Address of a variable: Difference between revisions

{{works with|BASIC|2.0 (C-64)}}

With older machines, there's no built-in mechanism in BASIC to find a variable's address, but you can use the internal state of the BASIC interpreter to achieve similar results. You have to work around the fact that the interpreter is actively interpreting the running program: for example, you can't store the result of the lookup directly into a variable, because the assignment statement will have changed the pointer to that of the variable being assigned to by the time BASIC executes the <tt>PEEK</tt>.

Here's a C-64 version that works the same as the above C-128 program:

<syntaxhighlight lang="basic">100 X=123:Y=456

110 SYS 45195 X:POKE 249,PEEK(71):POKE250,PEEK(72)

120 PX=PEEK(249)+256*PEEK(250)

130 SYS 45195 Y:POKE 249,PEEK(71):POKE250,PEEK(72)

140 PY=PEEK(249)+256*PEEK(250)

150 PRINT "BEFORE:X ="XCHR$(20)", Y ="Y

160 FOR I=0 TO 6

170 : T = PEEK(PY+I)

180 : POKE PY+I,PEEK(PX+I)

190 : POKE PX+I,T

200 NEXT I

210 PRINT "AFTER: X ="XCHR$(20)", Y ="Y</syntaxhighlight>

It also works on the VIC-20 if you change the <tt>SYS</tt> address to 53387, and with a few other adjustments the same idea will work on other Commodore machines.