Apple II Machine Language Monitor

This article concerns the memory manipulation program in ROM of the Apple II family, not display devices.

The Apple II's ROM contains a very simple program for the display, modification and execution of memory addresses. Notably, the original Apple II starts up directly to the monitor, while later Apples would boot into Applesoft BASIC. All later models have an Autostart ROM, and start up into Applesoft BASIC. An Autostart option is available with some expansions to the original Apple II and can start up into either Integer BASIC or Applesoft BASIC.

Either way, if you're in BASIC, you access the monitor by running

] CALL -151

And you will get the Monitor prompt

*

To exit back to BASIC, press Ctrl-C and return. This preserves any BASIC statements in memory.

>5 LET A = 12345
>10 PRINT "HELLO!"
>15 PRINT A
>20 END
>RUN
HELLO!
12345

>CALL -151

* ==> hit CTRL-C here and press return <==

>PRINT A
12345

>RUN
HELLO!
12345