>>722868653
This guy >>722868835
is exaggerating, but actual market shares in fiscal year 1990:

Consoles:
93.1% Nintendo
3.8% Sega
1.3% NEC
1.1% Atari

Computers:
85.8% IBM PC Compatible
5.4% Commodore 64
4.4% Mac OS
2.1% Amiga

Both industries were wildly monopolized and people didn't really seem to mind