The first computer I spent much time in front of was a 1mhz 6502 based computer with 12K of RAM and hooked up to a TV to display 256 by 192 black and white pixels and all programs were loaded and saved to cassette tapes.
Yet that was huge compared to the computer that controlled the Apollo Lunar Modules. The Apollo Guidance Computer (AGC) had 2048 words of magnetic core memory and 36 kilowords of read only core rope memory. The interface was a few rows of segmented displays that showed the contents of some registers, some blinking lights, and a tiny pushbutton keyboard.
That's it. That's what it took to get two people down to the moon and back up again.
It turns out there are simulators and emulators that allow you to use almost every variation of the AGC ever produced. You can marvel yourself at one very compact and amazing piece of programming.
In case you want to know a bit more about the AGC and what it took to make it work may I suggest Tales From The Lunar Module Guidance Computer by Don Eyles. If you want to understand the design, the programming, and the decisions that went into the AGC there is no better place to go.