Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!swrinde!ucsd!hub.ucsb.edu!mcm
From: mcm@maple.ucsb.edu (Marcelo Mourier)
Newsgroups: comp.os.minix
Subject: PC Minix 1.5 Kernel
Message-ID: <6288@hub.ucsb.edu>
Date: 16 Sep 90 01:15:23 GMT
Sender: news@hub.ucsb.edu
Reply-To: mcm@maple.ucsb.edu (Marcelo Mourier)
Organization: University of California, Santa Barbara
Lines: 49

Hi everybody!

The kernel in PC Minix 1.5 uses a global variable 'snow' to identify video
cards that produce snowy displays; i.e, that require their video memory to
be written during the vertical retrace period.  This variable is declared
in line 44 of kernel/glo.h as EXTERN int snow.  Space is assigned to it in
table.c, along with the default value of zero (FALSE).  However, I couldn't
find any piece of code where its value could be changed to TRUE.  Moreover,
the only reference to 'snow' is in klib.x, line 885: cmpb _snow,#0 , where
it is treated as a byte (char), instead of as a word (int)...???!!!!

The second question is regarding the keyboard hardware in the PC-AT.  In 
these machines it doesn't seem to be necessary to acknowledge the char to
the keyboard controller (i8042); at least not in the way it's done now by
scan_keyboard() in kernel/keyboard.c.  In that function the char read from
the keyboard is acknowledged by toggling bit 7 in the I/O port 0x061 (PORT_B).
In the PC and XT this I/O port corresponds to parallel port B of the 8255 PPI.
But in the AT (at least in the schematics I have) this I/O port addrerss is
assigned to a 4-bit latch connected to the lowest four bits of the data bus;
e.g., DB0 - DB3.  So, trying to toggle bit 7 is just a waist of CPU clocks...
I've modified the code in scan_keyboard() to look like this:

		.
		.
		.
	out_byte(0x66, val & ~0x10);
} else {
	code = in_byte(KEYBD);
	/* If machine is PC or XT, ack the char */
	if (!pc_at) {
		val = in_byte(PORT_B);
		out_byte(PORT_B, val | KBIT);
		out_byte(PORT_B, val);
	}
}
		.
		.
		.

I've tested the modified kernel successfully on two different AT's, but I'd
like to hear that the patch is 100% reliable.  Could you, AT Minixers, try it
please..?

Thanks,



--
Marcelo Mourier (mcm@cs.ucsb.edu)