--- /dev/null
+; -*- fundamental -*-
+; Boot sector for GeekOS
+; Copyright (c) 2001,2004 David H. Hovemeyer <daveho@cs.umd.edu>
+; Copyright (c) 2003, Jeffrey K. Hollingsworth <hollings@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"
+
+; Pad to desired offset from start symbol.
+; Usage: Pad_From_Symbol offset, symbol
+%macro Pad_From_Symbol 2
+ times (%1 - ($ - %2)) db 0
+%endmacro
+
+; ----------------------------------------------------------------------
+; 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 ax, word [setupStart]
+ mov word [sec_count], ax
+ add ax, [setupSize]
+ mov word [max_sector], ax
+.again:
+ mov ax, [sec_count]
+ push ax ; 1st param to ReadSector (log sec num)
+ push word SETUPSEG ; 2nd param to ReadSector (seg base)
+ sub ax, [setupStart] ; 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?
+ mov bx, word [max_sector]
+ cmp word [sec_count], bx
+ jl .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).
+
+ ; Figure out start sector and max sector
+
+ mov ax, word [kernelStart]
+ mov word [sec_count], ax
+ add ax, word [kernelSize]
+ mov word [max_sector], ax
+.again:
+ mov ax, [sec_count] ; logical sector on the floppy
+; mov dx, ax
+; call PrintHex
+; call PrintNL
+ push ax ; 1st param to ReadSector (log sec num)
+ sub ax, [kernelStart] ; 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
+; mov dx, ax
+; call PrintHex
+; call PrintNL
+ 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)
+; mov dx, cx
+; call PrintHex
+; call PrintNL
+
+
+ ; 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?
+ mov bx, word [max_sector]
+ cmp word [sec_count], bx
+ jl .again
+
+load_vm:
+ ; Load the guest image starting at 1MB
+ ; 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).
+
+ ; Figure out start sector and max sector
+ mov ax, word [vmStart]
+ mov word [sec_count], ax
+ add ax, word [vmSize]
+ mov word [max_sector], ax
+.again2:
+
+ mov ax, [sec_count] ; logical sector on the floppy
+ push ax ; 1st param to ReadSector (log sec num)
+
+; mov dx, ax
+; call PrintHex
+; call PrintNL
+
+ mov ax, VMSEG ; ...to get base relative to VMSEG
+ push ax ; 2nd param to ReadSector (seg base)
+
+; mov dx, ax ;
+; call PrintHex
+; call PrintNL
+
+ mov ax, 2000h ; Always write at the start of the segment
+ push ax ; 3rd parameter
+
+; mov dx, ax
+; call PrintHex
+; call PrintNL
+
+ ; read the sector from the floppy
+ call ReadSector
+ add sp, 6 ; clear 3 word params
+
+ push VMSEG
+ pop es
+
+; mov dx, word [es:2000h] ;
+; call PrintHex
+; call PrintNL
+
+
+
+; execute bios call
+
+ push cx
+ push si
+ push bx
+ push es ;
+
+ push cs
+ pop es
+ mov si, bootsect_gdt
+ mov ah, 87h
+
+ mov cx, 100h
+
+ int 0x15
+
+ adc ax, 0
+
+; mov dx, ax
+; call PrintHex
+; call PrintNL
+
+
+
+ pop es ;
+ pop bx
+ pop si
+ pop cx
+
+ ; on to next sector
+ inc word [sec_count]
+
+
+ ; update the low->high copy table for the bios
+; mov ax, word [bootsect_src_base] ;
+; add ax, 512
+; mov dx,ax;
+; call PrintHex
+; adc byte [bootsect_src_base+2], 0
+; mov word [bootsect_src_base],ax
+
+ mov ax, word [bootsect_dst_base] ;
+ add ax, 512
+ adc byte [bootsect_dst_base+2], 0
+ mov word [bootsect_dst_base],ax
+
+ ; have we loaded all of the sectors?
+
+ mov bx, word [max_sector]
+ cmp word [sec_count], bx
+
+
+.stall
+; jmp .skip
+ jl .again2
+
+.skip
+ ; 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"
+; REPLACED WITH FOLLOWING WHICH MUST BE COMPILED 16 FOR USE IN THIS CODE
+PrintHex:
+ pusha
+ mov cx, 4 ; 4 hex digits
+.PrintDigit:
+ rol dx, 4 ; rotate so that lowest 4 bits are used
+ mov ax, 0E0Fh ; ah = request, al = mask for nybble
+ and al, dl
+ add al, 90h ; convert al to ascii hex (four instructions)
+ daa ; I've spent 1 hour to understand how it works..
+ adc al, 40h
+ daa
+ int 10h
+ loop .PrintDigit
+ popa
+ ret
+
+; Print a newline.
+PrintNL: ; print CR and NL
+ push ax
+ mov ax, 0E0Dh ; CR
+ int 10h
+ mov al, 0Ah ; LF
+ int 10h
+ pop ax
+ ret
+
+
+
+; ----------------------------------------------------------------------
+; 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
+max_sector: dw 0
+
+
+
+
+bootsect_gdt:
+ dw 0,0,0,0
+ dw 0,0,0,0
+bootsect_src:
+ dw 0xffff
+bootsect_src_base:
+ db 0,0x20,0x08 ; ! base = 0x082000
+ db 0x93 ; ! typbyte
+ dw 0 ; ! limit16,base24 =0
+bootsect_dst:
+ dw 0xffff
+bootsect_dst_base:
+ db 0,0,0x10 ; ! base = 0x100000
+ db 0x93 ; ! typbyte
+ dw 0 ; ! limit16,base24 =0
+ dw 0,0,0,0 ; ! BIOS CS
+ dw 0,0,0,0 ; ! BIOS DS
+
+
+; Padding to make the PFAT Boot Record sit just before the BIOS signature.
+;Pad_From_Symbol PFAT_BOOT_RECORD_OFFSET, BeginText
+
+; PFAT boot record
+; Describes how to load the setup program and kernel.
+; The default values are appropriate for creating a boot
+; floppy by concatenating the boot sector, setup program,
+; and kernel image. The buildFat program will change
+; these values if the boot floppy is formatted as a PFAT
+; filesystem.
+ dw 0
+ dw 0
+
+ dw 0
+ dw 0
+
+ dw 0
+ dw 0
+
+ dw 0
+ dw 0
+
+ dw 0
+ dw 0
+
+;; part of pfat boot record
+setupStart:
+ dw 1 ; by default, setup is at first sector
+
+;; part of pfat boot record
+setupSize:
+ dw NUM_SETUP_SECTORS ; number of sectors in setup
+
+;; part of pfat boot record
+kernelStart:
+ dw 1+NUM_SETUP_SECTORS ; default start sector for kernel
+
+;; part of pfat boot record
+kernelSize:
+ dw NUM_KERN_SECTORS
+
+ ;; part of pfat boot record
+vmStart:
+ dw 1+NUM_SETUP_SECTORS+NUM_KERN_SECTORS
+ ;; part of pfat boot record
+
+vmSize:
+ dw NUM_VM_KERNEL_SECTORS
+
+
+ ; Finish by writing the BIOS signature to mark this as
+; a valid boot sector.
+Pad_From_Symbol BIOS_SIGNATURE_OFFSET, BeginText
+Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector
