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@ -1,11 +1,103 @@
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org 0x8200
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bits 16
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%include "defines.s"
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[org BOOT1_LOADPOINT]
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[bits 16]
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%macro copy_stack_var_to_globals 2
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mov %1, [bp - %2]
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mov [GLOBALS + %2], %1
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%endmacro
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; boot0 loads only our first sector into memory. We must load the rest.
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self_load:
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; Now that we're not doing instruction byte golf like we were in boot0, we can afford to move
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; the various boot0 stack variables to the globals section.
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copy_stack_var_to_globals ax, BOOT_DRIVE
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copy_stack_var_to_globals ax, SECTORS_PER_TRACK
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copy_stack_var_to_globals ax, N_HEADS
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copy_stack_var_to_globals ax, GPT_ENTRIES_START_LBA
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copy_stack_var_to_globals ax, GPT_N_ENTRIES_16
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copy_stack_var_to_globals ax, GPT_SECTOR_STRIDE
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copy_stack_var_to_globals ax, GPT_BYTE_STRIDE
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copy_stack_var_to_globals ax, GPT_ENTRIES_PER_SECTOR
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copy_stack_var_to_globals ax, GPT_CURRENT_ENTRY_IDX
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copy_stack_var_to_globals ax, GPT_SECTOR_ENTRY_IDX
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copy_stack_var_to_globals ax, GPT_SECTORS_LOADED
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copy_stack_var_to_globals ax, GPT_CURRENT_LBA
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copy_stack_var_to_globals ax, BOOT1_GPT_ENTRY_ADDR
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; Reset the stack, now we've got everything we need from it.
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mov sp, bp
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mov si, [GLOBALS + BOOT1_GPT_ENTRY_ADDR]
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mov eax, [si + 0x20] ; Partition / boot1 start LBA lower
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mov ebx, [si + 0x24] ; Partition / boot1 start LBA upper
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mov ecx, [si + 0x28] ; Partition end LBA lower
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mov edx, [si + 0x32] ; Partition LBA upper
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; Panic if the partition / boot1 starting LBA overflows 16 bits.
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or ebx, ebx
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jnz panic
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ror eax, 16
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or ax, ax
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jnz panic
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ror eax, 16
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; Calculate the boot1 end LBA and panic if it overflows 16 bits.
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; n.b. ebx is zero before this so both bx and ebx can be used as the boot1 end LBA.
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mov bx, ax
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add bx, BOOT1_TOTAL_SECTORS
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jc panic
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; Panic if the boot1 end LBA is after the partition end LBA.
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; If the upper 32 bits of the partition end LBA are nonzero, then it must be greater than our
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; 16-bit boot1 end LBA.
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or edx, edx
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jnz .end_lba_ok
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; Compare the boot1 end LBA to the lower 32 bits of the partition end LBA.
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cmp ebx, ecx
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ja panic
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.end_lba_ok:
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; The first sector has already been loaded (we're running it right now!) so increment the
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; current LBA.
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inc ax
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push ax ; Current LBA
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push bx ; boot1 end LBA
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mov ebx, BOOT1_LOADPOINT + 512 ; Current sector load address
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.self_load_loop:
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mov ax, [bp - 0x02] ; Load current LBA
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cmp word [bp - 0x04], ax ; Compare to boot1 end LBA
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jb .self_load_done
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mov ecx, ebx
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call read_sector
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jc panic
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add ebx, 512
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inc word [bp - 0x02]
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jmp .self_load_loop
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.self_load_done:
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mov ebx, cafebabe
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call addr32_to_addr16
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mov eax, es:[ebx]
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hlt
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call test_a20
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add al, 0x30
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mov byte fs:[0x0000], al
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hlt
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pr_womble:
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; Reveal the true nature of jen
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xor bx, bx
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loop:
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.loop:
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cmp bx, WOMBLE_LEN
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jae done
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jae .done
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lea si, [womble + bx]
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mov dh, [si]
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mov di, bx
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@ -13,9 +105,159 @@ loop:
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mov byte fs:[di], dh
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mov byte fs:[di + 0x01], 0x1f
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inc bx
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jmp loop
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done:
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jmp .loop
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.done:
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hlt
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; Converts a 32-bit address to a 16-bit sector and offset.
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; Arguments:
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; - ebx: 32-bit address
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; Return:
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; - es: 16-bit address segment (unchanged on failure)
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; - ebx: 16-bit address offset
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; - cf: unset on success, set on failure
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; Clobber: none
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addr32_to_addr16:
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push bp
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mov bp, sp
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push es
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push eax
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mov eax, ebx
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; Divide addr by 16 and saturate to 16 bits to get the segment.
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shr eax, 4
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ror eax, 16
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or ax, ax
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jz .segment_ok
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mov eax, 0xffff0000
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.segment_ok:
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ror eax, 16
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mov es, ax
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; Calculate offset = addr - (16 * segment), failing if the offset doesn't fit in 16 bits.
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shl eax, 4
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sub ebx, eax
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ror ebx, 16
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or bx, bx
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jnz .fail
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ror ebx, 16
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pop eax
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add sp, 2 ; Discard the original es from the stack
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pop bp
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clc
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ret
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.fail:
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pop eax
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pop es
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pop bp
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stc
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ret
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; Reads a single sector at the given LBA into memory.
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; Arguments:
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; - ax: start LBA
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; - ecx: address to read sector to
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; Return:
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; - cf: unset on success, set on failure
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; Clobber: eax, ecx, edx
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read_sector:
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; sector - 1 = LBA % sectors_per_track
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; temp = LBA / sectors_per_track
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; head = temp % n_heads
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; cylinder = temp / n_heads
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push bp
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mov bp, sp
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push es
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push ebx
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mov ebx, ecx
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call addr32_to_addr16
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jc .return
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; Calculate sector and temp
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xor dx, dx
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; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
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div word [GLOBALS + SECTORS_PER_TRACK]
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; Put the sector into cx (the bios call will use cl)
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mov cx, dx
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inc cx
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; Calculate head and cylinder
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xor dx, dx
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; Divide by number of heads. dx = mod (head), ax = div (cylinder)
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div word [GLOBALS + N_HEADS]
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mov dh, dl
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mov ch, al
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mov dl, byte [GLOBALS + BOOT_DRIVE]
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mov ah, 0x02
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mov al, 1
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; Read sector
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int 0x13
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.return:
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pop ebx
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pop es
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pop bp
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ret
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%if ($ - $$) > 512
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%error "boot1 self-loader exceeded sector size"
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%endif
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test_a20:
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push gs
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; Restore the boot sector identifier in case it was overwritten by anything.
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mov word [0x7dfe], 0xaa55
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mov ax, 0xffff
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mov gs, ax
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xor ax, ax
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; If the word at 0x107dfe (1 MiB after the boot sector identifier) is different to the boot
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; sector identifier, than A20 must be enabled.
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cmp word gs:[0x7e0e], 0xaa55
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setne al
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jne .return
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; Even if A20 was enabled, the two words may have been equal by chance, so we temporarily swap
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; the boot sector identifier bytes and test again.
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ror word [0x7dfe], 8
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cmp word gs:[0x7e0e], 0x55aa
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setne al
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ror word [0x7dfe], 8
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jmp .return
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.return:
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pop gs
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ret
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panic:
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mov ax, 0x0003
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int 0x10
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mov word fs:[0x0000], 0x4f21
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hlt
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womble db "jen is a womble!"
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WOMBLE_LEN equ $ - womble
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times 409600 db 0
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cafebabe:
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dd 0xfeedface
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BOOT1_TOTAL_LEN equ $ - $$
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BOOT1_TOTAL_SECTORS equ (BOOT1_TOTAL_LEN + 511) / 512
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%if (BOOT1_LOADPOINT + BOOT1_TOTAL_LEN) > EBDA_START
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%error "boot1 too large to be loaded"
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%endif
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pantonshire
1 year ago