Too much space needed; optimizing now
This commit is contained in:
Eric-Paul Ickhorn
parent
4887c61303
commit
49230d4c72
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@ -3,8 +3,6 @@ section .text
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org 0x7c00
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start:
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push dx
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mov ax, 0
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mov ds, ax
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mov es, ax
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@ -17,62 +15,196 @@ start:
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mov sp, 256
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mov bp, 256
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mov si, dx ; save the origin disk number
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mov ax, 0x7c00
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add ax, 446
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; ax now points to the start of the partition table
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mov di, 0
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xor di, di
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call initialize_video_mode
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.search_partition:
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mov bx, ax
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add bx, 4
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mov bx, ax ; get the offset of the current entry
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add bx, 4 ; go to the offset of the partition type
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mov cl, [bx]
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cmp cl, 0x9d
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je .found_partition
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je .load_partition
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inc di
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; leave and print debug message if the last partition entry
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; did not provide the wanted bootloader - partition
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cmp di, 4
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je .no_boot_partition
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add ax, 16
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jmp .search_partition
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.found_partition:
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.load_partition:
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; ax is the start of the bootloader's partition entry in the mbr
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; read the chs-entry and push it onto the stack
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; head
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xor dx, dx
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mov bx, ax
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add bx, 1
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mov dl, [bx]
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push dx
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; cylinder
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xor dx, dx
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mov bx, ax
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add bx, 3
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mov dh, [bx]
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mov bx, ax
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add bx, 2
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mov dl, [bx]
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shr dx, 6
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push dx
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; sector
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xor dx, dx
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mov bx, ax
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add bx, 2
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mov dl, [bx]
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and dl, 0b00111111
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push dx
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; push the partition's length
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mov bx, ax
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add bx, 12
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mov dx, [bx]
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push dx ; lower part
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mov bx, ax
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add bx, 14
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mov cx, [bx] ; more than 65536 sectors will NOT be
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; supported for the bootloader's partition!
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mov bx, 0x8000
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mov es, bx
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mov di, 0x0000
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mov bx, ax
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inc bx
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; bx now points to the CHS-address of the first sector
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; of the bootloader's partition.
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mov dx, [bx]
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mov ah, dh ; head
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mov bl, dl ; sector
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push dx ; higher part
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; load it all and read from disk
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mov bx, sp
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add bx, 4
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mov al, [bx] ; sector-count
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add bx, 2
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; bx now points to the CHS-cylinder of the first sector
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; of the bootloader's partition.
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mov cl, [bx] ; start-sector
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mov al, [bx] ; cylinder
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add bx, 2
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mov ch, [bx] ; start-cylinder
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call load_sectors
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mov dx, si ; drive
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mov bx, 0x0800
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mov es, bx ; output segment
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mov bx, sp
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add bx, 8
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mov dh, [bx] ; start-head
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call .write_hex
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mov cl, 2
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mov ah, 0x02
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mov al, 1
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mov bx, 0x0000 ; output address in es - segment
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int 13h
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mov cx, 8
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mov si, 0x8000
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mov dh, 12
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mov dl, 32
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call print_string
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jmp .keepalive
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.no_boot_partition:
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hlt
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mov cx, LEN_TEXT_NO_BOOT_PARTITION
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mov si, error_texts.no_boot_partition
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mov dh, 12
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mov dl, 32
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call print_string
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.keepalive:
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jmp .keepalive
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.write_hex:
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push bx
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push cx
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mov bx, error_texts.hextab
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and cx, 0x0f
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add bx, cx
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mov cl, [bx]
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mov bx, error_texts.hexbuf
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inc bx
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mov [bx], cl
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mov bx, error_texts.hextab
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and cx, 0xf0
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shr cx, 4
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add bx, cx
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mov cl, [bx]
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mov bx, error_texts.hexbuf
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mov [bx], cl
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mov cx, 2
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mov si, error_texts.hexbuf
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mov dh, 12
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mov dl, 32
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call print_string
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pop cx
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pop bx
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ret
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; %include "exfat.asm"
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%include "gdt.asm"
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%include "utility.asm"
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LEN_TEXT_NO_BOOT_PARTITION equ 17
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error_texts:
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.no_boot_partition:
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;db "NO_BOOT_PARTITION"
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.hextab:
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db "0123456789abcdef"
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.hexbuf:
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resb 4
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times 446 - ($-$$) db 0
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db 0x80
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db 0, 2, 0
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db 0x9d
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db 0, 3, 0
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dd 1
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dd 1
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times 510 - ($-$$) db 0
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db 0x55
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db 0xAA
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db "################"
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@ -198,14 +198,28 @@ mem_equals:
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; ax: start_sector (lower)
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; bl: start_sector (upper)
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; cx: sector_count
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; ax: start_sector (upper)
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; bl: start_sector (lower)
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; cx: wanted_sector_count
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; dh: drive
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; es: buffer_address (upper)
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; si: buffer_address (upper)
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; di: buffer_address (lower)
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load_sectors:
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; INNER WORKING:
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;
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; This function first checks if it needs to load sectors going over
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; a cylinder boundary and loads to that cylinder boundary first continuing
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; in 63-sector - parts after that until the last, < 63-sectors part is read.
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;
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; This is done because some systems don't like reading over the boundaries
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; between the cylinders.
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;
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; The actual part loading the data is in .read_from_disk, it's no different
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; from the first read to the other reads. It just gets different arguments
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; from 1).load_first_sectors, and 2) .load_further_sectors
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;
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.prolog:
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push ax
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push bx
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@ -214,101 +228,43 @@ load_sectors:
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push si
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xor si, si ; current sector index
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; calculate the distance (in sectors) to the next cylinder boundary
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; get the index of the current cylinder
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and ax, 0b00111111 ; extract only the sector index
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mov bx, 64 ; get the maximum number of sectors + 1
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sub bx, ax ; invert the result; 1 becomes 63; 63 becomes 1
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cmp ax, 63
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jne .is_not_on_full_cylinder_boundary
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; if it's indeed on a full cylinder boundary, the first_read-part is unimportant
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mov ax, 0
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set_load_args
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.is_not_on_full_cylinder_boundary:
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.load_first_sectors:
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; cx still contains the entry-point's wanted sector_count
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; ax now contains the distance to the next cylinder boundary
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cmp cx, ax
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jbe .load_data
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; it gets here if there are more sectors needed after the
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; next cylinder boundary
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; if the number of sectors
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; which are wanted (sector_count) is above the
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; distance to the next cylinder boundary
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; ah: EMPTY
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; al: num_sectors_to_read
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; bx: EMPTY
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; ch: cylinder
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; cl: sector
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; dh: head
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; dl: drive
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.read_from_disk:
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; if not that many sectors are needed, just read as
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; many sectors as are needed in total
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mov cx, bx
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; set es:bx to the address in bytes to which the output should be written.
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jmp .load_data
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mov bx, si
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shr bx, 9
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; sets the arguments for a run of '.load_data' after the
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; first one; aligned on a CHS - cylinder boundary
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.set_load_args:
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mov bx, sp
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add bx, 4
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mov cx, [bx]
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sub cx, si ; cx = num_missing_sectors
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cmp cx, 63
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jbe .add_num_missing_sectors_to_sector_index
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; if this goes here, there are more than 63 sectors
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; still missing to be loaded
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; -> clamp them to 63 so they can be loaded.
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mov cl, 63
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jmp .load_data
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.add_num_missing_sectors_to_sector_index:
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add si, cx
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; loads at most 64 sectors from the disk into the main memory
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; ax: scratchpad
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; bx: start_sector (high part)
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; ch: start_sector (low part)
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; cl: num_sectors
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.load_data:
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mov ah, 0x02
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mov al, cl
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mov dl, [bx] ; drive
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mov dh, bh ; head
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mov cl, ch ; sector
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mov ch, bl ; cylinder
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mov bx, di ; buffer address (lower)
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int 0x13
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.continue_if_needed:
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; check if there still are sectors missing to load
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mov bx, sp
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add bx, 4
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mov cx, [bx] ; sector_count
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cmp si, cx
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jb .set_load_args
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.load_further_sectors:
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.epilog:
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pop si
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pop dx
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pop cx
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pop bx
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pop ax
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ret
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