Sega SG-1000

Hardware Description

Sega SG-1000 II

The system is a simple construction with only a few integrated circuits which contains a Zilog Z80, a TMS9918 VDP, a 76489 PSG, a 74LS139 decoder, a 74LS32 OR gate, 1KB of static RAM accessible to the processor and 16KB of VRAM for the VDP. In the first model, all those components are in their respective packages but some units of the revised SG-1000 II integrated most of the logic plus the VDP and PSG in a single integrated circuit which, at the same time, was capable of outputting RGB video, althought this feature was not used (the only output this machine had was RF). The console has no ROM on it, therefore it expects any code to be introduced at the system via the cartridge or the expansion ports.

Logical Description

SG-1000 Memory Map

Being handled so basically by a couple of decoders and only four OR gates, the decoding is very simple - and by the choice of incomplete decoding also suboptimal. The decision causes the last 15KB of memory space to be wasted as a mirror of the work RAM or losing most of the I/O ports. The first three-quarters of the address space is expected to be filled with a ROM in a cartridge or from an expansion, while the last is dedicated to RAM althought only the first kilobyte should be accessed, as the rest of space is mirrored.

Units in collection

Inventory ID	Model	Submodel
235	SG-1000	II