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106 lines
4.1 KiB
106 lines
4.1 KiB
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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Enable_A20:
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; Enables A20 gate using:the BIOS, keyboard controller, or Fast A20
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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pushad ;Preserve registers
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pushfd ;Preserve EFLAGS because we might disable interrupts
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mov ax,0x2402 ;INT 15h AX=2402h: Query A20 status
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int 0x15 ;Check to see if A20 gate is enabled
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jc A20_No_BIOS ;Error? Don't use BIOS to enable gate
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test al,1 ;Test Bit 0 of AL
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je A20_Enabled ;A20 gate already enabled
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mov ax,0x2401 ;INT 15h AX=2401h: Enable A20 gate
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int 0x15 ;Use BIOS to enable A20 gate
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jc A20_No_BIOS ;Error? Don't use BIOS to enable gate
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or ah,ah ;Test AH
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jnz A20_No_BIOS ;Non-zero? A20 gate may not be enabled
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A20_Enabled:
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popfd ;Restore EFLAGS
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popad ;Restore registers
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ret ;Return
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A20_No_BIOS:
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mov ax,0x2403 ;INT 15h AX=2403h: Query A20 support
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int 0x15 ;Call BIOS to find out how the A20 gate can be enabled
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jc A20_KBD ;Error? Assume that keyboard controller is the only option
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test bx,1 ;Bit 0:Keyboard controller supported
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je A20_KBD ;BIOS indicates that keyboard controller is supported
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test bx,2 ;Bit 1:Fast A20 supported
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je FAST_A20 ;BIOS indicated that Fast A20 is supported
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call Check_A20 ;Test A20 gate manually
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jc A20_Enabled ;A20 gate already enabled
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A20_Fail:
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lea si,[A20_ERROR] ;"Unable to enable A20 gate!"
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call PrintString
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call A20_OUT_Wait ;Print string
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cli ;Disable interrupts
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hlt ;Halt machine
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FAST_A20:
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in al,0x92 ;Read System Control Port A
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test al,2 ;Test Fast A20 bit
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jnz A20_Fail ;Bit already set, failed to enable A20 gate
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or al,2 ;Set Bit 1:Enable Fast A20
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and al,0xFE ;Always clear Bit 0 so that machine does not reboot
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out 0x92,al ;Write to port 92h
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call Check_A20 ;Check A20 gate
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jc A20_Enabled ;Success?
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cli ;No? something must have gone wrong
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hlt ;Clear interrupts and halt machine
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A20_KBD:
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cli ;Disable interrupts
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mov cx,50 ;Number of attempts to enable gate
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Use_KBD:
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call A20_OUT_Wait ;Wait for keyboard
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mov al,0xAD ;Disable Keyboard
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out 0x64,al ;Wait for Keyboard
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mov al,0xD0 ;Read Controller Output Port
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out 0x64,al
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call A20_IN_Wait ;Wait for Keyboard
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in al,0x60 ;Read Data Port
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push ax ;Save
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call A20_OUT_Wait ;Wait for Keyboard
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mov al,0xD1 ;Write Controller Output Port
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out 0x64,al
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call A20_OUT_Wait ;Wait for Keyboard
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pop ax ;Get port data back
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or al,10b ;Bit 1: Enable A20 gate
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out 0x60,al ;Write to data port
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call A20_OUT_Wait ;Wait for Keyboard
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mov al,0xAE ;Enable Keyboard
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out 0x64,al
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call A20_OUT_Wait ;Wait for Keyboard
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call Check_A20 ;Verify that gate is enabled
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jc A20_Enabled ;Yes? Return
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loop Use_KBD ;No? Keep trying
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jmp A20_Fail ;Could not enable gate after 50 tries
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A20_OUT_Wait:
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in al,0x64 ;Read status register
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test al,10b ;Is port ready?
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jnz A20_OUT_Wait ;Nope, wait
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ret ;Yep, continue
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A20_IN_Wait:
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in al,0x64 ;Read status register
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test al,1b ;Is port ready?
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jz A20_IN_Wait ;Nope, wait
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ret ;Yep, continue
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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Check_A20:
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; Test if A20 gate is enabled by comaparing the value in FFFF:7E0E
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; with the VBR boot signature at 0000:7DFE
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; Out: Carry flag set if A20 gate is enabled
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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pushad ;Preserve registers
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push es
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mov ax, 0xFFFF
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mov es, ax ;Extra segment:FFFF
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cmp WORD [es:0x7DFE + 16], 0xAAFF ;See if boot signature is wrapped at FFFF:7E0E (Meaning FFFF:7E0E = 0:7DFE)
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stc ; Assume we didn't wrap, yes A20
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jne .End
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clc ; We did wrap, no A20
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.End:
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pop es
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popad
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ret
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A20_ERROR db "Unable to enable A20 gate!",0
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