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The Enterprise is a Zilog Z80-based home computer first produced in 1985. It was developed by British company Intelligent Software and marketed by Enterprise Computers. Its two variants are the Enterprise 64, with 64 kilobytes (kB) of Random Access Memory (RAM), and the Enterprise 128, with 128 kB of RAM.

The Enterprise has a 4 megahertz (MHz) Z80 Central processing unit (CPU), 64 kB or 128 kB of RAM, and 32 kB of internal read-only memory (ROM) that contains the EXOS operating system and a screen editor / word processor. The BASIC programming language was supplied on a 16 kB ROM module.

Two application-specific integrated circuit (ASIC) chips take some of the workload off of the central processor. They are named “Nick” and “Dave” after their designers, Nick Toop, who had previously worked on the Acorn Atom, and Dave Woodfield. “Nick” manages graphics, while “Dave” handles sound and memory paging (bank switching).

A bank switching scheme allows the memory to be expanded to a maximum of 4 megabytes (MB). The highest 2 address lines from the Z80 are used to select one of the four 8-bit Page Registers in Dave chip. The output from the selected register is used as the highest 8 bits of the 22-bit address bus, while the lowest 14 bits come directly from the Z80 address bus. Effectively, the 64 kB address space of the Z80 processor is divided into four 16k sections. Any 16k page from the 4 MB address space can be mapped to any of these sections. The lowest two pages (pages 0 and 1) of the 4 MB address space contain system ROM.

The next four pages (2 to 5) are reserved for a ROM cartridge (max 64 kB). The top four pages (pages 252 to 255, totaling 64 kB) are used as video RAM, but can be used for storage of program code and data as well. On the 128k model, the additional 64 kB of ram is mapped on pages 248 to 251. The remaining memory space can be used by external devices and memory modules connected to the expansion bus.

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source: wikipedia ep128.hu