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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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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 |