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