Editing Enterprise (computer) (section)

=== CPU, memory and ASIC chips ===

The Enterprise has a 4 [[megahertz]] (MHz) Z80 [[Central processing unit]] (CPU),<ref>{{cite journal |last=Mace |first=Scott |date=6 February 1984 |title=British firm unveils micros at Consumer Electronics Show |url=https://books.google.com/books?id=gC4EAAAAMBAJ&pg=PA3 |journal=[[InfoWorld]] |volume=6 |issue=6 |page=62 |issn=0199-6649 |quote=The Enterprise's Z80 runs at a speed of 4 MHz, [...]}}</ref> 64 KB (65,536 bytes) or 128 KB of RAM, and 32 KB (32,768 bytes) 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).<ref name="ElanStory" />

A [[bank switching]] scheme allows the memory to be expanded to a maximum of 4 megabytes (4,194,304 bytes). The highest 2 [[address line]]s from the Z80 are used to select one of the four 8-bit Page Registers in the 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.<ref>{{cite book | title=Enterprise Technical Information | year=1984 | publisher=Enterprise Computers Ltd.}}</ref> 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.