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; NOTICE; This was taken from the os2 bios sources and was slightly modified to; enable the a20 line. There's still some work to do and much clean-up to; bring the file upto coding standards. I'll do this when time permits.; TomP
;* _EnableA20;* Description: *;* This routine enables and disables the A20 address line, depending on *;* the value in ax *;* *;* In general when in real mode we want the A20 line disabled, *;* when in protected mode enabled. However if there is no high *;* memory installed we can optimise out unnecessary switching *;* of the A20 line. Unfortunately the PC/AT ROM does not allow *;* us to completely decouple mode switching the 286 from gating *;* the A20 line. *;* *;* In real mode we would want A20 enabled if we need to access *;* high memory, for example in a device driver. We want it *;* disabled while running arbitrary applications because they *;* may rely on the 1 meg address wrap feature which having the *;* A20 line off provides. *;* *;* This code is largely duplicated from the PC/AT ROM BIOS. *;* See Module "BIOS1" on page 5-155 of the PC/AT tech ref. *;* *;* WARNING: *;* *;* The performance characteristics of these routines *;* are not well understood. There may be worst case *;* scenarios where the routine could take a relatively *;* long time to complete. *;* *;* Linkage: *;* far call *;* *;* Input: *;* *;* Exit: *;* A20 line enabled/disabled *;* *;* Uses: *;* ax *;* *;* Internal References: *;* empty_8042 -- waits for 8042 input buffer to drain *
.386pinclude su.inc
IODelay macro jmp $+2 endm
extrn _puts:nearextrn _Empty_8042Failed:near
; Equates for cmos
CMOS_DATA equ 71h ; I/O word for cmos chipSHUT_ADDR equ 8fh ; shutdown byte address in cmosSHUT_CODE equ 9 ; block copy return code we use
; equates for 8042STATUS_PORT equ 64h ; 8042 com portPORT_A equ 60h ; 8042 data portBUF_FULL equ 2 ; 8042 busy bit
SHUT_CMD equ 0feh ; RESET 286 command
MSW_VIRTUAL equ 1 ; protected mode bit of MSW
MASTER_IMR equ 21h ; mask port for master 8259
;CONST SEGMENT WORD USE16 PUBLIC 'CONST';CONST ENDS
;_BSS SEGMENT WORD USE16 PUBLIC 'BSS';_BSS ENDS
;DGROUP GROUP CONST, _BSS, _DATA; ASSUME CS: _TEXT, DS: DGROUP, SS: DGROUP
_TEXT segment para use16 public 'CODE' ASSUME CS: _TEXT, DS: DGROUP, SS: DGROUP
;++; ;VOID;EnableA20(; VOID; );;Routine Description:;; Enables the A20 line for any machine. ;;Arguments:;; None;;Return Value:;; None.;; The A20 line is enabled.;;-- public _EnableA20
_EnableA20 proc near
; Check if empty_8042 has failed before; If so, skip this function. This would occur; on legacy free systems.
mov di,offset DGROUP:_Empty_8042Failed cmp byte ptr [di],1 jz EA2 ; cmp byte ptr [di],0 call empty_8042 ; ensure 8042 input buffer empty jnz EA2 ; 8042 error return
; Enable or disable the A20 line
mov al,0d1h ; 8042 cmd to write output port out STATUS_PORT,al ; send cmd to 8042 call empty_8042 ; wait for 8042 to accept cmd jnz EA2 ; 8042 error return mov al,0dfh ; 8042 port data out PORT_A,al ; output port data to 8042 call empty_8042
; We must wait for the a20 line to settle down, which (on an AT); may not happen until up to 20 usec after the 8042 has accepted; the command. We make use of the fact that the 8042 will not; accept another command until it is finished with the last one.; The 0FFh command does a NULL 'Pulse Output Port'. Total execution; time is on the order of 30 usec, easily satisfying the IBM 8042; settling requirement. (Thanks, CW!)
mov al,0FFh ;* Pulse Output Port (pulse no lines) out STATUS_PORT,al ;* send cmd to 8042 call empty_8042 ;* wait for 8042 to accept cmd
EA2: ret
_EnableA20 endp
;++; ;VOID;DisableA20(; VOID; );;Routine Description:;; Disables the A20 line for any machine. ;;Arguments:;; None;;Return Value:;; None.;; The A20 line is disabled.;;-- public _DisableA20
_DisableA20 proc near
; Check if empty_8042 has failed before; If so, skip this function. This would occur; on legacy free systems.
mov di,offset DGROUP:_Empty_8042Failed cmp byte ptr [di],1 jz EA2 cmp byte ptr [di],0 DA1: call empty_8042 ; ensure 8042 input buffer empty jnz DA2 ; 8042 error return
; Disable the A20 line
mov al,0d1h ; 8042 cmd to write output port out STATUS_PORT,al ; send cmd to 8042 call empty_8042 ; wait for 8042 to accept cmd jnz DA2 ; 8042 error return mov al,0ddh ; 8042 port data out PORT_A,al ; output port data to 8042 call empty_8042
; We must wait for the a20 line to settle down, which (on an AT); may not happen until up to 20 usec after the 8042 has accepted; the command. We make use of the fact that the 8042 will not; accept another command until it is finished with the last one.; The 0FFh command does a NULL 'Pulse Output Port'. Total execution; time is on the order of 30 usec, easily satisfying the IBM 8042; settling requirement. (Thanks, CW!)
mov al,0FFh ;* Pulse Output Port (pulse no lines) out STATUS_PORT,al ;* send cmd to 8042 call empty_8042 ;* wait for 8042 to accept cmd
DA2: ret
_DisableA20 endp;**; empty_8042 -- wait for 8042 input buffer to drain;; Input:; interrupts disabled;; Exit:; al=0, z=0 => 8042 input buffer empty;; Uses:; ax, flags
public Empty8042Empty8042 proc nearempty_8042: sub cx,cx ; cx = 0, timeout loop counter
emp1: in al,STATUS_PORT ; read 8042 status port IODelay IODelay IODelay IODelay and al,BUF_FULL ; test buffer full bit loopnz emp1
cmp cx,0 ; see if buffer is full jnz emp2
; if we reached this point this indicates an error mov di,offset DGROUP:_Empty_8042Failed mov byte ptr [di],1 ; mov [_Empty_8042Failed],1 ; set Empty_8042Failed global to "TRUE"; mov _Empty_8042Failed,1 ; set Empty_8042Failed global to "TRUE"; mov cx, offset _Empty_8042Failed; mov [cx],ah
emp2: and al,BUF_FULL ; reset the Z flag ret
Empty8042 endp
_TEXT ends
end
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