[9fans] any vga driver gurus about?

["Matt" <matt@proweb.co.uk>]

Hi,

Okay I've ended up having to try and write a vga driver for my laptop
a challenge to which I have never risen before
It's partiularly intriguing as I've not even got a running plan9 to compile
it with

I'm reading through stuff and I've come across something I don't understand

in http://offworld.fac.cs.cmu.edu/plan9/source/src/cmd/aux/vga/vga.h

the crt registers are defined as ushorts

125 typedef struct Vga {
129         ushort  crt[256];

in my cl-gd52x manual

the CRT registers are 8 bits

in http://offworld.fac.cs.cmu.edu/plan9/source/src/cmd/aux/vga/clgd542x.c
which is the driver file I'm studying

there is some stuff like this :

248         if(vga->crt[0x16] & 0x100)
249                 vga->crt[0x1A] |= 0x40;
250         if(vga->crt[0x16] & 0x200)
251                 vga->crt[0x1A] |= 0x80;
these set the vertical blank end overflows (whatever they are :)


253         if(vga->crt[0x13] & 0x100)
254                 vga->crt[0x1B] |= 0x10;
this sets the blanking control to 1 which enables the blanking set in CR1A

so what use is & ing an 8bit value with a 16bit value?

Matt