Sinclair ZX81 User Manual page 71

Type
POKE 17300,57
This makes the byte at address 17300 have the value 57. If you now type
PRINT PEEK 17300
you get your number 57 back. (Try poking in other values, to prove that there's no cheating.)
Note that the address has to be between 0 & 65535; & most of these will refer to bytes in ROM or
nowhere at all, & so have no effect. The value must be between -255 & +255, & if it is negative it gets 256
added to it.
The ability to poke gives you immense power over the computer if you know how to use it; however, the
necessary knowledge is rather more than can be imparted in an introductory manual like this.
The last big chip is the logic chip, or SCL (Sinclair Computer Logic) chip. Again, this was specially
designed & made for the ZX81, & it connects the other chips together in rather a clever way to make them
do more than they normally would.
The modulator converts the computer's television output into a form suitable for the television, & the
regulator converts the smoothed, but unregulated 9 volts of the power supply to a regulated 5 volts.
Summary
Chips
Statements: POKE
Functions: PEEK