Load kernel off of FAT12 disk
This commit is contained in:
Juhani Krekelä
parent
8da504ca2d
commit
9a829fc71c
4
Makefile
4
Makefile
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@ -3,10 +3,12 @@
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all: nor86.img
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nor86.img: bootsector.bin kernel.bin
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nor86.img: bootsector.bin kernel.bin CC0 README.md
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rm -f $@
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mkdosfs -C $@ 1440
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rw -i bootsector.bin -o $@
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mcopy -i $@ CC0 ::
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mcopy -i $@ README.md ::
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mcopy -i $@ kernel.bin ::
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.asm.bin:
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185
bootsector.asm
185
bootsector.asm
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@ -45,7 +45,7 @@ _start:
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; Save bootdrive
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mov [drivenumber], dl
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root_dir:
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calc_constants:
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; Disk organization:
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; Reserved sectors (MBR)
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; FAT1
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@ -58,16 +58,51 @@ root_dir:
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add ax, [reservedsectors]
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mov [rootdir], ax
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; Data area comes after root dir, so we need to add the size of
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; the root dir in sectors
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; One root dir entry is 32 bytes, and one sector has 512 bytes
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; Therefore 512 / 32 = 16 entries correspond to a sector
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; However, we can't just do a shift, since we need to round up
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; For integers, ceil(x/y) = floor((x+y-1)/y)
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mov ax, [rootdirentries]
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add ax, 15
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shr ax, 1
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shr ax, 1
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shr ax, 1
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shr ax, 1
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mov [rootdirsectors], ax
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fat:
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; FAT1 (the main one) follows right after bootsector(s)
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mov ax, [reservedsectors]
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call chs
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; TODO: handle possible crossing of track boundary
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mov ax, [sectorsperfat]
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mov ah, 2
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mov dl, [drivenumber]
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mov bx, 0x8000
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int 0x13
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root_dir:
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mov ax, [rootdir]
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call chs
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; TODO: handle possible crossing of track boundary
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mov ax, [rootdirsectors]
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mov ah, 2
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mov al, 14 ; TODO: don't hardcode
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mov dl, [drivenumber]
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mov bx, 0x500
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int 0x13
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print_root:
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mov byte [0x500 + 32*224], 0 ; TODO: don't hardcode
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search_root:
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mov bx, [rootdirentries]
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shr ax, 1
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shr ax, 1
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shr ax, 1
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shr ax, 1
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shr ax, 1
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mov byte [bx + 0x500], 0
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mov si, 0x500
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.entry:
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cmp byte [si], 0
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@ -80,36 +115,137 @@ print_root:
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jmp .entry
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.isfile:
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mov ah, 0xe
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mov cx, 8
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.name:
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mov cx, 11
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mov bx, kernel_name
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.compare:
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lodsb
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int 0x10
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loop .name
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cmp al, [bx]
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jne .nomatch
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inc bx
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loop .compare
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mov al, ' '
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int 0x10
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jmp found
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mov cx, 3
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.ext:
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lodsb
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int 0x10
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loop .ext
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mov al, 13
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int 0x10
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mov al, 10
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int 0x10
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add si, 32 - 11
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.nomatch:
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; Each entry is 32 bytes long
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; During each iteration of the compare loop, si is
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; incremented and cx is decremented
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; At the top of the loop, si + cx = entry_start + 11, but
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; when we jump here si has been incremented by one already
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; while cx is still the same
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; Therefore, si + cx is one more, so add extra - 1 here
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add si, 32 - 11 - 1
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add si, cx
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jmp .entry
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.end:
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notfound:
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; TODO: Better error messages
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mov ax, 0x0e00 + '?'
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int 0x10
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hang:
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hlt
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jmp hang
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found:
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; SI points to 11 bytes after the start of the entry, so adjust all
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; offsets
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; TODO: Handle zero-length files
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mov ax, [si - 11 + 26] ; First cluster
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push ax
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; Load OS at the start of the memory
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mov bx, 0x500
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push bx
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.tosector
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; Adjust the cluster number to account for first two
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; "clusters" in the FAT being used for metadata
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sub ax, 2
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; Scale by number of sectors per cluster
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xor bx, bx
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mov bl, [sectorspercluster]
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mul word bx
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; Offset by number of non-data sectors before data area
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add ax, [rootdir]
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add ax, [rootdirsectors]
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; Load sectors
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pop bx
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xor cx, cx
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mov cl, [sectorspercluster]
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mov dl, [drivenumber]
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.loop:
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push ax
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push cx
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call chs
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mov ah, 2
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mov al, 1
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int 0x13
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pop cx
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pop ax
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add bx, 512
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inc ax
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loop .loop
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.next:
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pop ax
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; Multiply by 1.5 to get offset into the table
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; This rounds down, which is what we want in order to get
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; the first byte of the address
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mov si, ax
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shl si, 1
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add si, ax
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shr si, 1
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; Load the entry from FAT
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mov dx, [si + 0x8000]
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; Two clusters, 0xABC and 0xXYZ, are stored as
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; BC ZA XY
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; ^ where we start reading on even cluster numbers
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; ^^^^^ loading word → 0xZABC
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; ^ where we start reading on odd cluster numbers
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; ^^^^^ loading word: 0xXYZA
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test ax, 1
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jnz .odd
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.even:
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and dx, 0x0fff
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jmp .check_cluster
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.odd:
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shr dx, 1
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shr dx, 1
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shr dx, 1
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shr dx, 1
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.check_cluster:
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mov ax, dx
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; End of chain
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cmp ax, 0xFF8
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jg execute_kernel
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push ax
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push bx
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jmp .tosector
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execute_kernel:
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mov dl, [drivenumber]
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jmp 0:0x500
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chs:
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push ax
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push bx
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@ -154,7 +290,10 @@ chs:
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%include "hexprint.inc"
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kernel_name: db "KERNEL BIN"
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times 510-($-$$) db 0
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db 0x55, 0xaa
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_end:
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rootdir equ _end
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rootdirsectors equ _end + 2
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@ -0,0 +1,8 @@
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org 0x500
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mov ax, 0x0e40
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int 0x10
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hang:
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hlt
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jmp hang
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