2024-01-11 22:03:52 +02:00
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.code16
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.section .stage2
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# checks whether A20 line is enabled or disabled
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# return
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# ax: 1 if enabled, 0 otherwise
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check_a20:
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pushf
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pushw %si
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pushw %di
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pushw %ds
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pushw %es
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cli
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xorw %ax, %ax
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movw %ax, %es
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notw %ax
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movw %ax, %ds
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movw $0x0500, %di
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movw $0x0510, %si
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movb %es:(%di), %al
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pushw %ax
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movb %ds:(%si), %al
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pushw %ax
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movb $0x00, %es:(%di)
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movb $0xFF, %ds:(%si)
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cmpb $0xFF, %es:(%di)
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pop %ax
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movb %al, %ds:(%si)
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pop %ax
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movb %al, %es:(%di)
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movw $0, %ax
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je .check_a20_done
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movw $1, %ax
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.check_a20_done:
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popw %es
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popw %ds
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popw %di
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popw %si
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popf
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ret
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# Try to enable A20 using PS2 controller
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enable_a20_ps2:
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pushf
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pushw %ax
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cli
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# disable first port
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call .enable_a20_ps2_wait1
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movb $0xAD, %al
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outb %al, $0x64
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# read controller output
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call .enable_a20_ps2_wait1
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movb $0xD0, %al
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outb %al, $0x64
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call .enable_a20_ps2_wait2
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inb $0x60, %al
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pushw %ax
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# write controller output
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call .enable_a20_ps2_wait1
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movb $0xD1, %al
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outb %al, $0x64
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call .enable_a20_ps2_wait1
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popw %ax
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orw $2, %ax
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outb %al, $0x60
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# enable first port
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call .enable_a20_ps2_wait1
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movb $0xAE, %al
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outb %al, $0x64
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call .enable_a20_ps2_wait1
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popw %ax
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popf
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ret
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.enable_a20_ps2_wait1:
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inb $0x64, %al
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test $2, %al
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jnz .enable_a20_ps2_wait1
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ret
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.enable_a20_ps2_wait2:
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inb $0x64, %al
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test $1, %al
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jnz .enable_a20_ps2_wait1
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ret
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# Check if A20 line is disabled. If it is, try to enable it
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.global enable_a20
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enable_a20:
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pushw %ax
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pushw %si
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call check_a20
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testw %ax, %ax
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jnz .enable_a20_done
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movw $a20_line_disabled_msg, %si
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call puts; call print_newline
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# Try to enable A20 line using bios interrupt
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movw $0x2401, %ax
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int $0x15
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call check_a20
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testw %ax, %ax
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jnz .enable_a20_done
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# Try to enable A20 line using ps2 controller
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call enable_a20_ps2
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call check_a20
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testw %ax, %ax
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jnz .enable_a20_done
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# Try to enable A20 line using fast A20 gate
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inb $0x92, %al
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testb $2, %al
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jnz .enable_a20_fast_done
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orb $2, %al
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outb %al, $0x92
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.enable_a20_fast_done:
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call check_a20
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testw %ax, %ax
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jnz .enable_a20_done
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movw $a20_could_not_enable_msg, %si
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call print_and_halt
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.enable_a20_done:
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movw $a20_line_enabled_msg, %si
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call puts; call print_newline
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popw %si
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popw %ax
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ret
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2024-01-12 19:25:07 +02:00
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.section .data
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2024-01-11 22:03:52 +02:00
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a20_line_disabled_msg:
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.asciz "A20 line disabled. Trying to enable it"
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a20_line_enabled_msg:
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.asciz "A20 line enabled"
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a20_could_not_enable_msg:
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.asciz "Could not enable A20 line"
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