The vertical scrolling effect in the original “The Legend of Zelda” relies on manipulating the NES graphics hardware in a manner likely that was unintended by its designers.[…]
Writing to a particular PPU register while a frame is being drawn can result in graphical artefacts. The Legend of Zelda intentionally causes an artefact which manifests itself as partial vertical scrolling. This post gives some background on NES graphics hardware, and explains how the partial vertical scrolling trick works.
Game developers on these older, constrained systems had to resort to some very clever thinking to work around said constraints.
Not just game developers. Programmers of every system until somewhat recently (read: late 90’s) had to deal with a lot of system constraints.
With the NES, more often than not they resorted to custom ASICs to integrate into the cartridge to accomplish these hacks.
An example is Super Mario Bros 3, which used the MMC3 mapper to allow both split-screen vertical scrolling, as well as diagonal scrolling.
Zelda, though, was designed without mappers, since it was originally released on the Famicom Disk System, which used a custom floppy disk rather than a ROM cartridge.
In the US and Europe, Zelda used the MMC1 mapper, which enables battery-backed save games, the first NES game to have this feature.
Unfortunately, the NES was not designed to accommodate battery saves in cartridges, and pressing the power switch shorted out the needed pins, causing data loss. That’s why you had to hold the reset switch in when you powered off in order to prevent losing your save data.
The redesigned top-loading NES console fixed the issue, so if you turned a game off without holding reset button you wouldn’t lose your data.