Michael W. Shaffer writes:
>
> I have made some progress getting ELKS up on my PCjr. When the BIOS (or
> anything else) accesses the area of 0xB8000 on a PCjr, there is special
> hardware which intercepts the access and redirects it to an arbitrary area
> of one or more 16K pages somewhere in conventional memory from 0-128K. The
> address used for this redirection to the 'real' video buffer is controlled
> by a register on the 6845 display controller. The value of this register can
> be obtained by reading location 0x48A and contains a defualt of 0x3F which
> breaks out as follows:
>
> 7654 3210
> ---------
> 0011 1111
>
> Bits 7-6 are 00 to indicate text mode (there are other values for graphics)
> Bits 5-3 are 0x7 which indicates the base address of the video buffer used
> by
> the processor (BIOS int 0x10, etc.)
> Bits 2-0 are 0x7 which indicates the base address of the video buffer used
> by
> the CRT display for refresh
>
> The actual base address is found by multipying the number in one of these
> sections by 16K to get the segment start of the area of memory. By default,
> 7 * 16K = 112K (leaving 16K for video buffer before the top of memory at
> 128K). Obviously, this is right in the path of the kernel as it loads in at
> 64K.
>
> To change these base addresses, you can do an OUT to IO-Port 0x3DF. I have
> gotten ELKs to boot alittle further by adding the following code to
> bootsect.S right before the 'load_setup:' label:
>
> mov dx,#0x3DF
> mov ax,#0x12 ! places the new buffer in segment 2 (32K)
> out dx,ax
>
This is going to cause some problems because the segment at 32K is used by
the BIOS disk driver. I think the driver only uses 1K as requests are only
ever 1k, and we do not glob requests together. I am considering hard coding
1k block requests into the kernel to simplify the request code,a nd reduce
the size of drivers.
> Now I need to figure out some reliable method for detecting the PCjr at boot
> time so that this can be excuted only when necessary. I also want to figure
> out how to put the console in 80 column mode and maybe find a better place
> in the boot sequence to put this code. I don't think it is interfering with
> any other setup, but the screen remains blank when you would normally see
> the 'ELKS boot....' message (the first message printed visibly is
> 'Calibrating delay loop..'), and I need to figure out how to perform the
> redirection before those screen writes take place.
>
> So, I now get a complete boot to the point of 'probing fd1' and then a
> kernel panic due to inability to read the superblock of the Minix
> filesystem. I think this is due to my lack of a proper floppy for writing
> the root disk.
>
You would eliminate many possible faults by not using a 1.2M drive to write
360K floppies.
How common are these PCjr machines? If they are quite rare it may only be
worth including support for them as an option. If there are alot about,
then we should support them out of the box.
Al
