Atari MEGA STe Technical Information download pdf (Page 46)

9.5 TOS Partition Structures

This section details the content of a TOS primary partition

9.5.1 TOS Boot sector

The Boot Sector is located in the first logical sector of a logical drive (standard partition) and it

occupies one logical sector. When a logical sector contains more than one physical (512-byte) sectors,

the Boot Sector will be bigger than 512 bytes. However, only the first 512 bytes of a Boot Sector are

used, no matter how big the Boot Sector might be. The rest of the Boot Sector is zero-filled.

This sector is read by the TOS to find important information about the disk. Some parameters are

loaded from this sector to be used by the BIOS and are stored in a TOS structure called the BPB

(Bios Parameter Block). Eventually the Boot Sector also contains a bootstrap routine that allow

starting a relocatable program at boot time.

The fields in the Boot Sector:

Name

Offset

Bytes

Contents

BRA

$00

This word contains a 680x0 BRA.S instruction to the bootstrap code in

this sector if the disk is executable, otherwise it is unused.

OEM

$02

These six bytes are reserved for use as any necessary filler information.

SERIAL

$08

The low 24-bits of this long represent a unique disk serial number.

BPS

$0B

This is an Intel format word (big-endian) which indicates the size of a

logical sector in number of bytes.

SPC

$0D

This is a byte which indicates the number of sectors per cluster (must be

a power of 2). The only value supported by GEMDOS is 2.

RES

$0E

This is an Intel format word which indicates the number of reserved

logical sectors at the beginning of the logical drive, including the boot

sector itself. This value is usually one.

NFATS

$10

This is a byte indicating the number of File Allocation Table's (FAT's)

stored on the logical drive. Usually the value is two.

NDIRS

$11

This is an Intel format word indicating the total number of file name

entries that can be stored in the root directory of the logical drive.

NSECTS

$13

This is an Intel format word indicating the total number of logical sectors

on a logical drive including the reserved sectors.

MEDIA

$15

This byte is the media descriptor. For hard disks this value is set to $F8.

It is not used by the ST BIOS.

SPF

$16

This is an Intel format word indicating the size occupied by each of the

FATs in number of logical sectors

SPT

$18

This is an Intel format word indicating the number of sectors per track.

Not applicable to Hard Disk.

NHEADS

$1A

This is an Intel format word indicating the number of heads on the

media. Not applicable to Hard Disk.

NHID

$1C

This is an Intel format word indicating the number of hidden sectors. Not

applicable to Hard Disk.

$1E

Boot Code if Any

$1FE

Checksum

The grayed areas are read from the boot sector and stored in the BPB.

The last word in the boot sector (at offset $1FE) is reserved for the sector checksum. To be bootable a

Boot Sector checksum must be equal to the magic number $1234. During system initialization, the

first 512 bytes of the boot sector from a logical drive are loaded into a buffer. If the checksum is

correct, the system JSRs the first byte of the buffer. Since location of the buffer is indeterminate, any

code contained in the boot sector must be position independent.

The Atari BPB is based on MS-DOS version 2.x BPB.

Given this information, together with the number of FATs and reserved sectors listed above, we can compute where the root

directory begins. Given the number of entries in the root directory, we can also compute where the user data area of the disk

begins.

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 68 69

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Atari MEGA STe Technical Information Page 46

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