Rework VBR
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c288bbffb9
commit
1259fe14fe
3
Makefile
3
Makefile
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@ -23,12 +23,13 @@ kernel.rom: kernel/*.asm kernel/*.inc lib/*.inc
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nasm -s -o $@ -I lib -I kernel kernel/main.asm && scripts/fix-rom.sh $@
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boot/%.bin: boot/%.asm lib/*.inc
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nasm -s -o $@ -I lib $<
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nasm -s -o $@ -I boot -I lib $<
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fdimage.img: boot/floppy.bin kernel.rom
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dd if=/dev/zero bs=512 count=$$(( $(FD_CYLINDERS) * $(FD_HEADS) * $(FD_SECTORS) )) of=$@
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mformat -i $@ -t $(FD_CYLINDERS) -h $(FD_HEADS) -n $(FD_SECTORS) -B boot/floppy.bin ::
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mcopy -i $@ kernel.rom ::kernel.rom
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mattrib -i $@ +s ::kernel.rom
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clean:
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rm -f *.com *.bin *.rom *.img *.log boot/*.bin
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@ -0,0 +1,47 @@
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fix_chs:
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mov ah, 0x08
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int 0x13
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jc chs_end ; skip if function does not exist
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inc dh
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mov [cs:fdc.nos], dh
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mov ax, cx
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and ax, 0x003F
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mov [cs:fdc.spt], ax ; no adjustment because sectors are 1-indexed
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mov ax, cx
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xchg al, ah
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mov cl,6
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shr ah,cl
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inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
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mul word [cs:fdc.nos] ; number of tracks = number of cylinders * heads
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mul word [cs:fdc.spt] ; number of sectors = number of tracks * sectors per track
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mov [cs:fdc.ts], ax
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jmp chs_end
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; Load a single sector into memory
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; Does not return on error
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; in dx:ax sector number (will be trashed)
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; es:bx buffer
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load_sector:
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push cx
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div word [cs:fdc.spt] ; ax:temp = (lba / spt)
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inc dx ; dx:sector = (lba % spt) + 1
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mov cl, dl ; sector number
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xor dx, dx
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div word [cs:fdc.nos] ; ax:cylinder = (tmp / heads)
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; dx:head = (tmp % heads)
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mov ch, al ; cylinder number
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mov dh, dl ; head number
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mov dl, [cs:fdc.drv] ; driver number
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mov ax, 0x0201 ; ah=0x02 al=0x01
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int 0x13
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jc disk_error
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; return
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pop cx
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ret
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disk_error:
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xchg al, ah
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call error
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db "DISK ERROR", 0
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chs_end:
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260
boot/floppy.asm
260
boot/floppy.asm
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@ -1,11 +1,9 @@
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org 0x0000
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fdc: ; FDC Descriptor as per ECMA-107
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jmp _startup
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times (0x03 - ($-$$)) nop
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.oem:
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db "2B" ; creating system identifier
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times (0x0B - ($-$$)) db " " ; space-padded
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jmp 0x07C0:main
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times (0x0B - ($-$$)) db 0 ; space-padded
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fdc:
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.ss:
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dw 0x200 ; sector size
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.sc:
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@ -43,62 +41,99 @@ fdc: ; FDC Descriptor as per ECMA-107
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db "FAT12"
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times (62 - ($-$$)) db " "
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_startup:
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xor ax, ax
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; setup stack area growing down from 0x10000
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mov ss, ax
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mov sp, ax
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; adjust CS
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jmp 0x07C0:main
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; non-fdc variables
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rootdir:
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.start:
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dw 0
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.length:
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dw 0
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main:
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cli
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; backup important values
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; setup stack area growing down from directly below us
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xor ax, ax
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mov ss, ax
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mov sp, 0x7C00
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; setup base segments
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mov ax, cs
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mov ds, ax
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mov es, ax
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; save initial environment
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push bx
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push cx
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push dx
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; setup buffer area directly at 4k
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mov ax, 0x0100
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mov ds, ax
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mov es, ax
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mov [fdc.drv], dl ; fix drive number in fdc
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mov [cs:fdc.drv], dl ; backup drive number
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%include "chs.inc"
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call fix_chs
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call calc_sectors
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call find_first_cluster
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xor bx, bx
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call load_cluster
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mov di, 0x1000 ; loading address for roms
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call search_rootdir
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; restore important variables
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xor ax, ax
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pop dx
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pop cx
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pop bx
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mov di, [0x001A]
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mov ax, [di+0x1A]
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push ds
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push ax
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retf
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mov ax, 0xFFFF
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call error
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db "ROM ERROR", 0
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db "THING", 0
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search_rootdir:
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mov ax, [fdc.sf]
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mul byte [fdc.fn]
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add ax, [fdc.rsc]
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mul word [fdc.ss]
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mov si, ax ; now pointing to start of rootdir
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mov cx, [fdc.rde]
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jmp .start
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.next:
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dec cx
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jz .end
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add si, 0x20
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.start:
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call lazy_load
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cmp byte [si], 0 ; end of directory
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je .end
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cmp byte [si], 0xe5 ; deleted entry
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je .next
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cmp word [si+8], 0x4F52 ; first 2 chars of extension: "RO"
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jne .next
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mov ax, [si+10] ; 3rd char of extension, file flags
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and ax, 0x14FF ; zero out flags except directory and system
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cmp ax, 0x044D ; "M" , system flag set, directory flag unset
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jne .next
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mov ax, [si+26]
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push si
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push cx
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call loadrom
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pop cx
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pop si
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jmp .next
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.end:
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ret
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; in ax cluster number
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loadrom:
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call print16
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ret
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; Makes sure the block starting with (or under SI) is
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; already fetched from disk
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; in ds:si data expected to be fetched from disk
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; trashes ax, bx and dx
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lazy_load:
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cmp si, [max_si]
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jc .end
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; calculate next sector number
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mov ax, [max_si]
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xor dx, dx
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div word [fdc.ss]
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; write to es:bx
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xor dx, dx
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push cs
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pop es
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mov bx, [max_si]
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call load_sector
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; update max_si counter
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mov ax, [max_si]
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add ax, [fdc.ss]
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mov [max_si], ax
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; loop until requirement is satisfied
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jmp lazy_load
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.end:
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ret
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; Write AX, a string and bail out
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; Return pointer is the string start
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; String must be 0 suffixed
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error:
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call print16
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push cs
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pop ds
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pop si
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mov bx, 0x0000
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mov al, 0x20
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@ -112,132 +147,13 @@ error:
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hlt
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jmp .hlt
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; Read CHS data from BIOS and fix fdc values
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fix_chs:
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mov ah, 0x08
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int 0x13
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jc .end ; skip if function does not exist
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inc dh
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mov [cs:fdc.nos], dh
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mov ax, cx
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and ax, 0x003F
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mov [cs:fdc.spt], ax ; no adjustment because sectors are 1-indexed
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mov ax, cx
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xchg al, ah
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mov cl,6
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shr ah,cl
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inc ax ; convert from maximum number (0-based) to total number (1-based) of cylinders
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mul word [cs:fdc.nos] ; number of tracks = number of cylinders * heads
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mul word [cs:fdc.spt] ; number of sectors = number of tracks * sectors per track
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mov [cs:fdc.ts], ax
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.end:
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ret
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calc_sectors:
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mov bx, [cs:fdc.ss] ; bytes per sector
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mov cl, 5
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shr bx, cl ; div by 2^5, 32 bytes per entry
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mov ax, [cs:fdc.rde] ; number of entries
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xor dx, dx
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div bx
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mov [cs:rootdir.length], ax
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mov ax, [cs:fdc.sf] ; sectors per fat
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mul byte [cs:fdc.fn] ; number of fats
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add ax, [cs:fdc.rsc] ; reserved sectors
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mov [cs:rootdir.start], ax
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ret
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; Load the root directory
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find_first_cluster:
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mov ax, [cs:rootdir.start]
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mov cx, [cs:rootdir.length]
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xor bx, bx
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.blkloop:
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call loadblk
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add bx, [cs:fdc.ss]
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add ax, 1
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loop .blkloop
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push cs
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pop es
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mov cx, [cs:fdc.rde]
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xor bx, bx
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.dirloop:
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push cx
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mov cx, 0x000B
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mov si, bx
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mov di, bootfilename
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repe cmpsb
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pop cx
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jz .found
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add bx, 0x0020 ; 32 bytes per directory entry
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loop .dirloop
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call error
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db "NOT FOUND", 0
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.found:
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push ds
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pop es
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add bx, 26
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mov ax, [bx]
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ret
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; Load a single FAT cluster into memory
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; in ax cluster number
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; es:bx buffer
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;
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load_cluster:
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xor ch, ch
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mov cl, [cs:fdc.sc] ; sectors per cluster
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xor dx, dx
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dec ax ; somehow 2 is first cluster
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div word cx
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add ax, [cs:rootdir.start]
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add ax, [cs:rootdir.length]
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.loop:
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call loadblk
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inc ax
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add bx, [cs:fdc.ss]
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loop .loop
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ret
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; Load a single block into memory
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; Does not return on error
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; in ax sector number
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; es:bx buffer
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loadblk:
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push ax
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push cx
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push dx
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xor dx, dx
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div word [cs:fdc.spt] ; ax:temp = (lba / spt)
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inc dx ; dx:sector = (lba % spt) + 1
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mov cl, dl ; sector number
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xor dx, dx
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div word [cs:fdc.nos] ; ax:cylinder = (tmp / heads)
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; dx:head = (tmp % heads)
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mov ch, al ; cylinder number
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mov dh, dl ; head number
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mov dl, [cs:fdc.drv] ; driver number
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mov ax, 0x0201 ; ah=0x02 al=0x01
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int 0x13
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jc .error
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pop dx
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pop cx
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pop ax
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ret
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.error:
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xchg al, ah
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call error
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db "DISK ERROR", 0
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%include "print.asm"
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times (0x1F0 - ($-$$)) db 0
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times (0x1FA - ($-$$)) db 0
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bootfilename:
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db "KERNEL"
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times (0x1F8 - ($-$$)) db " "
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db "ROM"
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dw 0x2B2B
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times (0x1FE - ($-$$)) db 0
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max_si:
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dw 0x200
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dw 0xAA55
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