Screen refresh rate

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.

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 

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 

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