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PEEK and POKE
In computing, PEEK and POKE are commands used in some high-level programming languages for accessing the contents of a specific memory cell referenced by its memory address. PEEK gets the byte located at the specified memory address. POKE sets the memory byte at the specified address. These commands originated with machine code monitors such as the DECsystem-10 monitor; these commands are particularly associated with the BASIC programming language, though some other languages such as Pascal and COMAL also have these commands. These commands are comparable in their roles to pointers in the C language and some other programming languages.
One of the earliest references to these commands in BASIC, if not the earliest, is in Altair BASIC. The PEEK and POKE commands were conceived in early personal computing systems to serve a variety of purposes, especially for modifying special memory-mapped hardware registers to control particular functions of the computer such as the input/output peripherals. Alternatively programmers might use these commands to copy software or even to circumvent the intent of a particular piece of software (e.g. manipulate a game program to allow the user to cheat). Today it is unusual to control computer memory at such a low level using a high-level language like BASIC. As such the notions of PEEK and POKE commands are generally seen as antiquated.
The terms peek and poke are sometimes used colloquially in computer programming to refer to memory access in general.
The PEEK function and POKE commands 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 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:
A similar example from the Atari 8-bit computers tells the ANTIC display driver to turn all text upside-down:
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.
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PEEK and POKE
In computing, PEEK and POKE are commands used in some high-level programming languages for accessing the contents of a specific memory cell referenced by its memory address. PEEK gets the byte located at the specified memory address. POKE sets the memory byte at the specified address. These commands originated with machine code monitors such as the DECsystem-10 monitor; these commands are particularly associated with the BASIC programming language, though some other languages such as Pascal and COMAL also have these commands. These commands are comparable in their roles to pointers in the C language and some other programming languages.
One of the earliest references to these commands in BASIC, if not the earliest, is in Altair BASIC. The PEEK and POKE commands were conceived in early personal computing systems to serve a variety of purposes, especially for modifying special memory-mapped hardware registers to control particular functions of the computer such as the input/output peripherals. Alternatively programmers might use these commands to copy software or even to circumvent the intent of a particular piece of software (e.g. manipulate a game program to allow the user to cheat). Today it is unusual to control computer memory at such a low level using a high-level language like BASIC. As such the notions of PEEK and POKE commands are generally seen as antiquated.
The terms peek and poke are sometimes used colloquially in computer programming to refer to memory access in general.
The PEEK function and POKE commands 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 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:
A similar example from the Atari 8-bit computers tells the ANTIC display driver to turn all text upside-down:
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.