PEEK and POKE

Statement syntax

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.

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 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 family 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. A canonical example is Mapping the Atari, which started at location zero and mapped out the entire 64 kB memory of the Atari systems location by location.

Pre and non-PC computers usually differ in the memory address areas designated for user programs, user data, operating system code and data, and memory-mapped hardware units. For these reasons, PEEK functions and POKE commands are inherently non-portable, meaning that a given sequence of those statements will almost certainly not work on any system other than the one for which the program was written.

POKEs as cheats

In the context of games for many 8-bit computers, users could load games into memory and, before launching them, modify specific memory addresses in order to cheat, getting an unlimited number of lives, immunity, invisibility, etc. Such modifications were performed using POKE statements. The Commodore 64, ZX Spectrum and Amstrad CPC also allowed players with the relevant cartridges or Multiface add-on to freeze the running program, enter POKEs, and resume.

For example, in Knight Lore for the ZX Spectrum, immunity can be achieved with the following command:

In this case, the value 201 corresponds to a RET instruction, so that the game returns from a subroutine early before triggering collision detection.

Magazines such as Your Sinclair published lists of such POKEs for games. Such codes were generally identified by reverse-engineering the machine code to locate the memory address containing the desired value that related to, for example, the number of lives, detection of collisions, etc.

Using a 'POKE' cheat is more difficult in modern games, as many include anti-cheat or copy-protection measures that inhibit modification of the game's memory space. Modern operating systems enforce virtual memory protection schemes to deny external program access to non-shared memory (for example, separate page tables for each application, hence inaccessible memory spaces).

16-bit PEEKs and POKEs

As most early home computers running BASIC used eight-bit processors, single peek or poke values were between 0 and 255. Setting or reading a 16-bit value on such machines therefore required two PEEK or two POKE as well as some algebra. Typically something like PEEK A+256*PEEK(A+1) in order to read a 16-bit integer value at address A, while you would use something like POKE A,(v AND 255) followed by POKE (A+1),trunc(v/256) in order to write the 16-bit integer v at address A.

However, BASIC on 16- or 32-bit machines, such as IBM PC or Commodore Amiga often had additional commands, such as DPEEK and DPOKE to read and set a 16-bit value in a single operation. Other 16/32-bit machines, such as Sinclair QL, had PEEK_W/L and POKE_W/L for reading and setting 16- and 32-bit values respectively, while Atari ST used the traditional names but had the possibility to define 8/16/32-bit memory segments and addresses that determined the size. Also some 8-bit machines had BASIC dialects with 16-bit PEEK and POKE, such as the East-German "Kleincomputer" KC85/1 (aka Z9001) and KC87, manufactured by VEB robotron - Meßelektronik "Otto Schön", which implemented DEEK and DOKE.

Peek and Poke in other BASICs

North Star Computers, a vendor from the early 1980s, offered their own dialect of BASIC with their NSDOS operating system. Concerned about possible legal issues, they renamed the commands EXAM and FILL.

BBC BASIC, used on the BBC Micro and other Acorn Computers machines, did not feature the keywords PEEK and POKE but used the question mark symbol (?), known as query in BBC BASIC, for both operations, as a function and command. For example:

32-bit values could be POKEd and PEEKed using the exclamation mark symbol (!), known as pling, with the least significant byte first (little-endian). In addition, the address could be offset by specifying either query or pling after the address and following it with the offset:

Strings of text could be PEEKed and POKEd in a similar way using the Dollar sign ($). The end of the string is marked with the Carriage return character (&0D in ASCII); when read back, this terminating character is not returned. Offsets cannot be used with the dollar sign.

Generic usage of "POKE"

"POKE" is sometimes used to refer to any direct manipulation of the contents of memory, rather than just via BASIC, particularly among people who learned computing on the 8-bit microcomputers of the late 1970s and early 1980s. BASIC was often the only language available on those machines (on home computers, usually present in ROM), and therefore the obvious, and simplest, way to program in machine language was to use BASIC to POKE the opcode values into memory. Doing much low-level coding like this usually came from lack of access to an assembler.

An example of the generic usage of POKE and PEEK is in Visual Basic for Windows, where DDE can be achieved with the LinkPoke keyword.

Cheats for eight-bit video games were sometimes referred to as pokes (see "POKEs as cheats" above).