the ZX81 and the Spectrum they pick one of the operands instead, so 5 AND 3 is 5 there and 1 on a Commodore. A condition built from 0 and 1 behaves the same either way, which is why the line that masks bits is the line nobody rewrites. The truth value differs with it: a true comparison is -1 on the Microsoft, BBC and Amstrad machines and the ZX80, and 1 on the ZX81, the Spectrum and the Atom — so the counting idiom X=X+(A>B) changes sign after a port. Exclusive-OR is EOR on the BBC, XOR on the Amstrad, : on the Atom, and absent elsewhere. The two Sinclairs are not a pair here: the ZX80 sits with the bitwise machines and the ZX81 does not.

The arguments themselves. A command that survives the port under the same spelling can still take different arguments, and nothing about the line will look wrong — it simply does something else. Three ways this bites. Some machines take a leading argument the others don't: PLOT is PLOT <x>, <y> on the Sinclair and Amstrad machines but PLOT <action>, <x>, <y> on the Acorn ones, where the first number chooses whether to draw, invert or move. Some take the same arguments in a different order: the BBC's SOUND <channel>, <amplitude>, <pitch>, <duration> puts duration last, the Amstrad's SOUND <channel>, <period>, <duration> puts it third, and the Spectrum's BEEP <duration>, <pitch> puts it first. And some measure the same thing differently — that Amstrad <period> is a tone period, not a pitch. Coordinates can be relative rather than absolute, too: the Spectrum's DRAW takes an offset from where the last one finished, where every other machine here draws to a point.

Optional arguments are the quiet ones. Dropping an argument the target does not have is usually easy; noticing that the target needs one is not. Each language reference gives the full argument list for every command, written the same way on every page, so two pages can be read side by side — and the porting guide reports the commands whose usage differs between the pair you picked.

Everything touching hardware. Addresses never travel: a POKE, USR, CALL or SYS aimed at one machine's screen, sound chip or system variables means nothing on another, and neither do its control codes. Graphics and sound have to be rewritten rather than translated — these machines range from no graphics commands at all to PLOT/DRAW/CIRCLE with sound.