Designing Embedded Hardware, 2nd Edition

Interacting with Hardware and Memory

Forth is great for debugging embedded hardware. You can use Forth to examine the contents of memory or peripheral registers:

2000 @

This places the content of address 2000 onto the stack. You can also use Forth to set memory or peripheral registers:

41 2000 !

This sets the content of address 2000 to 41. Remember that you can do this from within a word or interactively from the command prompt. This means that from the prompt, you can interactively probe and change peripheral registers—a very powerful debugging tool.

Warning

If you're using a version of Forth that is running on a desktop computer (such as a Mac or PC), don't try using store (!) or any other word that directly modifies memory, as it may have unforeseen (and possibly disastrous) results.

Fetch (@) and store (!) work with 16-bit-wide locations. To access 8-bit-wide locations, use the words c@ ("c-fetch") and c! ("c-store"), respectively. To access 32-bit-wide locations, use 2@ and 2!.

Warning

On some versions of Forth, such as gnu's gforth running on a PowerPC processor, @ and ! operate on 32-bit values rather than 16-bit values. Similarly, on the same platform, 2@ and 2! are 64-bit operations.

As an example, let's create a Forth program called status that will read the content of an 8-bit register located at address 2100 and place the result onto the stack.

: status 2100 c@ ;

This can be manually run from the prompt to examine the register during debugging:

status .

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