+++ /dev/null
-; Boot sector for GeekOS
-; Copyright (c) 2001, David H. Hovemeyer <daveho@cs.umd.edu>
-; $Revision: 1.1 $
-
-; This is free software. You are permitted to use,
-; redistribute, and modify it as specified in the file "COPYING".
-
-; Loads setup code and a program image from sectors 1..n of a floppy
-; and executes the setup code (which will in turn execute
-; the program image).
-
-; Some of this code is adapted from Kernel Toolkit 0.2
-; and Linux version 2.2.x, so the following copyrights apply:
-
-; Copyright (C) 1991, 1992 Linus Torvalds
-; modified by Drew Eckhardt
-; modified by Bruce Evans (bde)
-; adapted for Kernel Toolkit by Luigi Sgro
-
-%include "defs.asm"
-
-; This macro is used to calculate padding needed
-; to ensure that the boot sector is exactly 512 bytes
-; in size. The argument is the desired offset to be
-; padded to.
-%macro PadFromStart 1
- times (%1 - ($ - BeginText)) db 0
-%endmacro
-
-KERN_START_SEC equ (NUM_SETUP_SECTORS + 1)
-
-BIOS_SIGNATURE_OFFSET equ 510
-
-; ----------------------------------------------------------------------
-; The actual code
-; ----------------------------------------------------------------------
-
-[BITS 16]
-[ORG 0x0]
-
-BeginText: ; needed to calculate padding bytes to fill the sector
-
- ; Copy the boot sector into INITSEG.
- mov ax, BOOTSEG
- mov ds, ax ; source segment for string copy
- xor si, si ; source index for string copy
- mov ax, INITSEG
- mov es, ax ; destination segment for string copy
- xor di, di ; destination index for string copy
- cld ; clear direction flag
- mov cx, 256 ; number of words to copy
- rep movsw ; copy 512 bytes
-
- jmp INITSEG:after_move
-
-after_move:
- ; Now we're executing in INITSEG
-
- ; We want the data segment to refer to INITSEG
- ; (since we've defined variables in the same place as the code)
- mov ds, ax ; ax still contains INITSEG
-
- ; Put the stack in the place where we were originally loaded.
- ; By definition, there is nothing important there now.
- mov ax, 0
- mov ss, ax
- mov sp, (BOOTSEG << 4) + 512 - 2
-
-load_setup:
- ; Load the setup code.
- mov word [sec_count], 1
-.again:
- mov ax, [sec_count]
- push ax ; 1st param to ReadSector (log sec num)
- push word SETUPSEG ; 2nd param to ReadSector (seg base)
- dec ax ; convert to 0-indexed
- shl ax, 9 ; multiply by 512
- push ax ; ...to get 3rd param (byte offset)
-
- ; read the sector from the floppy
- call ReadSector
- add sp, 6 ; clear 3 word params
-
- ; on to next sector
- inc word [sec_count]
-
- ; are we done?
- cmp word [sec_count], NUM_SETUP_SECTORS
- jle .again
-
-load_kernel:
- ; Load the kernel image from sectors KERN_START_SEC..n of the
- ; floppy into memory at KERNSEG. Note that there are 128 sectors
- ; per 64K segment. So, when figuring out which segment to
- ; load the sector into, we shift right by 7 bits (which is
- ; equivalent to dividing by 128).
- mov word [sec_count], KERN_START_SEC
-.again:
- mov ax, [sec_count] ; logical sector on the floppy
- push ax ; 1st param to ReadSector (log sec num)
- sub ax, KERN_START_SEC ; convert to 0-indexed
- mov cx, ax ; save in cx
- shr ax, 7 ; divide by 128
- shl ax, 12 ; ...and multiply by 0x1000
- add ax, KERNSEG ; ...to get base relative to KERNSEG
- push ax ; 2nd param to ReadSector (seg base)
- and cx, 0x7f ; mod sector by 128
- shl cx, 9 ; ...and multiply by 512
- push cx ; to get offset in segment (3rd parm)
-
- ; read the sector from the floppy
- call ReadSector
- add sp, 6 ; clear 3 word params
-
- ; on to next sector
- inc word [sec_count]
-
- ; have we loaded all of the sectors?
- cmp word [sec_count], KERN_START_SEC+NUM_KERN_SECTORS
- jl .again
-
- ; Now we've loaded the setup code and the kernel image.
- ; Jump to setup code.
- jmp SETUPSEG:0
-
-; Read a sector from the floppy drive.
-; This code (and the rest of this boot sector) will have to
-; be re-written at some point so it reads more than one
-; sector at a time.
-;
-; Parameters:
-; - "logical" sector number [bp+8]
-; - destination segment [bp+6]
-; - destination offset [bp+4]
-ReadSector:
- push bp ; set up stack frame
- mov bp, sp ; "
- pusha ; save all registers
-
-%if 0
-; debug params
- mov dx, [bp+8]
- call PrintHex
- call PrintNL
- mov dx, [bp+6]
- call PrintHex
- call PrintNL
- mov dx, [bp+4]
- call PrintHex
- call PrintNL
-%endif
-
- ; Sector = log_sec % SECTORS_PER_TRACK
- ; Head = (log_sec / SECTORS_PER_TRACK) % HEADS
- mov ax, [bp+8] ; get logical sector number from stack
- xor dx, dx ; dx is high part of dividend (== 0)
- mov bx, SECTORS_PER_TRACK ; divisor
- div bx ; do the division
- mov [sec], dx ; sector is the remainder
- and ax, 1 ; same as mod by HEADS==2 (slight hack)
- mov [head], ax
-
- ; Track = log_sec / (SECTORS_PER_TRACK*HEADS)
- mov ax, [bp+8] ; get logical sector number again
- xor dx, dx ; dx is high part of dividend
- mov bx, SECTORS_PER_TRACK*2 ; divisor
- div bx ; do the division
- mov [track], ax ; track is quotient
-
-%if 0
-; debugging code
- mov dx, [sec]
- call PrintHex
- call PrintNL
- mov dx, [head]
- call PrintHex
- call PrintNL
- mov dx, [track]
- call PrintHex
- call PrintNL
-%endif
-
- ; Now, try to actually read the sector from the floppy,
- ; retrying up to 3 times.
-
- mov [num_retries], byte 0
-
-.again:
- mov ax, [bp+6] ; dest segment...
- mov es, ax ; goes in es
- mov ax, (0x02 << 8) | 1 ; function = 02h in ah,
- ; # secs = 1 in al
- mov bx, [track] ; track number...
- mov ch, bl ; goes in ch
- mov bx, [sec] ; sector number...
- mov cl, bl ; goes in cl...
- inc cl ; but it must be 1-based, not 0-based
- mov bx, [head] ; head number...
- mov dh, bl ; goes in dh
- xor dl, dl ; hard code drive=0
- mov bx, [bp+4] ; offset goes in bx
- ; (es:bx points to buffer)
-
- ; Call the BIOS Read Diskette Sectors service
- int 0x13
-
- ; If the carry flag is NOT set, then there was no error
- ; and we're done.
- jnc .done
-
- ; Error - code stored in ah
- mov dx, ax
- call PrintHex
- inc byte [num_retries]
- cmp byte [num_retries], 3
- jne .again
-
- ; If we got here, we failed thrice, so we give up
- mov dx, 0xdead
- call PrintHex
-.here: jmp .here
-
-.done:
- popa ; restore all regisiters
- pop bp ; leave stack frame
- ret
-
-; Include utility routines
-%include "util.asm"
-
-; ----------------------------------------------------------------------
-; Variables
-; ----------------------------------------------------------------------
-
-; These are used by ReadSector
-head: dw 0
-track: dw 0
-sec: dw 0
-num_retries: db 0
-
-
-; Used for loops reading sectors from floppy
-sec_count: dw 0
-
-
-PadFromStart BIOS_SIGNATURE_OFFSET
-Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector