The following is edited from several messages from Kees Bot dated May 4

and 5, 1998. 

From: Kees J Bot <kjb=731391@CS.VU.NL>

Subject: Re: 286 protected mode patch

Below is a patch to klib88.s for the changes I made after I got hold of

HIMEM.ASM, a somewhat old source of HIMEM.SYS.  The A20 en/disable code

is a bit better for the AT, and I also added code to en/disable the A20

line for a PC with an MCA bus, i.e.  most PS/2's.

So if any of you is running 16-bit Minix on a machine with an MCA bus

then this may be for you to get rid of processor=86.  

The PS/2 side of the patch is the great unknown, since I don't have a

PS/2 to test it on.  Who knows, maybe someone will tell us if it works

this time.

Apply the patch like this:

	cd /usr/src/kernel

	patch klib88.s klib88.dif

	cd ../tools

	make hdboot

	shutdown -x 'unset processor;boot'

[Please note that this is experimental, there is another patch in this

directory that attempts to solve the same problems.  Neither may be

correct for every machine. -- asw 12.06.98]

_._. .._ _   ._ ._.. ___ _. __.   _ .... .   _.. ___ _ _ . _..   ._.. .. _. .

*** /save/std/2.0.0/src/kernel/klib88.s	Wed Jul 10 20:17:12 1996

--- klib88.s	Mon May 04 12:20:59 1998

***************

*** 72,75 ****

--- 72,76 ----

  .extern	_vid_mask

  .extern	_level0_func

+ .extern	_ps_mca

  	.text

***************

*** 201,205 ****

  	ltr	ax			! set TSS register

! 	movb	ah, #0xDF

  	jmp	gate_A20		! enable the A20 address line

--- 202,206 ----

  	ltr	ax			! set TSS register

! 	movb	ah, #0x02

  	jmp	gate_A20		! enable the A20 address line

***************

*** 214,224 ****

  	jae	p2r386

  p2r286:

! 	mov	_gdt+ES_286_OFFSET+DESC_BASE, #0x0400

  	movb	_gdt+ES_286_OFFSET+DESC_BASE_MIDDLE, #0x00

  	mov	ax, #ES_286_SELECTOR

  	mov	es, ax			! BIOS data segment

!   eseg	mov	0x0067, #real		! set return from shutdown address

    cseg	mov	ax, kernel_cs

!   eseg	mov	0x0069, ax

  	movb	al, #0x8F

  	outb	0x70			! select CMOS byte 0x0F (disable NMI)

--- 215,225 ----

  	jae	p2r386

  p2r286:

! 	mov	_gdt+ES_286_OFFSET+DESC_BASE, #0x0000

  	movb	_gdt+ES_286_OFFSET+DESC_BASE_MIDDLE, #0x00

  	mov	ax, #ES_286_SELECTOR

  	mov	es, ax			! BIOS data segment

!   eseg	mov	0x0467, #real		! set return from shutdown address

    cseg	mov	ax, kernel_cs

!   eseg	mov	0x0469, ax

  	movb	al, #0x8F

  	outb	0x70			! select CMOS byte 0x0F (disable NMI)

***************

*** 254,273 ****

  	mov	sp, save_sp		! restore stack

! 	movb	ah, #0xDD

  	!jmp	gate_A20		! disable the A20 address line

! ! Enable (ah = 0xDF) or disable (ah = 0xDD) the A20 address line.

  gate_A20:

  	call	kb_wait

  	movb	al, #0xD1	! Tell keyboard that a command is coming

  	outb	0x64

  	call	kb_wait

! 	movb	al, ah		! Enable or disable code

  	outb	0x60

  	call	kb_wait

! 	mov	ax, #25		! 25 microsec delay for slow keyboard chip

! 0:	out	0xED		! Write to an unused port (1us)

! 	dec	ax

! 	jne	0b

  	ret

  kb_wait:

--- 255,276 ----

  	mov	sp, save_sp		! restore stack

! 	xorb	ah, ah

  	!jmp	gate_A20		! disable the A20 address line

! ! Enable (ah = 0x02) or disable (ah = 0x00) the A20 address line.

  gate_A20:

+ 	cmp	_ps_mca, #0	! PS/2 bus?

+ 	jnz	gate_PS_A20

  	call	kb_wait

  	movb	al, #0xD1	! Tell keyboard that a command is coming

  	outb	0x64

  	call	kb_wait

! 	movb	al, #0xDD	! 0xDD = A20 disable code if ah = 0x00

! 	orb	al, ah		! 0xDF = A20 enable code if ah = 0x02

  	outb	0x60

  	call	kb_wait

! 	movb	al, #0xFF	! Pulse output port

! 	outb	0x64

! 	call	kb_wait		! Wait for the A20 line to settle down

  	ret

  kb_wait:

***************

*** 275,278 ****

--- 278,293 ----

  	testb	al, #0x02	! Keyboard input buffer full?

  	jnz	kb_wait		! If so, wait

+ 	ret

+ 

+ gate_PS_A20:		! The PS/2 can twiddle A20 using port A

+ 	inb	0x92		! Read port A

+ 	andb	al, #0xFD

+ 	orb	al, ah		! Set A20 bit to the required state

+ 	outb	0x92		! Write port A

+ 	jmp	.+2		! Small delay

+ A20ok:	inb	0x92		! Check port A

+ 	andb	al, #0x02

+ 	cmpb	al, ah		! A20 line settled down to the new state?

+ 	jne	A20ok		! If not then wait

  	ret