Sega Master System / Mark III / Game Gear
The original Master System hardware includes a Reset button whose purpose is to restart the currently running software. While it might be expected that this would be directly connected to the CPU's RESET line, it is in fact mapped via the I/O chip to the peripheral port interface and thus must be handled in the same way as controller inputs - software must poll it to detect when it is pressed and is free to ignore it or perform some other action when it is pressed.
Its value is read from port $DD; see the linked page for more information.
The Mark III, Japanese Master System, Master System II, Game Gear and Mega Drive Power Base Converter do not have a reset button and it is always read as unpressed.
The SG-1000 does not have a reset button, but the SC-3000's keyboard includes one that is connected to the Z80's NMI line; thus, SC-3000 software must include an interrupt handler at offset $0066 to handle it. A sufficient handler would be