Editing PEEK and POKE (section)

==Memory cells and hardware registers==

The address locations that are POKEd or PEEKed at may refer either to ordinary memory cells or to [[memory-mapped I/O|memory-mapped]] [[hardware register]]s of [[input/output|I/O]] units or support chips such as [[sound chip]]s and video graphics chips, or even to memory-mapped [[processor register|register]]s of the CPU itself (which makes software implementations of powerful [[machine code monitor]]s and [[debugging]]/simulation tools possible). As an example of a POKE-driven support chip control scheme, the following POKE command is directed at a specific register of the [[Commodore 64]]'s built-in [[MOS Technology VIC-II|VIC-II]] graphics chip, which will make the screen border turn black:
<syntaxhighlight lang="qbasic">POKE 53280, 0</syntaxhighlight>

A similar example from the [[Atari 8-bit computers]] tells the [[ANTIC]] display driver to turn all text upside-down:
<syntaxhighlight lang="qbasic">POKE 755, 4</syntaxhighlight>

The difference between machines, and the importance and utility of the hard-wired memory locations, meant that "memory maps" of various machines were important documents. An example is ''[[Mapping the Atari]]'', which starts at location zero and maps out the entire 64 kB address space, location by location.