Development and Design
The S-1990 was especially developed for "MSX turbo R computers" which are outfitted with an ASCII R800-processor. To retain backward compatibility with the older MSX 1, MSX 2 and MSX 2+ systems the Turbo R also uses the Z80 CPU inside the T9769. The S-1990 is used to control the bus access so that either the R800 or the Z80 inside the T9769 can access the memory and peripherals. Because of the comparatively greater speed of the R800 (a relative speed of 28.636360 MHz compared to the 3.58 MHz; or 7.16 MHz in MSX2+ machines; of the Z80) the S1990 also implements a "wait state" mechanism to slow down access to slower legacy devices (especially for the peripherals in the T9769, in particular the video display controller) that otherwise could not keep up with the R800. It also implements hardware assisted software debugging.
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