[MBR] Add A20 line
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1f88f01c33
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1 changed files with 125 additions and 10 deletions
135
mbr/mbr.asm
135
mbr/mbr.asm
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@ -93,11 +93,106 @@ print_loop:
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RET ; return
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; function check_a20: get status of address line 20 (21st bit of any address)
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; Returns:
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; AX=0 if A20 off, AX=1 if A20 on
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check_a20:
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PUSHF ; save FLAGS
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PUSH DS ; save DS
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PUSH ES ; save ES
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PUSH DI ; save DI
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PUSH SI ; save SI
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CLI ; disable interrupts
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XOR AX, AX ; AX = 0
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MOV ES, AX ; ES = 0
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NOT AX ; AX = 0xFFFF
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MOV DS, AX ; DS = 0xFFFF
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MOV DI, 0x0500 ; DI = 0x0500
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MOV SI, 0x0510 ; SI = 0x0510
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MOV AL, [ES:DI] ; AL = Memory[0x0500]
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PUSH AX ; save original value at Memory[0x500]
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MOV AL, [DS:SI] ; AL = Memory[0xFFFF:0x0510]
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PUSH AX ; save original value at Memory[0xFFFF:0x0510]
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MOV BYTE [ES:DI], 0x00 ; Memory[0x0500] = 0x00
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MOV BYTE [DS:SI], 0xFF ; Memory[0xFFFF:0x0510] = 0xFF
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CMP BYTE [ES:DI], 0xFF ; Compare Memory[0x0500] to 0xFF
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; if the bytes match, the memory is wrapping around
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; therefore, the A20 line is disabled
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POP AX ; Restore memory at [0xFFFF:0x0510]
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MOV [DS:SI], AL ; ^^^^
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POP AX ; Restore memory at [0x0500]
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MOV [ES:DI], AL ; ^^^^
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MOV AX, 0 ; AX (return value) = 0 (A20 disabled)
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JE check_a20_exit ; If the last CMP was equal (A20 disabled), exit
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MOV AX, 1 ; AX (return value) = 1 (A20 enabled)
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check_a20_exit:
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POP SI ; restore SI
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POP DI ; restore DI
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POP ES ; restore ES
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POP DS ; restore DS
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POPF ; restore FLAGS
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RET ; return
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enable_a20:
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CALL a20_wait ; wait for keyboard controller
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MOV AL, 0xAD ; 0xAD = disable keyboard
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OUT 0x64, AL ; send command 0xAD to keyboard controller
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CALL a20_wait ; wait for keyboard controller
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MOV AL, 0xD0 ; 0xD0 = read from input
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OUT 0x64, AL ; send command 0xD0
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CALL a20_wait2 ; wait for keyboard input
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IN AL, 0x60 ; read input from keyboard controller
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PUSH AX ; save input
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CALL a20_wait ; wait for keybaord controller
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MOV AL, 0xD1 ; 0xD1 = write to output
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OUT 0x64, AL ; send command 0xD1
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CALL a20_wait ; wait for keyboard controller
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POP AX ; restore input value
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OR AL, 2 ; enable bit 1
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OUT 0x60, AL ; send output to keyboard
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CALL a20_wait ; wait for keyboard controller
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MOV AL, 0xAE ; 0xAE = enable keyboard
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OUT 0x64, AL ; send command 0xAE
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RET
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a20_wait:
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IN AL, 0x64 ; read status of keyboard controller
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TEST AL, 2 ; read bit 1 of status
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JNZ a20_wait ; if not zero, keyboard controller isn't ready. Retry
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RET ; return
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a20_wait2:
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IN AL, 0x64 ; read status of keyboard controller
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TEST AL, 1 ; read bit 0 of status
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JZ a20_wait2 ; if zero, input isn't ready
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RET ; return
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entry:
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; Set up stack and segments
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MOV AX, 0 ; set up stack segment
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MOV SS, AX ; ^^^
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MOV SP, 0x7C00 ; set up stack pointer (0x6c00-x7BFF)
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MOV SP, 0x7C00 ; set up stack pointer (0x6c00-0x7BFF)
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; Sp HAS TO be set in the instruction after settings SS
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; as setting SS disables interrupts for the next instruction
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; Not doing this may cause an interrupt to use an old SP and new SS
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@ -106,8 +201,13 @@ entry:
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MOV FS, AX ; ^^^
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MOV GS, AX ; ^^^
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PUSH DX ; save drive number (DL)
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; Load drive parameters
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MOV AX, 4800h ; load AH=48h, AL=0h
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MOV SI, disk_info_packet ; load address of Disk_Info_Packet
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INT 13H ; INT 14h AH=48h: get drive parameters
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PUSH DX ; save drive number (DL)
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; Print message
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MOV AX, msg ; load address of msg
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@ -116,17 +216,31 @@ entry:
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PUSH AX ; push parameter 'size of string'
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CALL print ; call function 'print'
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over:
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JMP over ; loop forever
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SUB SP, 4 ; clear previous call parameters
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CALL check_a20 ; call check_a20. Result in Ax
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CMP AX, 0 ; compare AX to 0 (0 = A20 disabled)
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JNE after_a20 ; if AX != 0, a20 is enabled, skip enabling it
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CALL enable_a20 ; enable A20 line via keyboard controller
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CALL check_a20 ; check status of A20 line (again)
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CMP AX, 0 ; compare AX to 0 (0 = A20 disabled)
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JNE after_a20 ; if AX != 0, a20 is enabled
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MOV AX, a20_msg ; load address of 'a20_msg'
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PUSH AX ; push parameter 'address of string'
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MOV AX, 6 ; load size of 'a20_msg'
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PUSH AX ; push parameter 'size of string'
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CALL print ; print 'a20_msg'
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JMP endless_loop ; loop endlessly
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; Load drive parameters
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MOV AX, 4800h ; load AH=48h, AL=0h
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MOV SI, disk_info_packet ; load address of Disk_Info_Packet
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INT 13H ; INT 14h AH=48h: get drive parameters
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after_a20:
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endless_loop:
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JMP endless_loop ; loop endlessly
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section .data
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disk_info_packet:
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@ -150,4 +264,5 @@ build_settings_magic_0 dd 0x49494949
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build_settings_bytes dw 0x0
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build_settings_magic_1 dd 0x49494949
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msg db "OS3_BOOT_OK!"
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msg db "OS3_BOOT_OK!"
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a20_msg db "NO_A20"
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