PEEK and POKE

In computing, PEEK is a BASIC programming language extension used for reading the contents of a memory cell at a specified address. The corresponding command to set the contents of a memory cell is POKE. Peek and poke are also implemented in popular versions of Pascal as well as in a few lesser known programming languages, such as COMAL, a structured Pascal-like BASIC dialect.

The PEEK function and POKE command are usually invoked as follows, either in direct mode (entered and executed at the BASIC prompt) or in indirect mode (as part of a program):

The address and value parameters may contain complex expressions, as long as the evaluated expressions correspond to valid memory addresses or values, respectively. A valid address in this context is an address within the computer's address space, while a valid value is (typically) an unsigned value between zero and the maximum unsigned number that the minimum addressable unit (memory cell) may hold.

The address locations that are POKEd or PEEKed at may refer either to ordinary memory cells or to memory-mapped hardware registers of I/O units or support chips such as sound chips and video graphics chips, or even to memory-mapped registers of the CPU itself (which makes software implementations of powerful machine code monitors 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 VIC-II graphics chip, which will make the screen border turn black:

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