182 lines
7.0 KiB
NASM
182 lines
7.0 KiB
NASM
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; sector.asm
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;
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; writes a boot sector out to the floppy disk in the A: drive
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;
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; The input file is assumed to be a binary file with the relevant code
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; at offset 7C00h relative to the beginning of the file. This is because
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; when the sector loader transfers control to the boot sector, it's at
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; address 0:7C00h. Rather than clutter up the boot loader source with
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; a bunch of ugly offsets, we simply use an ORG 7C00h instead and let
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; the linker insert a bunch of empty space which this program skips over.
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;
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; Style Note:
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; There aren't any hardwired numbers in this code. That is to say,
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; equates and macros are used to render gibberish like this:
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; mov ax,4c00h
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; int 33
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;
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; into somewhat self-documenting code like this:
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; DosInt DOS_TERMINATE, 0
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;
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; This is done to make the code more readable, and comprehensible, and
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; to aid in maintenance by not littering mysterious constants throughout
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; the code. Please be kind to animals (specifically your fellow
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; programmers) and use this practice in your own code.
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;
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;
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STACKSIZE = 200h ; this is how much stack we'll allocate
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SECTORSIZE = 200h ; size of the boot sector on a floppy
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CMD_LINE_LEN = 80h ; offset of the command line length (one byte)
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; relative to the beginning of the PSP.
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CMD_LINE = 81h ; the offset relative to the beginning of the
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; PSP that is the start of the command line
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; arguments.
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DOS_OPEN_HANDLE = 03dh ; open file
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READ_ONLY_DENY_NONE = 020h ; file open mode
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DOS_MOVE_HANDLE = 042h ; move file pointer
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WHENCE_BEGIN = 0 ; move pointer relative to beginning
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WHENCE_CURRENT = 1 ; move pointer relative to current location
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WHENCE_EOF = 2 ; move pointer relative to end of file
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DOS_READ_HANDLE = 03fh ; read from an open file handle
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DOS_CLOSE_HANDLE = 03eh ; close an open file handle
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DOS_WRITE_HANDLE = 040h ; write to open file
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DOS_TERMINATE = 04ch ; terminate and exit
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DOS_INT = 021h
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; various named character constants
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NUL = 0
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CR = 13
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LF = 10
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SPACE = ' '
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GenericInt macro function, subfunction
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ifb <subfunction>
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mov ah,function
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else
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mov ax,(function SHL 8) OR (subfunction AND 0ffh)
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endif
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endm
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DosInt macro function, subfunction
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GenericInt <function>,<subfunction>
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int DOS_INT
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endm
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BDISK_WRITE_SECTOR = 03h
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BDISK_INT = 013h
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; constants unique to this program
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FILE_OFFS_LO = 7C00h ;
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FILE_OFFS_HI = 0000h ;
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BOOT_DRIVE = 0 ; we'll be writing to drive A:
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BOOT_HEAD = 0 ; head 0 is the boot head
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BOOT_CYLSECT = 0001h ; a word value with the following format
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; bits 15-8 low bits of cylinder
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; bits 7-6 high two bits of cylinder
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; bits 5-0 sector
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NUM_SECTORS = 1 ; number of sector to write to disk
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model small
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.386
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.stack STACKSIZE
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.code
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;**********************************************************************
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; program code start
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;**********************************************************************
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Start:
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; parse the command line args
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mov cl,byte ptr [DGROUP:CMD_LINE_LEN] ; read the length byte
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; NOTE that the command line length isn't really part of the
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; DGROUP group, but DS currently points to the PSP, and if we
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; omit the DGROUP override, the assembler thinks we're trying
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; to load a constant instead of the contents of the memory loc.
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; In other words, it's ugly but it has a purpose.
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or cl,cl ; check for zero
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jz Usage ; no command line args
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mov si,CMD_LINE ;
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mov al,' ' ;
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repe cmpsb ; burn off leading spaces
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mov dx,si ; save that starting point
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repne cmpsb ; scan for next space (if any)
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cmp byte ptr [si],SPACE ; if it's > space char,
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ja skip ; skip the nul termination
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mov byte ptr [si],NUL ; terminate with a NUL char
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skip:
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; first, open the file
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DosInt DOS_OPEN_HANDLE, READ_ONLY_DENY_NONE
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mov si,seg DGROUP ;
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mov ds,si ;
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mov es,si ; point 'em all over there
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mov si,offset err_fopen ; can't open input file
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jc ErrorExit
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; the file's open, so move the file pointer to offset 7C00h
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mov bx,ax ; fetch the file handle
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mov cx,FILE_OFFS_HI
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mov dx,FILE_OFFS_LO
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DosInt DOS_MOVE_HANDLE, WHENCE_BEGIN
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mov si,offset err_fmove ;
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jc ErrorExit ;
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; read the data
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mov cx,SECTORSIZE ; max number of bytes to read
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mov dx,offset buffer ; point ds:dx to buffer
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DosInt DOS_READ_HANDLE ;
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mov si,offset err_fread ;
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jc ErrorExit ;
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; close the file
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DosInt DOS_CLOSE_HANDLE ; close this file
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; now write it out to the floppy disk's boot sector
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mov bx,offset buffer ;
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mov cx,BOOT_CYLSECT ;
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mov dx,(BOOT_HEAD SHL 8) OR (BOOT_DRIVE)
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GenericInt BDISK_WRITE_SECTOR, NUM_SECTORS
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int BDISK_INT
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mov si,offset err_write ;
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jc ErrorExit ;
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mov si,offset msg_ok ;
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ErrorExit:
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mov cx,[si] ;
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inc si ;
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inc si ;
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mov dx,si ;
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mov bx,1 ; write to stdout
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DosInt DOS_WRITE_HANDLE ; write to that file
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DosInt DOS_TERMINATE, 0
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Usage:
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mov si,seg DGROUP ;
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mov ds,si ; load correct data segment
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mov si,offset use_msg
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jmp ErrorExit ;
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;**********************************************************************
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; program data starts
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;**********************************************************************
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.data
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msgstruc macro msglabel, msgstring
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local alpha
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msglabel dw (alpha - $) - 2
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db msgstring
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alpha = $
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endm
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msgstruc err_fopen ,<"ERROR: couldn't open input file",CR,LF>
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msgstruc err_fmove ,<"ERROR: unable to move file pointer",CR,LF>
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msgstruc err_fread ,<"ERROR: couldn't read from input file",CR,LF>
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msgstruc err_write ,<"ERROR: unable to write to floppy disk",CR,LF>
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msgstruc msg_ok ,<"Boot sector was successfully written to floppy",CR,LF>
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msgstruc use_msg ,<"Usage: SECTOR infile.bin",CR,LF>
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buffer db SECTORSIZE dup (?) ; sector buffer
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end Start
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