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The architecture of this console is very similar to its predecessor, the Sega SG-1000, so the Master System managed to gain backwards compatibility with the SG-1000. Thus, it had to fit everything in the memory map. The way SEGA interconnected the CPU with the rest of the components enables not only to access values but also to show/hide certain components from appearing in the memory map.Ĭuriously enough, the Game Boy had a Z80 ‘variant’ that completely omitted the I/O ports. In the case of an IN instruction, the CPU will store the received value on a pre-defined register. The addressed peripheral must manually check for the address bus and the I/O pins and perform the required operation. When an IN or OUT instruction is executed, the Z80 sets up the address lines pointing to the peripheral (which could be, for instance, a keyboard), flags its IORQ pin indicating that an I/O request has been initiated and also flags the RD pin or the WR pin whether it’s an IN or OUT instruction, respectively. The difference, however, is that ports are read and written using IN and OUT instructions, respectively - as opposed to the traditional load/store instruction ( LD). For this, there’s a separate address space for ‘I/O devices’ called ports and both share the same data and address bus. This is because the Z80 family contains an interesting feature called I/O ports which enables the CPU to communicate with other hardware without running out of memory addresses. Instead, some peripherals are found on the I/O space. Accessing the rest of the componentsĪs you can read from the previous paragraph, only main RAM and some cartridge ROM is found on the address space, so how can the program access other components? Well, unlike Nintendo’s Famicom/NES, not all the hardware of the Master System is mapped using memory locations. Finally, up to 48 KB of game ROM are mapped as well. In the memory map you’ll find 8 KB of RAM for general purpose use, this is mirrored in another 8 KB block. The Z80 has a 16-bit address bus, so the CPU can find up to 64 KB worth of memory. But for this article, it doesn’t matter who fabricated the CPU, as the internal features remain the same. The motherboard picture at the start of the article shows a NEC D780C-1 CPU, that’s just SEGA second-sourcing the chip to different manufacturers, other revisions even included the chip manufactured by Zilog. The Z80 has an instruction set compatible with the Intel 8080 but expanded with lots of more instructions. A popular choice by other machines like the ZX Spectrum and the Amstrad CPC. Sega decided on a fully-fledged Zilog Z80 CPU running at ~3.58 MHz. However, it lacks the RESET button.įrom now on I’ll use the term ‘Master System’ or ‘SMS’ to refer to all of these, except when talking about exclusive features from a particular model. Sega Master System (Japan): An European/American Master system with the Mark III’s cartridge slot, a new FM chip and a jack port for ‘3D glasses’.Sega Master System (Europe and America): A rebranded Mark III with a new case, a BIOS ROM chip and a different cartridge slot.Sega Mark III: The first console featuring this architecture, only released in Japan.I was a bit confused at first while reading about the different models that Sega ended up shipping, so here is a summary of the main models discussed to avoid further confusions:
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What started as a collection of off-the-shelf components, has now gained a new identity thanks to Sega’s engineering. The Master System comes from a long line of succession. Remember that the sound chip is embedded in the video display processor Motherboard with important parts labelled Main architecture diagram