Screen refresh rate
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It is possible to set different screen refresh rates by changing some CRTC registers. With the following lines of code you can change the refresh rate to 50Hz, 60Hz or 75Hz.
50 Hz
ld bc,&bc04 out (c),c ld bc,&bd26 out (c),c ld bc,&bc05 out (c),c ld bc,&bd00 out (c),c ld bc,&bc07 out (c),c ld bc,&bd1e out (c),c
60 Hz
ld bc,&bc04 out (c),c ld bc,&bd1f out (c),c ld bc,&bc05 out (c),c ld bc,&bd06 out (c),c ld bc,&bc07 out (c),c ld bc,&bd1b out (c),c
75 Hz
ld bc,&bc04 out (c),c ld bc,&bd1b out (c),c ld bc,&bc05 out (c),c ld bc,&bd02 out (c),c ld bc,&bc07 out (c),c ld bc,&bd19 out (c),c