BootSector

The Boot sector

GeneralDesign

loaded and run by the bios at PC boot up
16 bits RealMode
nasm source code
BiosBased text output
BiosBased floppy access
no keyboard access

Boot sector Structure

Offset	Len	Description
0000	3	jmp XXXX floowed by a NOP
0003	?	DiskParameterTable
XXXX	?	BootSector code
YYYY	?	filled with 0 (not necessary)
1FFE	2	55AA for a bootable floppy flag

Disk parameter table

This table describes the format of the FloppyDisk on which it resides.

It is situated at offset 0003 in the BootSector binary:

Description	Len	360	720	1200	1440	All Formats
OEM signature	8					'TCOS 0.0'
bytes per sector	2					512
sectors per cluster	1					1
reserved sectors	2					1
number of fats	1					2
number of root entries	2					224
number of sectors	2	720	1440	2400	2880
Media Type	1	FD	F9	F9	F0
Sectors per fat	2					9
Sectors per track	2	9	9	15	18
Number of heads	2					2
Hidden sectors	4					0
Large Sectors	4					0
Drive number	1					0 A:
NT "dirty" flags	1					0
Extended BIOS paramter block	1					29h
Serial Number	4					0
Disk Label	11					'TCOS '
Fat system	8					'FAT12 '

The BootSector source code (boot12.asm) is written for a 1440 kb FloppyDisk, hence its binary (boot12.bin) must be patched according to the table above to give you boot360.bin, boot720.bin, boot1220.bin and boot1440.bin. This patching is made via a DebugScript.

Execution scheme

segments registers setup
registers setup
eventually copy the boot sector code to a safe place
Display a welcome message
loads the org 0000h flat 16 bit RealMode KernelLoader binary from file 'LOADER.SYS' into memory at segment $1000, offset $0000 (linear address $10000)

find the loader file entry in the directory. BootSectorPanic if it can not be found.
load the afferent clusters/sectors into memory. BootSectorPanic if an error occurs.

run the loader (absolute jump to $1000:$0000)

BootMedium

The BootSector is designed to work on any fat12 formatted floppy with any DiskGeometry, which is HardCoded into a DiskParameterTable in the BootSector code.

Thus, the BootSector binary must be patched with the debug script to be usable from any DiskGeometry.

Installation

The boot sector is copied onto a floppy image during the BuildProcess.

Done

fat12 FileSystem based
loads its TestSuite from the LOADER.SYS file
TestSuite

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